32 Concurrency support library [thread]

32.1 General [thread.general]

The following subclauses describe components to create and manage threads, perform mutual exclusion, and communicate conditions and values between threads, as summarized in Table 149 .

Table 149: Concurrency support library summary [tab:thread.summary]

🔗		Subclause	Header
🔗	[thread.req]	Requirements
🔗	[thread.stoptoken]	Stop tokens	<stop_token>
🔗	[thread.threads]	Threads	<thread>
🔗	[atomics]	Atomic operations	<atomic>, <stdatomic.h>
🔗	[thread.mutex]	Mutual exclusion	<mutex>, <shared_mutex>
🔗	[thread.condition]	Condition variables	<condition_variable>
🔗	[thread.sema]	Semaphores	<semaphore>
🔗	[thread.coord]	Coordination types	<latch>, <barrier>
🔗	[futures]	Futures	<future>
🔗	[saferecl]	Safe reclamation	<rcu>, <hazard_pointer>

32.2 Requirements [thread.req]

32.2.1 Template parameter names [thread.req.paramname]

Throughout this Clause, the names of template parameters are used to express type requirements.

Predicate is a function object type ([function.objects]).

Let pred denote an lvalue of type Predicate.

Then the expression pred() shall be well-formed and the type decltype(pred()) shall model boolean-testable ([concept.booleantestable]).

The return value of pred(), converted to bool, yields true if the corresponding test condition is satisfied, and false otherwise.

If a template parameter is named Clock, the corresponding template argument shall be a type C that meets the Cpp17Clock requirements ([time.clock.req]); the program is ill-formed if is_clock_v<C> is false.

32.2.2 Exceptions [thread.req.exception]

Some functions described in this Clause are specified to throw exceptions of type system_error ([syserr.syserr]).

Such exceptions are thrown if any of the function's error conditions is detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications.

Failure to allocate storage is reported as described in [res.on.exception.handling].

[Example 1:

Consider a function in this Clause that is specified to throw exceptions of type system_error and specifies error conditions that include operation_not_permitted for a thread that does not have the privilege to perform the operation.

Assume that, during the execution of this function, an errno of EPERM is reported by a POSIX API call used by the implementation.

Since POSIX specifies an errno of EPERM when “the caller does not have the privilege to perform the operation”, the implementation maps EPERM to an error_condition of operation_not_permitted ([syserr]) and an exception of type system_error is thrown.

— end example]

The error_code reported by such an exception's code() member function compares equal to one of the conditions specified in the function's error condition element.

32.2.3 Native handles [thread.req.native]

Several classes described in this Clause have members native_handle_type and native_handle.

The presence of these members and their semantics is implementation-defined.

[Note 1:

These members allow implementations to provide access to implementation details.

Their names are specified to facilitate portable compile-time detection.

Actual use of these members is inherently non-portable.

— end note]

32.2.4 Timing specifications [thread.req.timing]

Several functions described in this Clause take an argument to specify a timeout.

These timeouts are specified as either a duration or a time_point type as specified in [time].

Implementations necessarily have some delay in returning from a timeout.

Any overhead in interrupt response, function return, and scheduling induces a “quality of implementation” delay, expressed as duration

D_{i}

Ideally, this delay would be zero.

Further, any contention for processor and memory resources induces a “quality of management” delay, expressed as duration

D_{m}

The delay durations may vary from timeout to timeout, but in all cases shorter is better.

The functions whose names end in _for take an argument that specifies a duration.

These functions produce relative timeouts.

Implementations should use a steady clock to measure time for these functions.297

Given a duration argument

D_{t}

, the real-time duration of the timeout is

D_{t} + D_{i} + D_{m}

The functions whose names end in _until take an argument that specifies a time point.

These functions produce absolute timeouts.

Implementations should use the clock specified in the time point to measure time for these functions.

Given a clock time point argument

C_{t}

, the clock time point of the return from timeout should be

C_{t} + D_{i} + D_{m}

when the clock is not adjusted during the timeout.

If the clock is adjusted to the time

C_{a}

during the timeout, the behavior should be as follows:

(4.1)
If $C_{a} > C_{t}$ , the waiting function should wake as soon as possible, i.e., $C_{a} + D_{i} + D_{m}$ , since the timeout is already satisfied.

This specification may result in the total duration of the wait decreasing when measured against a steady clock.
(4.2)
If $C_{a} \leq C_{t}$ , the waiting function should not time out until Clock::now() returns a time $C_{n} \geq C_{t}$ , i.e., waking at $C_{t} + D_{i} + D_{m}$ .

[Note 1:
When the clock is adjusted backwards, this specification can result in the total duration of the wait increasing when measured against a steady clock.

When the clock is adjusted forwards, this specification can result in the total duration of the wait decreasing when measured against a steady clock.
— end note]

An implementation returns from such a timeout at any point from the time specified above to the time it would return from a steady-clock relative timeout on the difference between

C_{t}

and the time point of the call to the _until function.

Recommended practice: Implementations should decrease the duration of the wait when the clock is adjusted forwards.

[Note 2:

If the clock is not synchronized with a steady clock, e.g., a CPU time clock, these timeouts can fail to provide useful functionality.

— end note]

The resolution of timing provided by an implementation depends on both operating system and hardware.

The finest resolution provided by an implementation is called the native resolution.

Implementation-provided clocks that are used for these functions meet the Cpp17TrivialClock requirements ([time.clock.req]).

A function that takes an argument which specifies a timeout will throw if, during its execution, a clock, time point, or time duration throws an exception.

Such exceptions are referred to as timeout-related exceptions.

[Note 3:

Instantiations of clock, time point and duration types supplied by the implementation as specified in [time.clock] do not throw exceptions.

— end note]

297)297)

Implementations for which standard time units are meaningful will typically have a steady clock within their hardware implementation.

32.2.5 Requirements for Cpp17Lockable types [thread.req.lockable]

32.2.5.1 General [thread.req.lockable.general]

An execution agent is an entity such as a thread that may perform work in parallel with other execution agents.

[Note 1:

Implementations or users can introduce other kinds of agents such as processes or thread-pool tasks.

— end note]

The calling agent is determined by context, e.g., the calling thread that contains the call, and so on.

[Note 2:

Some lockable objects are “agent oblivious” in that they work for any execution agent model because they do not determine or store the agent's ID (e.g., an ordinary spin lock).

— end note]

The standard library templates unique_lock ([thread.lock.unique]), shared_lock ([thread.lock.shared]), scoped_lock ([thread.lock.scoped]), lock_guard ([thread.lock.guard]), lock, try_lock ([thread.lock.algorithm]), and condition_variable_any ([thread.condition.condvarany]) all operate on user-supplied lockable objects.

The Cpp17BasicLockable requirements, the Cpp17Lockable requirements, the Cpp17TimedLockable requirements, the Cpp17SharedLockable requirements, and the Cpp17SharedTimedLockable requirements list the requirements imposed by these library types in order to acquire or release ownership of a lock by a given execution agent.

[Note 3:

The nature of any lock ownership and any synchronization it entails are not part of these requirements.

— end note]

A lock on an object m is said to be

(4.1)
a non-shared lock if it is acquired by a call to lock, try_lock, try_lock_for, or try_lock_until on m, or
(4.2)
a shared lock if it is acquired by a call to lock_shared, try_lock_shared, try_lock_shared_for, or try_lock_shared_until on m.

[Note 4:

Only the method of lock acquisition is considered; the nature of any lock ownership is not part of these definitions.

— end note]

32.2.5.2 Cpp17BasicLockable requirements [thread.req.lockable.basic]

A type L meets the Cpp17BasicLockable requirements if the following expressions are well-formed and have the specified semantics (m denotes a value of type L).

🔗

m.lock()

Effects: Blocks until a lock can be acquired for the current execution agent.

If an exception is thrown then a lock shall not have been acquired for the current execution agent.

🔗

m.unlock()

Preconditions: The current execution agent holds a non-shared lock on m.

Effects: Releases a non-shared lock on m held by the current execution agent.

Throws: Nothing.

32.2.5.3 Cpp17Lockable requirements [thread.req.lockable.req]

A type L meets the Cpp17Lockable requirements if it meets the Cpp17BasicLockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L).

🔗

m.try_lock()

Effects: Attempts to acquire a lock for the current execution agent without blocking.

If an exception is thrown then a lock shall not have been acquired for the current execution agent.

Return type: bool.

Returns: true if the lock was acquired, otherwise false.

32.2.5.4 Cpp17TimedLockable requirements [thread.req.lockable.timed]

A type L meets the Cpp17TimedLockable requirements if it meets the Cpp17Lockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L, rel_time denotes a value of an instantiation of duration, and abs_time denotes a value of an instantiation of time_point).

🔗

m.try_lock_for(rel_time)

Effects: Attempts to acquire a lock for the current execution agent within the relative timeout ([thread.req.timing]) specified by rel_time.

The function will not return within the timeout specified by rel_time unless it has obtained a lock on m for the current execution agent.

If an exception is thrown then a lock has not been acquired for the current execution agent.

Return type: bool.

Returns: true if the lock was acquired, otherwise false.

🔗

m.try_lock_until(abs_time)

Effects: Attempts to acquire a lock for the current execution agent before the absolute timeout ([thread.req.timing]) specified by abs_time.

The function will not return before the timeout specified by abs_time unless it has obtained a lock on m for the current execution agent.

If an exception is thrown then a lock has not been acquired for the current execution agent.

Return type: bool.

Returns: true if the lock was acquired, otherwise false.

32.2.5.5 Cpp17SharedLockable requirements [thread.req.lockable.shared]

A type L meets the Cpp17SharedLockable requirements if the following expressions are well-formed, have the specified semantics, and the expression m.try_lock_shared() has type bool (m denotes a value of type L):

🔗

m.lock_shared()

Effects: Blocks until a lock can be acquired for the current execution agent.

If an exception is thrown then a lock shall not have been acquired for the current execution agent.

🔗

m.try_lock_shared()

Effects: Attempts to acquire a lock for the current execution agent without blocking.

If an exception is thrown then a lock shall not have been acquired for the current execution agent.

Returns: true if the lock was acquired, false otherwise.

🔗

m.unlock_shared()

Preconditions: The current execution agent holds a shared lock on m.

Effects: Releases a shared lock on m held by the current execution agent.

Throws: Nothing.

32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]

A type L meets the Cpp17SharedTimedLockable requirements if it meets the Cpp17SharedLockable requirements, and the following expressions are well-formed, have type bool, and have the specified semantics (m denotes a value of type L, rel_time denotes a value of a specialization of chrono::duration, and abs_time denotes a value of a specialization of chrono::time_point).

🔗

m.try_lock_shared_for(rel_time)

Effects: Attempts to acquire a lock for the current execution agent within the relative timeout ([thread.req.timing]) specified by rel_time.

The function will not return within the timeout specified by rel_time unless it has obtained a lock on m for the current execution agent.

If an exception is thrown then a lock has not been acquired for the current execution agent.

Returns: true if the lock was acquired, false otherwise.

🔗

m.try_lock_shared_until(abs_time)

Effects: Attempts to acquire a lock for the current execution agent before the absolute timeout ([thread.req.timing]) specified by abs_time.

The function will not return before the timeout specified by abs_time unless it has obtained a lock on m for the current execution agent.

If an exception is thrown then a lock has not been acquired for the current execution agent.

Returns: true if the lock was acquired, false otherwise.

32.3 Stop tokens [thread.stoptoken]

32.3.1 Introduction [thread.stoptoken.intro]

Subclause [thread.stoptoken] describes components that can be used to asynchronously request that an operation stops execution in a timely manner, typically because the result is no longer required.

Such a request is called a stop request.

The concepts stoppable-source, stoppable_token, and stoppable-callback-for specify the required syntax and semantics of shared access to a stop state.

Any object modeling stoppable-source, stoppable_token, or stoppable-callback-for that refers to the same stop state is an associated stoppable-source, stoppable_token, or stoppable-callback-for, respectively.

An object of a type that models stoppable_token can be passed to an operation that can either

(3.1)
actively poll the token to check if there has been a stop request, or
(3.2)
register a callback that will be called in the event that a stop request is made.

A stop request made via an object whose type models stoppable-source will be visible to all associated stoppable_token and stoppable-source objects.

Once a stop request has been made it cannot be withdrawn (a subsequent stop request has no effect).

Callbacks registered via an object whose type models stoppable-callback-for are called when a stop request is first made by any associated stoppable-source object.

The types stop_source and stop_token and the class template stop_callback implement the semantics of shared ownership of a stop state.

The last remaining owner of the stop state automatically releases the resources associated with the stop state.

An object of type inplace_stop_source is the sole owner of its stop state.

An object of type inplace_stop_token or of a specialization of the class template inplace_stop_callback does not participate in ownership of its associated stop state.

[Note 1:

They are for use when all uses of the associated token and callback objects are known to nest within the lifetime of the inplace_stop_source object.

— end note]

32.3.2 Header <stop_token> synopsis [thread.stoptoken.syn]

🔗

namespace std { // [stoptoken.concepts], stop token concepts template<class CallbackFn, class Token, class Initializer = CallbackFn> concept stoppable-callback-for = see below; // exposition only template<class Token> concept stoppable_token = see below; template<class Token> concept unstoppable_token = see below; template<class Source> concept stoppable-source = see below; // exposition only // [stoptoken], class stop_token class stop_token; // [stopsource], class stop_source class stop_source; // no-shared-stop-state indicator struct nostopstate_t { explicit nostopstate_t() = default; }; inline constexpr nostopstate_t nostopstate{}; // [stopcallback], class template stop_callback template<class Callback> class stop_callback; // [stoptoken.never], class never_stop_token class never_stop_token; // [stoptoken.inplace], class inplace_stop_token class inplace_stop_token; // [stopsource.inplace], class inplace_stop_source class inplace_stop_source; // [stopcallback.inplace], class template inplace_stop_callback template<class CallbackFn> class inplace_stop_callback; template<class T, class CallbackFn> using stop_callback_for_t = T::template callback_type<CallbackFn>; }

32.3.3 Stop token concepts [stoptoken.concepts]

The exposition-only stoppable-callback-for concept checks for a callback compatible with a given Token type.

template<class CallbackFn, class Token, class Initializer = CallbackFn> concept stoppable-callback-for = // exposition only invocable<CallbackFn> && constructible_from<CallbackFn, Initializer> && requires { typename stop_callback_for_t<Token, CallbackFn>; } && constructible_from<stop_callback_for_t<Token, CallbackFn>, const Token&, Initializer>;

Let t and u be distinct, valid objects of type Token that reference the same logical stop state; let init be an expression such that same_as<decltype(init), Initializer> is true; and let SCB denote the type stop_callback_for_t<Token, CallbackFn>.

The concept stoppable-callback-for<CallbackFn, Token, Initializer> is modeled only if:

(3.1)
The following concepts are modeled:
- (3.1.1)
  constructible_from<SCB, Token, Initializer>
- (3.1.2)
  constructible_from<SCB, Token&, Initializer>
- (3.1.3)
  constructible_from<SCB, const Token, Initializer>
(3.2)
An object of type SCB has an associated callback function of type CallbackFn.

Let scb be an object of type SCB and let callback_fn denote scb's associated callback function.
Direct-non-list-initializing scb from arguments t and init shall execute a stoppable callback registration as follows:
- (3.2.1)
  If t.stop_possible() is true:
  - (3.2.1.1)
    callback_fn shall be direct-initialized with init.
  - (3.2.1.2)
    Construction of scb shall only throw exceptions thrown by the initialization of callback_fn from init.
  - (3.2.1.3)
    The callback invocation std::forward<CallbackFn>(callback_fn)() shall be registered with t's associated stop state as follows:
    (3.2.1.3.1)
    If t.stop_requested() evaluates to false at the time of registration, the callback invocation is added to the stop state's list of callbacks such that std::forward<CallbackFn>(
    callback_fn)() is evaluated if a stop request is made on the stop state.
    (3.2.1.3.2)
    Otherwise, std::forward<CallbackFn>(callback_fn)() shall be immediately evaluated on the thread executing scb's constructor, and the callback invocation shall not be added to the list of callback invocations.
    
    If the callback invocation was added to stop state's list of callbacks, scb shall be associated with the stop state.
- (3.2.2)
  [Note 1:
  If t.stop_possible() is false, there is no requirement that the initialization of scb causes the initialization of callback_fn.
  — end note]
(3.3)
Destruction of scb shall execute a stoppable callback deregistration as follows (in order):
- (3.3.1)
  If the constructor of scb did not register a callback invocation with t's stop state, then the stoppable callback deregistration shall have no effect other than destroying callback_fn if it was constructed.
- (3.3.2)
  Otherwise, the invocation of callback_fn shall be removed from the associated stop state.
- (3.3.3)
  If callback_fn is concurrently executing on another thread, then the stoppable callback deregistration shall block ([defns.block]) until the invocation of callback_fn returns such that the return from the invocation of callback_fn strongly happens before ([intro.races]) the destruction of callback_fn.
- (3.3.4)
  If callback_fn is executing on the current thread, then the destructor shall not block waiting for the return from the invocation of callback_fn.
- (3.3.5)
  A stoppable callback deregistration shall not block on the completion of the invocation of some other callback registered with the same logical stop state.
- (3.3.6)
  The stoppable callback deregistration shall destroy callback_fn.

The stoppable_token concept checks for the basic interface of a stop token that is copyable and allows polling to see if stop has been requested and also whether a stop request is possible.

The unstoppable_token concept checks for a stoppable_token type that does not allow stopping.

template<template<class> class> struct check-type-alias-exists; // exposition only template<class Token> concept stoppable_token = requires (const Token tok) { typename check-type-alias-exists<Token::template callback_type>; { tok.stop_requested() } noexcept -> same_as<bool>; { tok.stop_possible() } noexcept -> same_as<bool>; { Token(tok) } noexcept; // see implicit expression variations ([concepts.equality]) } && copyable<Token> && equality_comparable<Token>; template<class Token> concept unstoppable_token = stoppable_token<Token> && requires (const Token tok) { requires bool_constant<(!tok.stop_possible())>::value; };

An object whose type models stoppable_token has at most one associated logical stop state.

A stoppable_token object with no associated stop state is said to be disengaged.

Let SP be an evaluation of t.stop_possible() that is false, and let SR be an evaluation of t.stop_requested() that is true.

The type Token models stoppable_token only if:

(7.1)
Any evaluation of u.stop_possible() or u.stop_requested() that happens after ([intro.races]) SP is false.
(7.2)
Any evaluation of u.stop_possible() or u.stop_requested() that happens after SR is true.
(7.3)
For any types CallbackFn and Initializer such that stoppable-callback-for<CallbackFn, Token, Initializer> is satisfied, stoppable-callback-for<CallbackFn, Token, Initializer> is modeled.
(7.4)
If t is disengaged, evaluations of t.stop_possible() and t.stop_requested() are false.
(7.5)
If t and u reference the same stop state, or if both t and u are disengaged, t == u is true; otherwise, it is false.

An object whose type models the exposition-only stoppable-source concept can be queried whether stop has been requested (stop_requested) and whether stop is possible (stop_possible).

It is a factory for associated stop tokens (get_token), and a stop request can be made on it (request_stop).

It maintains a list of registered stop callback invocations that it executes when a stop request is first made.

template<class Source> concept stoppable-source = // exposition only requires (Source& src, const Source csrc) { // see implicit expression variations ([concepts.equality]) { csrc.get_token() } -> stoppable_token; { csrc.stop_possible() } noexcept -> same_as<bool>; { csrc.stop_requested() } noexcept -> same_as<bool>; { src.request_stop() } -> same_as<bool>; };

An object whose type models stoppable-source has at most one associated logical stop state.

If it has no associated stop state, it is said to be disengaged.

Let s be an object whose type models stoppable-source and that is disengaged.

s.stop_possible() and s.stop_requested() shall be false.

Let t be an object whose type models stoppable-source.

If t is disengaged, t.get_token() shall return a disengaged stop token; otherwise, it shall return a stop token that is associated with the stop state of t.

Calls to the member functions request_stop, stop_requested, and stop_possible and similarly named member functions on associated stoppable_token objects do not introduce data races.

A call to request_stop that returns true synchronizes with a call to stop_requested on an associated stoppable_token or stoppable-source object that returns true.

Registration of a callback synchronizes with the invocation of that callback.

If the stoppable-source is disengaged, request_stop shall have no effect and return false.

Otherwise, it shall execute a stop request operation on the associated stop state.

A stop request operation determines whether the stop state has received a stop request, and if not, makes a stop request.

The determination and making of the stop request shall happen atomically, as-if by a read-modify-write operation ([intro.races]).

If the request was made, the stop state's registered callback invocations shall be synchronously executed.

If an invocation of a callback exits via an exception then terminate shall be invoked ([except.terminate]).

[Note 2:

No constraint is placed on the order in which the callback invocations are executed.

— end note]

request_stop shall return true if a stop request was made, and false otherwise.

After a call to request_stop either a call to stop_possible shall return false or a call to stop_requested shall return true.

[Note 3:

A stop request includes notifying all condition variables of type condition_variable_any temporarily registered during an interruptible wait ([thread.condvarany.intwait]).

— end note]

32.3.4 Class stop_token [stoptoken]

32.3.4.1 General [stoptoken.general]

The class stop_token models the concept stoppable_token .

It shares ownership of its stop state, if any, with its associated stop_source object ([stopsource]) and any stop_token objects to which it compares equal.

namespace std { class stop_token { public: template<class CallbackFn> using callback_type = stop_callback<CallbackFn>; stop_token() noexcept = default; // [stoptoken.mem], member functions void swap(stop_token&) noexcept; bool stop_requested() const noexcept; bool stop_possible() const noexcept; bool operator==(const stop_token& rhs) noexcept = default; private: shared_ptr<unspecified> stop-state; // exposition only }; }

stop-state refers to the stop_token's associated stop state.

A stop_token object is disengaged when stop-state is empty.

32.3.4.2 Member functions [stoptoken.mem]

🔗

void swap(stop_token& rhs) noexcept;

Effects: Equivalent to: stop-state.swap(rhs.stop-state);

🔗

bool stop_requested() const noexcept;

Returns: true if stop-state refers to a stop state that has received a stop request; otherwise, false.

🔗

bool stop_possible() const noexcept;

Returns: false if

(3.1)
*this is disengaged, or
(3.2)
a stop request was not made and there are no associated stop_source objects;

otherwise, true.

32.3.5 Class stop_source [stopsource]

32.3.5.1 General [stopsource.general]

namespace std { class stop_source { public: // [stopsource.cons], constructors, copy, and assignment stop_source(); explicit stop_source(nostopstate_t) noexcept {} // [stopsource.mem], member functions void swap(stop_source&) noexcept; bool request_stop() noexcept; bool operator==(const stop_source& rhs) noexcept = default; private: shared_ptr<unspecified> stop-state; // exposition only }; }

stop-state refers to the stop_source's associated stop state.

A stop_source object is disengaged when stop-state is empty.

stop_source models stoppable-source, copyable, equality_comparable, and swappable .

32.3.5.2 Constructors, copy, and assignment [stopsource.cons]

🔗

stop_source();

Effects: Initializes stop-state with a pointer to a new stop state.

Postconditions: stop_possible() is true and stop_requested() is false.

Throws: bad_alloc if memory cannot be allocated for the stop state.

32.3.5.3 Member functions [stopsource.mem]

🔗

void swap(stop_source& rhs) noexcept;

Effects: Equivalent to: stop-state.swap(rhs.stop-state);

🔗

stop_token get_token() const noexcept;

Returns: stop_token() if stop_possible() is false; otherwise a new associated stop_token object; i.e., its stop-state member is equal to the stop-state member of *this.

🔗

bool stop_possible() const noexcept;

Returns: stop-state != nullptr.

🔗

bool stop_requested() const noexcept;

Returns: true if stop-state refers to a stop state that has received a stop request; otherwise, false.

🔗

bool request_stop() noexcept;

Effects: Executes a stop request operation ([stoptoken.concepts]) on the associated stop state, if any.

32.3.6 Class template stop_callback [stopcallback]

32.3.6.1 General [stopcallback.general]

🔗

namespace std { template<class CallbackFn> class stop_callback { public: using callback_type = CallbackFn; // [stopcallback.cons], constructors and destructor template<class Initializer> explicit stop_callback(const stop_token& st, Initializer&& init) noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>); template<class Initializer> explicit stop_callback(stop_token&& st, Initializer&& init) noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>); ~stop_callback(); stop_callback(const stop_callback&) = delete; stop_callback(stop_callback&&) = delete; stop_callback& operator=(const stop_callback&) = delete; stop_callback& operator=(stop_callback&&) = delete; private: CallbackFn callback-fn; // exposition only }; template<class CallbackFn> stop_callback(stop_token, CallbackFn) -> stop_callback<CallbackFn>; }

Mandates: stop_callback is instantiated with an argument for the template parameter CallbackFn that satisfies both invocable and destructible .

Remarks: For a type Initializer, if stoppable-callback-for<CallbackFn, stop_token, Initializer> is satisfied, then stoppable-callback-for<CallbackFn, stop_token, Initializer> is modeled.

The exposition-only callback-fn member is the associated callback function ([stoptoken.concepts]) of stop_callback<
CallbackFn> objects.

32.3.6.2 Constructors and destructor [stopcallback.cons]

🔗

template<class Initializer>
  explicit stop_callback(const stop_token& st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);

template<class Initializer>
  explicit stop_callback(stop_token&& st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);

Constraints: CallbackFn and Initializer satisfy constructible_from<CallbackFn, Initializer>.

Effects: Initializes callback-fn with std::forward<Initializer>(init) and executes a stoppable callback registration ([stoptoken.concepts]).

If a callback is registered with st's shared stop state, then *this acquires shared ownership of that stop state.

🔗

~stop_callback();

Effects: Executes a stoppable callback deregistration ([stoptoken.concepts]) and releases ownership of the stop state, if any.

32.3.7 Class never_stop_token [stoptoken.never]

The class never_stop_token models the unstoppable_token concept.

It provides a stop token interface, but also provides static information that a stop is never possible nor requested.

namespace std { class never_stop_token { struct callback-type { // exposition only explicit callback-type(never_stop_token, auto&&) noexcept {} }; public: template<class> using callback_type = callback-type; static constexpr bool stop_requested() noexcept { return false; } static constexpr bool stop_possible() noexcept { return false; } bool operator==(const never_stop_token&) const = default; }; }

32.3.8 Class inplace_stop_token [stoptoken.inplace]

32.3.8.1 General [stoptoken.inplace.general]

The class inplace_stop_token models the concept stoppable_token .

It references the stop state of its associated inplace_stop_source object ([stopsource.inplace]), if any.

namespace std { class inplace_stop_token { public: template<class CallbackFn> using callback_type = inplace_stop_callback<CallbackFn>; inplace_stop_token() = default; bool operator==(const inplace_stop_token&) const = default; // [stoptoken.inplace.mem], member functions bool stop_requested() const noexcept; bool stop_possible() const noexcept; void swap(inplace_stop_token&) noexcept; private: const inplace_stop_source* stop-source = nullptr; // exposition only }; }

32.3.8.2 Member functions [stoptoken.inplace.mem]

🔗

void swap(inplace_stop_token& rhs) noexcept;

Effects: Exchanges the values of stop-source and rhs.stop-source.

🔗

bool stop_requested() const noexcept;

Effects: Equivalent to: return stop-source != nullptr && stop-source->stop_requested();

[Note 1:

As specified in [basic.life], the behavior of stop_requested is undefined unless the call strongly happens before the start of the destructor of the associated inplace_stop_source object, if any.

— end note]

🔗

stop_possible() const noexcept;

Returns: stop-source != nullptr.

[Note 2:

As specified in [basic.stc.general], the behavior of stop_possible is implementation-defined unless the call strongly happens before the end of the storage duration of the associated inplace_stop_source object, if any.

— end note]

32.3.9 Class inplace_stop_source [stopsource.inplace]

32.3.9.1 General [stopsource.inplace.general]

The class inplace_stop_source models stoppable-source.

namespace std { class inplace_stop_source { public: // [stopsource.inplace.cons], constructors constexpr inplace_stop_source() noexcept; inplace_stop_source(inplace_stop_source&&) = delete; inplace_stop_source(const inplace_stop_source&) = delete; inplace_stop_source& operator=(inplace_stop_source&&) = delete; inplace_stop_source& operator=(const inplace_stop_source&) = delete; ~inplace_stop_source(); // [stopsource.inplace.mem], stop handling constexpr inplace_stop_token get_token() const noexcept; static constexpr bool stop_possible() noexcept { return true; } bool stop_requested() const noexcept; bool request_stop() noexcept; }; }

32.3.9.2 Constructors [stopsource.inplace.cons]

🔗

constexpr inplace_stop_source() noexcept;

Effects: Initializes a new stop state inside *this.

Postconditions: stop_requested() is false.

32.3.9.3 Member functions [stopsource.inplace.mem]

🔗

constexpr inplace_stop_token get_token() const noexcept;

Returns: A new associated inplace_stop_token object whose stop-source member is equal to this.

🔗

bool stop_requested() const noexcept;

Returns: true if the stop state inside *this has received a stop request; otherwise, false.

🔗

bool request_stop() noexcept;

Effects: Executes a stop request operation ([stoptoken.concepts]).

Postconditions: stop_requested() is true.

32.3.10 Class template inplace_stop_callback [stopcallback.inplace]

32.3.10.1 General [stopcallback.inplace.general]

namespace std { template<class CallbackFn> class inplace_stop_callback { public: using callback_type = CallbackFn; // [stopcallback.inplace.cons], constructors and destructor template<class Initializer> explicit inplace_stop_callback(inplace_stop_token st, Initializer&& init) noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>); ~inplace_stop_callback(); inplace_stop_callback(inplace_stop_callback&&) = delete; inplace_stop_callback(const inplace_stop_callback&) = delete; inplace_stop_callback& operator=(inplace_stop_callback&&) = delete; inplace_stop_callback& operator=(const inplace_stop_callback&) = delete; private: CallbackFn callback-fn; // exposition only }; template<class CallbackFn> inplace_stop_callback(inplace_stop_token, CallbackFn) -> inplace_stop_callback<CallbackFn>; }

Mandates: CallbackFn satisfies both invocable and destructible .

Remarks: For a type Initializer, if stoppable-callback-for<CallbackFn, inplace_stop_token, Initializer> is satisfied, then stoppable-callback-for<CallbackFn, inplace_stop_token, Initializer> is modeled.

For an inplace_stop_callback<CallbackFn> object, the exposition-only callback-fn member is its associated callback function ([stoptoken.concepts]).

32.3.10.2 Constructors and destructor [stopcallback.inplace.cons]

🔗

template<class Initializer>
  explicit inplace_stop_callback(inplace_stop_token st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);

Constraints: constructible_from<CallbackFn, Initializer> is satisfied.

Effects: Initializes callback-fn with std::forward<Initializer>(init) and executes a stoppable callback registration ([stoptoken.concepts]).

🔗

~inplace_stop_callback();

Effects: Executes a stoppable callback deregistration ([stoptoken.concepts]).

32.4 Threads [thread.threads]

32.4.1 General [thread.threads.general]

[thread.threads] describes components that can be used to create and manage threads.

[Note 1:

These threads are intended to map one-to-one with operating system threads.

— end note]

32.4.2 Header <thread> synopsis [thread.syn]

🔗

#include <compare> // see [compare.syn] namespace std { // [thread.thread.class], class thread class thread; void swap(thread& x, thread& y) noexcept; // [thread.jthread.class], class jthread class jthread; // [thread.thread.this], namespace this_thread namespace this_thread { thread::id get_id() noexcept; void yield() noexcept; template<class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time); template<class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time); } }

32.4.3 Class thread [thread.thread.class]

32.4.3.1 General [thread.thread.class.general]

The class thread provides a mechanism to create a new thread of execution, to join with a thread (i.e., wait for a thread to complete), and to perform other operations that manage and query the state of a thread.

A thread object uniquely represents a particular thread of execution.

That representation may be transferred to other thread objects in such a way that no two thread objects simultaneously represent the same thread of execution.

A thread of execution is detached when no thread object represents that thread.

Objects of class thread can be in a state that does not represent a thread of execution.

[Note 1:

A thread object does not represent a thread of execution after default construction, after being moved from, or after a successful call to detach or join.

— end note]

🔗

namespace std { class thread { public: // [thread.thread.id], class thread::id class id; using native_handle_type = implementation-defined; // see [thread.req.native] // construct/copy/destroy thread() noexcept; template<class F, class... Args> explicit thread(F&& f, Args&&... args); ~thread(); thread(const thread&) = delete; thread(thread&&) noexcept; thread& operator=(const thread&) = delete; thread& operator=(thread&&) noexcept; // [thread.thread.member], members void swap(thread&) noexcept; bool joinable() const noexcept; void join(); void detach(); id get_id() const noexcept; native_handle_type native_handle(); // see [thread.req.native] // static members static unsigned int hardware_concurrency() noexcept; }; }

32.4.3.2 Class thread::id [thread.thread.id]

🔗

namespace std { class thread::id { public: id() noexcept; }; bool operator==(thread::id x, thread::id y) noexcept; strong_ordering operator<=>(thread::id x, thread::id y) noexcept; template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, thread::id id); template<class charT> struct formatter<thread::id, charT>; // hash support template<class T> struct hash; template<> struct hash<thread::id>; }

An object of type thread::id provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.thread.class]).

Each thread of execution has an associated thread::id object that is not equal to the thread::id object of any other thread of execution and that is not equal to the thread::id object of any thread object that does not represent threads of execution.

The text representation for the character type charT of an object of type thread::id is an unspecified sequence of charT such that, for two objects of type thread::id x and y, if x == y is true, the thread::id objects have the same text representation, and if x != y is true, the thread::id objects have distinct text representations.

thread::id is a trivially copyable class ([class.prop]).

The library may reuse the value of a thread::id of a terminated thread that can no longer be joined.

[Note 1:

Relational operators allow thread::id objects to be used as keys in associative containers.

— end note]

🔗

id() noexcept;

Postconditions: The constructed object does not represent a thread of execution.

🔗

bool operator==(thread::id x, thread::id y) noexcept;

Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.

🔗

strong_ordering operator<=>(thread::id x, thread::id y) noexcept;

Let P(x, y) be an unspecified total ordering over thread::id as described in [alg.sorting].

Returns: strong_ordering::less if P(x, y) is true.

Otherwise, strong_ordering::greater if P(y, x) is true.

Otherwise, strong_ordering::equal.

🔗

template<class charT, class traits>
  basic_ostream<charT, traits>&
    operator<< (basic_ostream<charT, traits>& out, thread::id id);

Effects: Inserts the text representation for charT of id into out.

Returns: out.

🔗

template<class charT> struct formatter<thread::id, charT>;

formatter<thread::id, charT> interprets format-spec as a thread-id-format-spec.

The syntax of format specifications is as follows:

thread-id-format-spec :
fill-and-align

_{o p t}

width

_{o p t}

[Note 2:

The productions fill-and-align and width are described in [format.string.std].

— end note]

If the align option is omitted it defaults to >.

A thread::id object is formatted by writing its text representation for charT to the output with additional padding and adjustments as specified by the format specifiers.

🔗

template<> struct hash<thread::id>;

The specialization is enabled ([unord.hash]).

32.4.3.3 Constructors [thread.thread.constr]

🔗

thread() noexcept;

Effects: The object does not represent a thread of execution.

Postconditions: get_id() == id().

🔗

template<class F, class... Args> explicit thread(F&& f, Args&&... args);

Constraints: remove_cvref_t<F> is not the same type as thread.

Mandates: The following are all true:

(4.1)
is_constructible_v<decay_t<F>, F>,
(4.2)
(is_constructible_v<decay_t<Args>, Args> && ...), and
(4.3)
is_invocable_v<decay_t<F>, decay_t<Args>...>.

Effects: The new thread of execution executes invoke(auto(std::forward<F>(f)), // for invoke, see [func.invoke] auto(std::forward<Args>(args))...) with the values produced by auto being materialized ([conv.rval]) in the constructing thread.

Any return value from this invocation is ignored.

[Note 1:

This implies that any exceptions not thrown from the invocation of the copy of f will be thrown in the constructing thread, not the new thread.

— end note]

If the invocation of invoke terminates with an uncaught exception, terminate is invoked ([except.terminate]).

Synchronization: The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f.

Postconditions: get_id() != id().

*this represents the newly started thread.

Throws: system_error if unable to start the new thread.

Error conditions:

(9.1)
resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.

🔗

thread(thread&& x) noexcept;

Postconditions: x.get_id() == id() and get_id() returns the value of x.get_id() prior to the start of construction.

32.4.3.4 Destructor [thread.thread.destr]

🔗

~thread();

Effects: If joinable(), invokes terminate ([except.terminate]).

Otherwise, has no effects.

[Note 1:

Either implicitly detaching or joining a joinable() thread in its destructor can result in difficult to debug correctness (for detach) or performance (for join) bugs encountered only when an exception is thrown.

These bugs can be avoided by ensuring that the destructor is never executed while the thread is still joinable.

— end note]

32.4.3.5 Assignment [thread.thread.assign]

🔗

thread& operator=(thread&& x) noexcept;

Effects: If joinable(), invokes terminate ([except.terminate]).

Otherwise, assigns the state of x to *this and sets x to a default constructed state.

Postconditions: x.get_id() == id() and get_id() returns the value of x.get_id() prior to the assignment.

Returns: *this.

32.4.3.6 Members [thread.thread.member]

🔗

void swap(thread& x) noexcept;

Effects: Swaps the state of *this and x.

🔗

bool joinable() const noexcept;

Returns: get_id() != id().

🔗

void join();

Effects: Blocks until the thread represented by *this has completed.

Synchronization: The completion of the thread represented by *this synchronizes with ([intro.multithread]) the corresponding successful join() return.

[Note 1:

Operations on *this are not synchronized.

— end note]

Postconditions: The thread represented by *this has completed.

get_id() == id().

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(7.1)
resource_deadlock_would_occur — if deadlock is detected or get_id() == this_thread::get_id().
(7.2)
no_such_process — if the thread is not valid.
(7.3)
invalid_argument — if the thread is not joinable.

🔗

void detach();

Effects: The thread represented by *this continues execution without the calling thread blocking.

When detach() returns, *this no longer represents the possibly continuing thread of execution.

When the thread previously represented by *this ends execution, the implementation releases any owned resources.

Postconditions: get_id() == id().

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(11.1)
no_such_process — if the thread is not valid.
(11.2)
invalid_argument — if the thread is not joinable.

🔗

id get_id() const noexcept;

Returns: A default constructed id object if *this does not represent a thread, otherwise this_thread::get_id() for the thread of execution represented by *this.

32.4.3.7 Static members [thread.thread.static]

🔗

unsigned hardware_concurrency() noexcept;

Returns: The number of hardware thread contexts.

[Note 1:

This value should only be considered to be a hint.

— end note]

If this value is not computable or well-defined, an implementation should return 0.

32.4.3.8 Specialized algorithms [thread.thread.algorithm]

🔗

void swap(thread& x, thread& y) noexcept;

Effects: As if by x.swap(y).

32.4.4 Class jthread [thread.jthread.class]

32.4.4.1 General [thread.jthread.class.general]

The class jthread provides a mechanism to create a new thread of execution.

The functionality is the same as for class thread ([thread.thread.class]) with the additional abilities to provide a stop_token ([thread.stoptoken]) to the new thread of execution, make stop requests, and automatically join.

🔗

namespace std { class jthread { public: // types using id = thread::id; using native_handle_type = thread::native_handle_type; // [thread.jthread.cons], constructors, move, and assignment jthread() noexcept; template<class F, class... Args> explicit jthread(F&& f, Args&&... args); ~jthread(); jthread(const jthread&) = delete; jthread(jthread&&) noexcept; jthread& operator=(const jthread&) = delete; jthread& operator=(jthread&&) noexcept; // [thread.jthread.mem], members void swap(jthread&) noexcept; bool joinable() const noexcept; void join(); void detach(); id get_id() const noexcept; native_handle_type native_handle(); // see [thread.req.native] // [thread.jthread.stop], stop token handling stop_source get_stop_source() noexcept; stop_token get_stop_token() const noexcept; bool request_stop() noexcept; // [thread.jthread.special], specialized algorithms friend void swap(jthread& lhs, jthread& rhs) noexcept; // [thread.jthread.static], static members static unsigned int hardware_concurrency() noexcept; private: stop_source ssource; // exposition only }; }

32.4.4.2 Constructors, move, and assignment [thread.jthread.cons]

🔗

jthread() noexcept;

Effects: Constructs a jthread object that does not represent a thread of execution.

Postconditions: get_id() == id() is true and ssource.stop_possible() is false.

🔗

template<class F, class... Args> explicit jthread(F&& f, Args&&... args);

Constraints: remove_cvref_t<F> is not the same type as jthread.

Mandates: The following are all true:

(4.1)
is_constructible_v<decay_t<F>, F>,
(4.2)
(is_constructible_v<decay_t<Args>, Args> && ...), and
(4.3)
is_invocable_v<decay_t<F>, decay_t<Args>...> ||

is_invocable_v<decay_t<F>, stop_token, decay_t<Args>...>.

Effects: Initializes ssource.

The new thread of execution executes invoke(auto(std::forward<F>(f)), get_stop_token(), // for invoke, see [func.invoke] auto(std::forward<Args>(args))...) if that expression is well-formed, otherwise invoke(auto(std::forward<F>(f)), auto(std::forward<Args>(args))...) with the values produced by auto being materialized ([conv.rval]) in the constructing thread.

Any return value from this invocation is ignored.

[Note 1:

This implies that any exceptions not thrown from the invocation of the copy of f will be thrown in the constructing thread, not the new thread.

— end note]

If the invoke expression exits via an exception, terminate is called.

Synchronization: The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f.

Postconditions: get_id() != id() is true and ssource.stop_possible() is true and *this represents the newly started thread.

[Note 2:

The calling thread can make a stop request only once, because it cannot replace this stop token.

— end note]

Throws: system_error if unable to start the new thread.

Error conditions:

(9.1)
resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.

🔗

jthread(jthread&& x) noexcept;

Postconditions: x.get_id() == id() and get_id() returns the value of x.get_id() prior to the start of construction.

ssource has the value of x.ssource prior to the start of construction and x.ssource.stop_possible() is false.

🔗

~jthread();

Effects: If joinable() is true, calls request_stop() and then join().

[Note 3:

Operations on *this are not synchronized.

— end note]

🔗

jthread& operator=(jthread&& x) noexcept;

Effects: If &x == this is true, there are no effects.

Otherwise, if joinable() is true, calls request_stop() and then join(), then assigns the state of x to *this and sets x to a default constructed state.

Postconditions: get_id() returns the value of x.get_id() prior to the assignment.

ssource has the value of x.ssource prior to the assignment.

Returns: *this.

32.4.4.3 Members [thread.jthread.mem]

🔗

void swap(jthread& x) noexcept;

Effects: Exchanges the values of *this and x.

🔗

bool joinable() const noexcept;

Returns: get_id() != id().

🔗

void join();

Effects: Blocks until the thread represented by *this has completed.

Synchronization: The completion of the thread represented by *this synchronizes with ([intro.multithread]) the corresponding successful join() return.

[Note 1:

Operations on *this are not synchronized.

— end note]

Postconditions: The thread represented by *this has completed.

get_id() == id().

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(7.1)
resource_deadlock_would_occur — if deadlock is detected or get_id() == this_thread::get_id().
(7.2)
no_such_process — if the thread is not valid.
(7.3)
invalid_argument — if the thread is not joinable.

🔗

void detach();

Effects: The thread represented by *this continues execution without the calling thread blocking.

When detach() returns, *this no longer represents the possibly continuing thread of execution.

When the thread previously represented by *this ends execution, the implementation releases any owned resources.

Postconditions: get_id() == id().

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(11.1)
no_such_process — if the thread is not valid.
(11.2)
invalid_argument — if the thread is not joinable.

🔗

id get_id() const noexcept;

Returns: A default constructed id object if *this does not represent a thread, otherwise this_thread::get_id() for the thread of execution represented by *this.

32.4.4.4 Stop token handling [thread.jthread.stop]

🔗

stop_source get_stop_source() noexcept;

Effects: Equivalent to: return ssource;

🔗

stop_token get_stop_token() const noexcept;

Effects: Equivalent to: return ssource.get_token();

🔗

bool request_stop() noexcept;

Effects: Equivalent to: return ssource.request_stop();

32.4.4.5 Specialized algorithms [thread.jthread.special]

🔗

friend void swap(jthread& x, jthread& y) noexcept;

Effects: Equivalent to: x.swap(y).

32.4.4.6 Static members [thread.jthread.static]

🔗

static unsigned int hardware_concurrency() noexcept;

Returns: thread::hardware_concurrency().

32.4.5 Namespace this_thread [thread.thread.this]

namespace std::this_thread { thread::id get_id() noexcept; void yield() noexcept; template<class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time); template<class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time); }

🔗

thread::id this_thread::get_id() noexcept;

Returns: An object of type thread::id that uniquely identifies the current thread of execution.

Every invocation from this thread of execution returns the same value.

The object returned does not compare equal to a default-constructed thread::id.

🔗

void this_thread::yield() noexcept;

Effects: Offers the implementation the opportunity to reschedule.

Synchronization: None.

🔗

template<class Clock, class Duration>
  void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);

Effects: Blocks the calling thread for the absolute timeout ([thread.req.timing]) specified by abs_time.

Synchronization: None.

Throws: Timeout-related exceptions ([thread.req.timing]).

🔗

template<class Rep, class Period>
  void sleep_for(const chrono::duration<Rep, Period>& rel_time);

Effects: Blocks the calling thread for the relative timeout ([thread.req.timing]) specified by rel_time.

Synchronization: None.

Throws: Timeout-related exceptions ([thread.req.timing]).

32.5 Atomic operations [atomics]

32.5.1 General [atomics.general]

Subclause [atomics] describes components for fine-grained atomic access.

This access is provided via operations on atomic objects.

32.5.2 Header <atomic> synopsis [atomics.syn]

🔗

namespace std { // [atomics.order], order and consistency enum class memory_order : unspecified; // freestanding inline constexpr memory_order memory_order_relaxed = memory_order::relaxed; // freestanding inline constexpr memory_order memory_order_consume = memory_order::consume; // freestanding inline constexpr memory_order memory_order_acquire = memory_order::acquire; // freestanding inline constexpr memory_order memory_order_release = memory_order::release; // freestanding inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel; // freestanding inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst; // freestanding template<class T> constexpr T kill_dependency(T y) noexcept; // freestanding } // [atomics.lockfree], lock-free property #define ATOMIC_BOOL_LOCK_FREE unspecified // freestanding #define ATOMIC_CHAR_LOCK_FREE unspecified // freestanding #define ATOMIC_CHAR8_T_LOCK_FREE unspecified // freestanding #define ATOMIC_CHAR16_T_LOCK_FREE unspecified // freestanding #define ATOMIC_CHAR32_T_LOCK_FREE unspecified // freestanding #define ATOMIC_WCHAR_T_LOCK_FREE unspecified // freestanding #define ATOMIC_SHORT_LOCK_FREE unspecified // freestanding #define ATOMIC_INT_LOCK_FREE unspecified // freestanding #define ATOMIC_LONG_LOCK_FREE unspecified // freestanding #define ATOMIC_LLONG_LOCK_FREE unspecified // freestanding #define ATOMIC_POINTER_LOCK_FREE unspecified // freestanding namespace std { // [atomics.ref.generic], class template atomic_ref template<class T> struct atomic_ref; // freestanding // [atomics.ref.pointer], partial specialization for pointers template<class T> struct atomic_ref<T*>; // freestanding // [atomics.types.generic], class template atomic template<class T> struct atomic; // freestanding // [atomics.types.pointer], partial specialization for pointers template<class T> struct atomic<T*>; // freestanding // [atomics.nonmembers], non-member functions template<class T> bool atomic_is_lock_free(const volatile atomic<T>*) noexcept; // freestanding template<class T> bool atomic_is_lock_free(const atomic<T>*) noexcept; // freestanding template<class T> void atomic_store(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr void atomic_store(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> void atomic_store_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr void atomic_store_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> T atomic_load(const volatile atomic<T>*) noexcept; // freestanding template<class T> constexpr T atomic_load(const atomic<T>*) noexcept; // freestanding template<class T> T atomic_load_explicit(const volatile atomic<T>*, memory_order) noexcept; // freestanding template<class T> constexpr T atomic_load_explicit(const atomic<T>*, memory_order) noexcept; // freestanding template<class T> T atomic_exchange(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_exchange(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_exchange_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_exchange_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> bool atomic_compare_exchange_weak(volatile atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> constexpr bool atomic_compare_exchange_weak(atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> bool atomic_compare_exchange_strong(volatile atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> constexpr bool atomic_compare_exchange_strong(atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type) noexcept; template<class T> bool atomic_compare_exchange_weak_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> constexpr bool atomic_compare_exchange_weak_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> bool atomic_compare_exchange_strong_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> constexpr bool atomic_compare_exchange_strong_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type*, typename atomic<T>::value_type, memory_order, memory_order) noexcept; template<class T> T atomic_fetch_add(volatile atomic<T>*, // freestanding typename atomic<T>::difference_type) noexcept; template<class T> constexpr T atomic_fetch_add(atomic<T>*, // freestanding typename atomic<T>::difference_type) noexcept; template<class T> T atomic_fetch_add_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::difference_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_add_explicit(atomic<T>*, // freestanding typename atomic<T>::difference_type, memory_order) noexcept; template<class T> T atomic_fetch_sub(volatile atomic<T>*, // freestanding typename atomic<T>::difference_type) noexcept; template<class T> constexpr T atomic_fetch_sub(atomic<T>*, // freestanding typename atomic<T>::difference_type) noexcept; template<class T> T atomic_fetch_sub_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::difference_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_sub_explicit(atomic<T>*, // freestanding typename atomic<T>::difference_type, memory_order) noexcept; template<class T> T atomic_fetch_and(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_fetch_and(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_and_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_and_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> T atomic_fetch_or(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_fetch_or(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_or_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_or_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> T atomic_fetch_xor(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_fetch_xor(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_xor_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_xor_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> T atomic_fetch_max(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_fetch_max(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_max_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_max_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> T atomic_fetch_min(volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr T atomic_fetch_min(atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> T atomic_fetch_min_explicit(volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr T atomic_fetch_min_explicit(atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> void atomic_wait(const volatile atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> constexpr void atomic_wait(const atomic<T>*, // freestanding typename atomic<T>::value_type) noexcept; template<class T> void atomic_wait_explicit(const volatile atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> constexpr void atomic_wait_explicit(const atomic<T>*, // freestanding typename atomic<T>::value_type, memory_order) noexcept; template<class T> void atomic_notify_one(volatile atomic<T>*) noexcept; // freestanding template<class T> constexpr void atomic_notify_one(atomic<T>*) noexcept; // freestanding template<class T> void atomic_notify_all(volatile atomic<T>*) noexcept; // freestanding template<class T> constexpr void atomic_notify_all(atomic<T>*) noexcept; // freestanding // [atomics.alias], type aliases using atomic_bool = atomic<bool>; // freestanding using atomic_char = atomic<char>; // freestanding using atomic_schar = atomic<signed char>; // freestanding using atomic_uchar = atomic<unsigned char>; // freestanding using atomic_short = atomic<short>; // freestanding using atomic_ushort = atomic<unsigned short>; // freestanding using atomic_int = atomic<int>; // freestanding using atomic_uint = atomic<unsigned int>; // freestanding using atomic_long = atomic<long>; // freestanding using atomic_ulong = atomic<unsigned long>; // freestanding using atomic_llong = atomic<long long>; // freestanding using atomic_ullong = atomic<unsigned long long>; // freestanding using atomic_char8_t = atomic<char8_t>; // freestanding using atomic_char16_t = atomic<char16_t>; // freestanding using atomic_char32_t = atomic<char32_t>; // freestanding using atomic_wchar_t = atomic<wchar_t>; // freestanding using atomic_int8_t = atomic<int8_t>; // freestanding using atomic_uint8_t = atomic<uint8_t>; // freestanding using atomic_int16_t = atomic<int16_t>; // freestanding using atomic_uint16_t = atomic<uint16_t>; // freestanding using atomic_int32_t = atomic<int32_t>; // freestanding using atomic_uint32_t = atomic<uint32_t>; // freestanding using atomic_int64_t = atomic<int64_t>; // freestanding using atomic_uint64_t = atomic<uint64_t>; // freestanding using atomic_int_least8_t = atomic<int_least8_t>; // freestanding using atomic_uint_least8_t = atomic<uint_least8_t>; // freestanding using atomic_int_least16_t = atomic<int_least16_t>; // freestanding using atomic_uint_least16_t = atomic<uint_least16_t>; // freestanding using atomic_int_least32_t = atomic<int_least32_t>; // freestanding using atomic_uint_least32_t = atomic<uint_least32_t>; // freestanding using atomic_int_least64_t = atomic<int_least64_t>; // freestanding using atomic_uint_least64_t = atomic<uint_least64_t>; // freestanding using atomic_int_fast8_t = atomic<int_fast8_t>; // freestanding using atomic_uint_fast8_t = atomic<uint_fast8_t>; // freestanding using atomic_int_fast16_t = atomic<int_fast16_t>; // freestanding using atomic_uint_fast16_t = atomic<uint_fast16_t>; // freestanding using atomic_int_fast32_t = atomic<int_fast32_t>; // freestanding using atomic_uint_fast32_t = atomic<uint_fast32_t>; // freestanding using atomic_int_fast64_t = atomic<int_fast64_t>; // freestanding using atomic_uint_fast64_t = atomic<uint_fast64_t>; // freestanding using atomic_intptr_t = atomic<intptr_t>; // freestanding using atomic_uintptr_t = atomic<uintptr_t>; // freestanding using atomic_size_t = atomic<size_t>; // freestanding using atomic_ptrdiff_t = atomic<ptrdiff_t>; // freestanding using atomic_intmax_t = atomic<intmax_t>; // freestanding using atomic_uintmax_t = atomic<uintmax_t>; // freestanding using atomic_signed_lock_free = see below; using atomic_unsigned_lock_free = see below; // [atomics.flag], flag type and operations struct atomic_flag; // freestanding bool atomic_flag_test(const volatile atomic_flag*) noexcept; // freestanding constexpr bool atomic_flag_test(const atomic_flag*) noexcept; // freestanding bool atomic_flag_test_explicit(const volatile atomic_flag*, // freestanding memory_order) noexcept; constexpr bool atomic_flag_test_explicit(const atomic_flag*, // freestanding memory_order) noexcept; bool atomic_flag_test_and_set(volatile atomic_flag*) noexcept; // freestanding constexpr bool atomic_flag_test_and_set(atomic_flag*) noexcept; // freestanding bool atomic_flag_test_and_set_explicit(volatile atomic_flag*, // freestanding memory_order) noexcept; constexpr bool atomic_flag_test_and_set_explicit(atomic_flag*, // freestanding memory_order) noexcept; void atomic_flag_clear(volatile atomic_flag*) noexcept; // freestanding constexpr void atomic_flag_clear(atomic_flag*) noexcept; // freestanding void atomic_flag_clear_explicit(volatile atomic_flag*, memory_order) noexcept; // freestanding constexpr void atomic_flag_clear_explicit(atomic_flag*, memory_order) noexcept; // freestanding void atomic_flag_wait(const volatile atomic_flag*, bool) noexcept; // freestanding constexpr void atomic_flag_wait(const atomic_flag*, bool) noexcept; // freestanding void atomic_flag_wait_explicit(const volatile atomic_flag*, // freestanding bool, memory_order) noexcept; constexpr void atomic_flag_wait_explicit(const atomic_flag*, // freestanding bool, memory_order) noexcept; void atomic_flag_notify_one(volatile atomic_flag*) noexcept; // freestanding constexpr void atomic_flag_notify_one(atomic_flag*) noexcept; // freestanding void atomic_flag_notify_all(volatile atomic_flag*) noexcept; // freestanding constexpr void atomic_flag_notify_all(atomic_flag*) noexcept; // freestanding #define ATOMIC_FLAG_INIT see below // freestanding // [atomics.fences], fences extern "C" constexpr void atomic_thread_fence(memory_order) noexcept; // freestanding extern "C" constexpr void atomic_signal_fence(memory_order) noexcept; // freestanding }

32.5.3 Type aliases [atomics.alias]

The type aliases atomic_intN_t, atomic_uintN_t, atomic_intptr_t, and atomic_uintptr_t are defined if and only if intN_t, uintN_t, intptr_t, and uintptr_t are defined, respectively.

The type aliases atomic_signed_lock_free and atomic_unsigned_lock_free name specializations of atomic whose template arguments are integral types, respectively signed and unsigned, and whose is_always_lock_free property is true.

[Note 1:

These aliases are optional in freestanding implementations ([compliance]).

— end note]

Implementations should choose for these aliases the integral specializations of atomic for which the atomic waiting and notifying operations ([atomics.wait]) are most efficient.

32.5.4 Order and consistency [atomics.order]

namespace std { enum class memory_order : unspecified { relaxed, consume, acquire, release, acq_rel, seq_cst }; }

The enumeration memory_order specifies the detailed regular (non-atomic) memory synchronization order as defined in [intro.multithread] and may provide for operation ordering.

Its enumerated values and their meanings are as follows:

(1.1)
memory_order::relaxed: no operation orders memory.
(1.2)
memory_order::release, memory_order::acq_rel, and memory_order::seq_cst: a store operation performs a release operation on the affected memory location.
(1.3)
memory_order::consume: a load operation performs a consume operation on the affected memory location.

[Note 1:
Prefer memory_order::acquire, which provides stronger guarantees than memory_order::consume.

Implementations have found it infeasible to provide performance better than that of memory_order::acquire.

Specification revisions are under consideration.
— end note]
(1.4)
memory_order::acquire, memory_order::acq_rel, and memory_order::seq_cst: a load operation performs an acquire operation on the affected memory location.

[Note 2:

Atomic operations specifying memory_order::relaxed are relaxed with respect to memory ordering.

Implementations must still guarantee that any given atomic access to a particular atomic object be indivisible with respect to all other atomic accesses to that object.

— end note]

An atomic operation A that performs a release operation on an atomic object M synchronizes with an atomic operation B that performs an acquire operation on M and takes its value from any side effect in the release sequence headed by A.

An atomic operation A on some atomic object M is coherence-ordered before another atomic operation B on M if

(3.1)
A is a modification, and B reads the value stored by A, or
(3.2)
A precedes B in the modification order of M, or
(3.3)
A and B are not the same atomic read-modify-write operation, and there exists an atomic modification X of M such that A reads the value stored by X and X precedes B in the modification order of M, or
(3.4)
there exists an atomic modification X of M such that A is coherence-ordered before X and X is coherence-ordered before B.

There is a single total order S on all memory_order::seq_cst operations, including fences, that satisfies the following constraints.

First, if A and B are memory_order::seq_cst operations and A strongly happens before B, then A precedes B in S.

Second, for every pair of atomic operations A and B on an object M, where A is coherence-ordered before B, the following four conditions are required to be satisfied by S:

(4.1)
if A and B are both memory_order::seq_cst operations, then A precedes B in S; and
(4.2)
if A is a memory_order::seq_cst operation and B happens before a memory_order::seq_cst fence Y, then A precedes Y in S; and
(4.3)
if a memory_order::seq_cst fence X happens before A and B is a memory_order::seq_cst operation, then X precedes B in S; and
(4.4)
if a memory_order::seq_cst fence X happens before A and B happens before a memory_order::seq_cst fence Y, then X precedes Y in S.

[Note 3:

This definition ensures that S is consistent with the modification order of any atomic object M.

It also ensures that a memory_order::seq_cst load A of M gets its value either from the last modification of M that precedes A in S or from some non-memory_order::seq_cst modification of M that does not happen before any modification of M that precedes A in S.

— end note]

[Note 4:

We do not require that S be consistent with “happens before” ([intro.races]).

This allows more efficient implementation of memory_order::acquire and memory_order::release on some machine architectures.

It can produce surprising results when these are mixed with memory_order::seq_cst accesses.

— end note]

[Note 5:

memory_order::seq_cst ensures sequential consistency only for a program that is free of data races and uses exclusively memory_order::seq_cst atomic operations.

Any use of weaker ordering will invalidate this guarantee unless extreme care is used.

In many cases, memory_order::seq_cst atomic operations are reorderable with respect to other atomic operations performed by the same thread.

— end note]

Implementations should ensure that no “out-of-thin-air” values are computed that circularly depend on their own computation.

[Note 6:

For example, with x and y initially zero, // Thread 1: r1 = y.load(memory_order::relaxed); x.store(r1, memory_order::relaxed);

// Thread 2: r2 = x.load(memory_order::relaxed); y.store(r2, memory_order::relaxed); this recommendation discourages producing r1 == r2 == 42, since the store of 42 to y is only possible if the store to x stores 42, which circularly depends on the store to y storing 42.

Note that without this restriction, such an execution is possible.

— end note]

[Note 7:

The recommendation similarly disallows r1 == r2 == 42 in the following example, with x and y again initially zero:

// Thread 1: r1 = x.load(memory_order::relaxed); if (r1 == 42) y.store(42, memory_order::relaxed);

// Thread 2: r2 = y.load(memory_order::relaxed); if (r2 == 42) x.store(42, memory_order::relaxed); — end note]

Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write associated with the read-modify-write operation.

Recommended practice: The implementation should make atomic stores visible to atomic loads, and atomic loads should observe atomic stores, within a reasonable amount of time.

🔗

template<class T>
  constexpr T kill_dependency(T y) noexcept;

Effects: The argument does not carry a dependency to the return value ([intro.multithread]).

Returns: y.

32.5.5 Lock-free property [atomics.lockfree]

🔗

#define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR8_T_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_POINTER_LOCK_FREE unspecified

The ATOMIC_..._LOCK_FREE macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together.

The properties also apply to the corresponding (partial) specializations of the atomic template.

A value of 0 indicates that the types are never lock-free.

A value of 1 indicates that the types are sometimes lock-free.

A value of 2 indicates that the types are always lock-free.

On a hosted implementation ([compliance]), at least one signed integral specialization of the atomic template, along with the specialization for the corresponding unsigned type ([basic.fundamental]), is always lock-free.

The functions atomic<T>::is_lock_free and atomic_is_lock_free ([atomics.types.operations]) indicate whether the object is lock-free.

In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.

Atomic operations that are not lock-free are considered to potentially block ([intro.progress]).

Recommended practice: Operations that are lock-free should also be address-free.298

The implementation of these operations should not depend on any per-process state.

[Note 1:

This restriction enables communication by memory that is mapped into a process more than once and by memory that is shared between two processes.

— end note]

298)298)

That is, atomic operations on the same memory location via two different addresses will communicate atomically.

32.5.6 Waiting and notifying [atomics.wait]

Atomic waiting operations and atomic notifying operations provide a mechanism to wait for the value of an atomic object to change more efficiently than can be achieved with polling.

An atomic waiting operation may block until it is unblocked by an atomic notifying operation, according to each function's effects.

[Note 1:

Programs are not guaranteed to observe transient atomic values, an issue known as the A-B-A problem, resulting in continued blocking if a condition is only temporarily met.

— end note]

[Note 2:

The following functions are atomic waiting operations:

(2.1)
atomic<T>::wait,
(2.2)
atomic_flag::wait,
(2.3)
atomic_wait and atomic_wait_explicit,
(2.4)
atomic_flag_wait and atomic_flag_wait_explicit, and
(2.5)
atomic_ref<T>::wait.

— end note]

[Note 3:

The following functions are atomic notifying operations:

(3.1)
atomic<T>::notify_one and atomic<T>::notify_all,
(3.2)
atomic_flag::notify_one and atomic_flag::notify_all,
(3.3)
atomic_notify_one and atomic_notify_all,
(3.4)
atomic_flag_notify_one and atomic_flag_notify_all, and
(3.5)
atomic_ref<T>::notify_one and atomic_ref<T>::notify_all.

— end note]

A call to an atomic waiting operation on an atomic object M is eligible to be unblocked by a call to an atomic notifying operation on M if there exist side effects X and Y on M such that:

(4.1)
the atomic waiting operation has blocked after observing the result of X,
(4.2)
X precedes Y in the modification order of M, and
(4.3)
Y happens before the call to the atomic notifying operation.

32.5.7 Class template atomic_ref [atomics.ref.generic]

32.5.7.1 General [atomics.ref.generic.general]

🔗

namespace std { template<class T> struct atomic_ref { private: T* ptr; // exposition only public: using value_type = remove_cv_t<T>; static constexpr size_t required_alignment = implementation-defined; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr explicit atomic_ref(T&); constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete; constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator=(value_type) const noexcept; constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept; constexpr operator value_type() const noexcept; constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void notify_one() const noexcept; constexpr void notify_all() const noexcept; constexpr T* address() const noexcept; }; }

An atomic_ref object applies atomic operations ([atomics.general]) to the object referenced by *ptr such that, for the lifetime ([basic.life]) of the atomic_ref object, the object referenced by *ptr is an atomic object ([intro.races]).

The program is ill-formed if is_trivially_copyable_v<T> is false.

The lifetime ([basic.life]) of an object referenced by *ptr shall exceed the lifetime of all atomic_refs that reference the object.

While any atomic_ref instances exist that reference the *ptr object, all accesses to that object shall exclusively occur through those atomic_ref instances.

No subobject of the object referenced by atomic_ref shall be concurrently referenced by any other atomic_ref object.

Atomic operations applied to an object through a referencing atomic_ref are atomic with respect to atomic operations applied through any other atomic_ref referencing the same object.

[Note 1:

Atomic operations or the atomic_ref constructor can acquire a shared resource, such as a lock associated with the referenced object, to enable atomic operations to be applied to the referenced object.

— end note]

The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

32.5.7.2 Operations [atomics.ref.ops]

🔗

static constexpr size_t required_alignment;

The alignment required for an object to be referenced by an atomic reference, which is at least alignof(T).

[Note 1:

Hardware could require an object referenced by an atomic_ref to have stricter alignment ([basic.align]) than other objects of type T.

Further, whether operations on an atomic_ref are lock-free could depend on the alignment of the referenced object.

For example, lock-free operations on std::complex<double> could be supported only if aligned to 2*alignof(double).

— end note]

🔗

static constexpr bool is_always_lock_free;

The static data member is_always_lock_free is true if the atomic_ref type's operations are always lock-free, and false otherwise.

🔗

bool is_lock_free() const noexcept;

Returns: true if operations on all objects of the type atomic_ref<T> are lock-free, false otherwise.

🔗

constexpr atomic_ref(T& obj);

Preconditions: The referenced object is aligned to required_alignment.

Postconditions: *this references obj.

Throws: Nothing.

🔗

constexpr atomic_ref(const atomic_ref& ref) noexcept;

Postconditions: *this references the object referenced by ref.

🔗

constexpr void store(value_type desired,
                     memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically replaces the value referenced by *ptr with the value of desired.

Memory is affected according to the value of order.

🔗

constexpr value_type operator=(value_type desired) const noexcept;

Constraints: is_const_v<T> is false.

Effects: Equivalent to: store(desired); return desired;

🔗

constexpr value_type load(memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value referenced by *ptr.

🔗

constexpr operator value_type() const noexcept;

Effects: Equivalent to: return load();

🔗

constexpr value_type exchange(value_type desired,
                              memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Effects: Atomically replaces the value referenced by *ptr with desired.

Memory is affected according to the value of order.

This operation is an atomic read-modify-write operation ([intro.multithread]).

Returns: Atomically returns the value referenced by *ptr immediately before the effects.

🔗

constexpr bool compare_exchange_weak(value_type& expected, value_type desired,
                           memory_order success, memory_order failure) const noexcept;

constexpr bool compare_exchange_strong(value_type& expected, value_type desired,
                             memory_order success, memory_order failure) const noexcept;

constexpr bool compare_exchange_weak(value_type& expected, value_type desired,
                           memory_order order = memory_order::seq_cst) const noexcept;

constexpr bool compare_exchange_strong(value_type& expected, value_type desired,
                             memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<T> is false.

Preconditions: failure is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

Effects: Retrieves the value in expected.

It then atomically compares the value representation of the value referenced by *ptr for equality with that previously retrieved from expected, and if true, replaces the value referenced by *ptr with that in desired.

If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure.

When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order::acq_rel shall be replaced by the value memory_order::acquire and a value of memory_order::release shall be replaced by the value memory_order::relaxed.

If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value read from the value referenced by *ptr during the atomic comparison.

If the operation returns true, these operations are atomic read-modify-write operations ([intro.races]) on the value referenced by *ptr.

Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

Remarks: A weak compare-and-exchange operation may fail spuriously.

That is, even when the contents of memory referred to by expected and ptr are equal, it may return false and store back to expected the same memory contents that were originally there.

[Note 2:

This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines.

A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.

When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

— end note]

🔗

constexpr void wait(value_type old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(27.1)
Evaluates load(order) and compares its value representation for equality against that of old.
(27.2)
If they compare unequal, returns.
(27.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]) on atomic object *ptr.

🔗

constexpr void notify_one() const noexcept;

Constraints: is_const_v<T> is false.

Effects: Unblocks the execution of at least one atomic waiting operation on *ptr that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

🔗

constexpr void notify_all() const noexcept;

Constraints: is_const_v<T> is false.

Effects: Unblocks the execution of all atomic waiting operations on *ptr that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

🔗

constexpr T* address() const noexcept;

Returns: ptr.

32.5.7.3 Specializations for integral types [atomics.ref.int]

There are specializations of the atomic_ref class template for all integral types except cv bool.

For each such type integral-type, the specialization atomic_ref<integral-type> provides additional atomic operations appropriate to integral types.

[Note 1:

The specialization atomic_ref<bool> uses the primary template ([atomics.ref.generic]).

— end note]

The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std { template<> struct atomic_ref<integral-type> { private: integral-type* ptr; // exposition only public: using value_type = remove_cv_t<integral-type>; using difference_type = value_type; static constexpr size_t required_alignment = implementation-defined; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr explicit atomic_ref(integral-type&); constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete; constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator=(value_type) const noexcept; constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept; constexpr operator value_type() const noexcept; constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_add(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_sub(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_and(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_or(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_xor(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_max(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_min(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator++(int) const noexcept; constexpr value_type operator--(int) const noexcept; constexpr value_type operator++() const noexcept; constexpr value_type operator--() const noexcept; constexpr value_type operator+=(value_type) const noexcept; constexpr value_type operator-=(value_type) const noexcept; constexpr value_type operator&=(value_type) const noexcept; constexpr value_type operator|=(value_type) const noexcept; constexpr value_type operator^=(value_type) const noexcept; constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void notify_one() const noexcept; constexpr void notify_all() const noexcept; constexpr integral-type* address() const noexcept; }; }

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 150 .

🔗

constexpr value_type fetch_key(value_type operand,
  memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<integral-type> is false.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptr and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptr immediately before the effects.

Remarks: Except for fetch_max and fetch_min, for signed integer types the result is as if the object value and parameters were converted to their corresponding unsigned types, the computation performed on those types, and the result converted back to the signed type.

[Note 2:

There are no undefined results arising from the computation.

— end note]

For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

🔗

constexpr value_type operator op=(value_type operand) const noexcept;

Constraints: is_const_v<integral-type> is false.

Effects: Equivalent to: return fetch_key(operand) op operand;

32.5.7.4 Specializations for floating-point types [atomics.ref.float]

There are specializations of the atomic_ref class template for all floating-point types.

For each such type floating-point-type, the specialization atomic_ref<floating-point> provides additional atomic operations appropriate to floating-point types.

The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std { template<> struct atomic_ref<floating-point-type> { private: floating-point-type* ptr; // exposition only public: using value_type = remove_cv_t<floating-point-type>; using difference_type = value_type; static constexpr size_t required_alignment = implementation-defined; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr explicit atomic_ref(floating-point-type&); constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete; constexpr void store(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator=(value_type) const noexcept; constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept; constexpr operator floating-point-type() const noexcept; constexpr value_type exchange(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_weak(value_type&, floating-point-type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_strong(value_type&, floating-point-type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_weak(value_type&, floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_strong(value_type&, floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator+=(value_type) const noexcept; constexpr value_type operator-=(value_type) const noexcept; constexpr void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; constexpr void notify_one() const noexcept; constexpr void notify_all() const noexcept; constexpr floating-point-type* address() const noexcept; }; }

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 150 .

🔗

constexpr value_type fetch_key(value_type operand,
                          memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<floating-point-type> is false.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptr and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptr immediately before the effects.

Remarks: If the result is not a representable value for its type ([expr.pre]), the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on floating-point-type should conform to the std::numeric_limits<value_type> traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-
point-type may be different than the calling thread's floating-point environment.

🔗

constexpr value_type operator op=(value_type operand) const noexcept;

Constraints: is_const_v<floating-point-type> is false.

Effects: Equivalent to: return fetch_key(operand) op operand;

32.5.7.5 Partial specialization for pointers [atomics.ref.pointer]

There are specializations of the atomic_ref class template for all pointer-to-object types.

For each such type pointer-type, the specialization atomic_ref<pointer-type> provides additional atomic operations appropriate to pointer types.

The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std { template<class T> struct atomic_ref<pointer-type> { private: pointer-type* ptr; // exposition only public: using value_type = remove_cv_t<pointer-type>; using difference_type = ptrdiff_t; static constexpr size_t required_alignment = implementation-defined; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr explicit atomic_ref(pointer-type&); constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete; constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator=(value_type) const noexcept; constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept; constexpr operator value_type() const noexcept; constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_add(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_sub(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_max(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_min(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator++(int) const noexcept; constexpr value_type operator--(int) const noexcept; constexpr value_type operator++() const noexcept; constexpr value_type operator--() const noexcept; constexpr value_type operator+=(difference_type) const noexcept; constexpr value_type operator-=(difference_type) const noexcept; constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void notify_one() const noexcept; constexpr void notify_all() const noexcept; constexpr pointer-type* address() const noexcept; }; }

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 151 .

🔗

constexpr value_type fetch_key(difference_type operand,
                     memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<pointer-type> is false.

Mandates: remove_pointer_t<pointer-type> is a complete object type.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptr and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptr immediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

[Note 1:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— end note]

🔗

constexpr value_type operator op=(difference_type operand) const noexcept;

Constraints: is_const_v<pointer-type> is false.

Effects: Equivalent to: return fetch_key(operand) op operand;

32.5.7.6 Member operators common to integers and pointers to objects [atomics.ref.memop]

Let referred-type be pointer-type for the specializations in [atomics.ref.pointer] and be integral-type for the specializations in [atomics.ref.int].

🔗

constexpr value_type operator++(int) const noexcept;

Constraints: is_const_v<referred-type> is false.

Effects: Equivalent to: return fetch_add(1);

🔗

constexpr value_type operator--(int) const noexcept;

Constraints: is_const_v<referred-type> is false.

Effects: Equivalent to: return fetch_sub(1);

🔗

constexpr value_type operator++() const noexcept;

Constraints: is_const_v<referred-type> is false.

Effects: Equivalent to: return fetch_add(1) + 1;

🔗

constexpr value_type operator--() const noexcept;

Constraints: is_const_v<referred-type> is false.

Effects: Equivalent to: return fetch_sub(1) - 1;

32.5.8 Class template atomic [atomics.types.generic]

32.5.8.1 General [atomics.types.generic.general]

🔗

namespace std { template<class T> struct atomic { using value_type = T; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const volatile noexcept; bool is_lock_free() const noexcept; // [atomics.types.operations], operations on atomic types constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>); constexpr atomic(T) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T load(memory_order = memory_order::seq_cst) const volatile noexcept; constexpr T load(memory_order = memory_order::seq_cst) const noexcept; operator T() const volatile noexcept; constexpr operator T() const noexcept; void store(T, memory_order = memory_order::seq_cst) volatile noexcept; constexpr void store(T, memory_order = memory_order::seq_cst) noexcept; T operator=(T) volatile noexcept; constexpr T operator=(T) noexcept; T exchange(T, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T exchange(T, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_weak(T&, T, memory_order, memory_order) noexcept; bool compare_exchange_strong(T&, T, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_strong(T&, T, memory_order, memory_order) noexcept; bool compare_exchange_weak(T&, T, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_weak(T&, T, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(T&, T, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_strong(T&, T, memory_order = memory_order::seq_cst) noexcept; void wait(T, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(T, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; }; }

The template argument for T shall meet the Cpp17CopyConstructible and Cpp17CopyAssignable requirements.

The program is ill-formed if any of

(1.1)
is_trivially_copyable_v<T>,
(1.2)
is_copy_constructible_v<T>,
(1.3)
is_move_constructible_v<T>,
(1.4)
is_copy_assignable_v<T>,
(1.5)
is_move_assignable_v<T>, or
(1.6)
same_as<T, remove_cv_t<T>>,

is false.

[Note 1:

Type arguments that are not also statically initializable can be difficult to use.

— end note]

The specialization atomic<bool> is a standard-layout struct.

It has a trivial destructor.

[Note 2:

The representation of an atomic specialization need not have the same size and alignment requirement as its corresponding argument type.

— end note]

32.5.8.2 Operations on atomic types [atomics.types.operations]

🔗

constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);

Constraints: is_default_constructible_v<T> is true.

Effects: Initializes the atomic object with the value of T().

Initialization is not an atomic operation ([intro.multithread]).

🔗

constexpr atomic(T desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[Note 1:

It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A to another thread via memory_order::relaxed operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread.

This results in undefined behavior.

— end note]

🔗

static constexpr bool is_always_lock_free = implementation-defined;

The static data member is_always_lock_free is true if the atomic type's operations are always lock-free, and false otherwise.

[Note 2:

The value of is_always_lock_free is consistent with the value of the corresponding ATOMIC_..._LOCK_FREE macro, if defined.

— end note]

🔗

bool is_lock_free() const volatile noexcept;
bool is_lock_free() const noexcept;

Returns: true if the object's operations are lock-free, false otherwise.

[Note 3:

The return value of the is_lock_free member function is consistent with the value of is_always_lock_free for the same type.

— end note]

🔗

void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr void store(T desired, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically replaces the value pointed to by this with the value of desired.

Memory is affected according to the value of order.

🔗

T operator=(T desired) volatile noexcept;
constexpr T operator=(T desired) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to store(desired).

Returns: desired.

🔗

T load(memory_order order = memory_order::seq_cst) const volatile noexcept;
constexpr T load(memory_order order = memory_order::seq_cst) const noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by this.

🔗

operator T() const volatile noexcept;
constexpr operator T() const noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return load();

🔗

T exchange(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr T exchange(T desired, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Atomically replaces the value pointed to by this with desired.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically returns the value pointed to by this immediately before the effects.

🔗

bool compare_exchange_weak(T& expected, T desired,
                           memory_order success, memory_order failure) volatile noexcept;
constexpr bool compare_exchange_weak(T& expected, T desired,
                           memory_order success, memory_order failure) noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order success, memory_order failure) volatile noexcept;
constexpr bool compare_exchange_strong(T& expected, T desired,
                             memory_order success, memory_order failure) noexcept;
bool compare_exchange_weak(T& expected, T desired,
                           memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr bool compare_exchange_weak(T& expected, T desired,
                           memory_order order = memory_order::seq_cst) noexcept;
bool compare_exchange_strong(T& expected, T desired,
                             memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr bool compare_exchange_strong(T& expected, T desired,
                             memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Preconditions: failure is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

Effects: Retrieves the value in expected.

It then atomically compares the value representation of the value pointed to by this for equality with that previously retrieved from expected, and if true, replaces the value pointed to by this with that in desired.

If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure.

If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value pointed to by this during the atomic comparison.

If the operation returns true, these operations are atomic read-modify-write operations ([intro.multithread]) on the memory pointed to by this.

Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

[Note 4:

For example, the effect of compare_exchange_strong on objects without padding bits ([basic.types.general]) is if (memcmp(this, &expected, sizeof(*this)) == 0) memcpy(this, &desired, sizeof(*this)); else memcpy(&expected, this, sizeof(*this));

— end note]

[Example 1:

The expected use of the compare-and-exchange operations is as follows.

The compare-and-exchange operations will update expected when another iteration of the loop is needed.

expected = current.load(); do { desired = function(expected); } while (!current.compare_exchange_weak(expected, desired)); — end example]

[Example 2:

Because the expected value is updated only on failure, code releasing the memory containing the expected value on success will work.

For example, list head insertion will act atomically and would not introduce a data race in the following code: do { p->next = head; // make new list node point to the current head } while (!head.compare_exchange_weak(p->next, p)); // try to insert

— end example]

Implementations should ensure that weak compare-and-exchange operations do not consistently return false unless either the atomic object has value different from expected or there are concurrent modifications to the atomic object.

Remarks: A weak compare-and-exchange operation may fail spuriously.

That is, even when the contents of memory referred to by expected and this are equal, it may return false and store back to expected the same memory contents that were originally there.

[Note 5:

This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines.

A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.

When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

— end note]

[Note 6:

Under cases where the memcpy and memcmp semantics of the compare-and-exchange operations apply, the comparisons can fail for values that compare equal with operator== if the value representation has trap bits or alternate representations of the same value.

Notably, on implementations conforming to ISO/IEC 60559, floating-point -0.0 and +0.0 will not compare equal with memcmp but will compare equal with operator==, and NaNs with the same payload will compare equal with memcmp but will not compare equal with operator==.

— end note]

[Note 7:

Because compare-and-exchange acts on an object's value representation, padding bits that never participate in the object's value representation are ignored.

As a consequence, the following code is guaranteed to avoid spurious failure: struct padded { char clank = 0x42; // Padding here. unsigned biff = 0xC0DEFEFE; }; atomic<padded> pad = {}; bool zap() { padded expected, desired{0, 0}; return pad.compare_exchange_strong(expected, desired); }

— end note]

[Note 8:

For a union with bits that participate in the value representation of some members but not others, compare-and-exchange might always fail.

This is because such padding bits have an indeterminate value when they do not participate in the value representation of the active member.

As a consequence, the following code is not guaranteed to ever succeed: union pony { double celestia = 0.; short luna; // padded }; atomic<pony> princesses = {}; bool party(pony desired) { pony expected; return princesses.compare_exchange_strong(expected, desired); }

— end note]

🔗

void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept;
constexpr void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(30.1)
Evaluates load(order) and compares its value representation for equality against that of old.
(30.2)
If they compare unequal, returns.
(30.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]).

🔗

void notify_one() volatile noexcept;
constexpr void notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

🔗

void notify_all() volatile noexcept;
constexpr void notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

32.5.8.3 Specializations for integers [atomics.types.int]

There are specializations of the atomic class template for the integral types char, signed char, unsigned char, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long, char8_t, char16_t, char32_t, wchar_t, and any other types needed by the typedefs in the header <cstdint>.

For each such type integral-type, the specialization atomic<integral-type> provides additional atomic operations appropriate to integral types.

[Note 1:

The specialization atomic<bool> uses the primary template ([atomics.types.generic]).

— end note]

namespace std { template<> struct atomic<integral-type> { using value_type = integral-type; using difference_type = value_type; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const volatile noexcept; bool is_lock_free() const noexcept; constexpr atomic() noexcept; constexpr atomic(integral-type) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; void store(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr void store(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type operator=(integral-type) volatile noexcept; constexpr integral-type operator=(integral-type) noexcept; integral-type load(memory_order = memory_order::seq_cst) const volatile noexcept; constexpr integral-type load(memory_order = memory_order::seq_cst) const noexcept; operator integral-type() const volatile noexcept; constexpr operator integral-type() const noexcept; integral-type exchange(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type exchange(integral-type, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(integral-type&, integral-type, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_weak(integral-type&, integral-type, memory_order, memory_order) noexcept; bool compare_exchange_strong(integral-type&, integral-type, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_strong(integral-type&, integral-type, memory_order, memory_order) noexcept; bool compare_exchange_weak(integral-type&, integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_weak(integral-type&, integral-type, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(integral-type&, integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_strong(integral-type&, integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_add(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_add(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_sub(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_sub(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_and(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_and(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_or(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_or(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_xor(integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_xor(integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_max( integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_max( integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type fetch_min( integral-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr integral-type fetch_min( integral-type, memory_order = memory_order::seq_cst) noexcept; integral-type operator++(int) volatile noexcept; constexpr integral-type operator++(int) noexcept; integral-type operator--(int) volatile noexcept; constexpr integral-type operator--(int) noexcept; integral-type operator++() volatile noexcept; constexpr integral-type operator++() noexcept; integral-type operator--() volatile noexcept; constexpr integral-type operator--() noexcept; integral-type operator+=(integral-type) volatile noexcept; constexpr integral-type operator+=(integral-type) noexcept; integral-type operator-=(integral-type) volatile noexcept; constexpr integral-type operator-=(integral-type) noexcept; integral-type operator&=(integral-type) volatile noexcept; constexpr integral-type operator&=(integral-type) noexcept; integral-type operator|=(integral-type) volatile noexcept; constexpr integral-type operator|=(integral-type) noexcept; integral-type operator^=(integral-type) volatile noexcept; constexpr integral-type operator^=(integral-type) noexcept; void wait(integral-type, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(integral-type, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; }; }

The atomic integral specializations are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 150 .

Table 150: Atomic arithmetic computations [tab:atomic.types.int.comp]

🔗	*key*	Op	Computation	*key*	Op	Computation
🔗	add	+	addition	and	&	bitwise and
🔗	sub	-	subtraction	or	\|	bitwise inclusive or
🔗	max		maximum	xor	^	bitwise exclusive or
🔗	min		minimum

🔗

T fetch_key(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr T fetch_key(T operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

[Note 2:

There are no undefined results arising from the computation.

— end note]

🔗

T operator op=(T operand) volatile noexcept;
constexpr T operator op=(T operand) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_key(operand) op operand;

32.5.8.4 Specializations for floating-point types [atomics.types.float]

There are specializations of the atomic class template for all cv-unqualified floating-point types.

For each such type floating-point-type, the specialization atomic<floating-point-type> provides additional atomic operations appropriate to floating-point types.

namespace std { template<> struct atomic<floating-point-type> { using value_type = floating-point-type; using difference_type = value_type; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const volatile noexcept; bool is_lock_free() const noexcept; constexpr atomic() noexcept; constexpr atomic(floating-point-type) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; void store(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr void store(floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type operator=(floating-point-type) volatile noexcept; constexpr floating-point-type operator=(floating-point-type) noexcept; floating-point-type load(memory_order = memory_order::seq_cst) volatile noexcept; constexpr floating-point-type load(memory_order = memory_order::seq_cst) noexcept; operator floating-point-type() volatile noexcept; constexpr operator floating-point-type() noexcept; floating-point-type exchange(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr floating-point-type exchange(floating-point-type, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order, memory_order) noexcept; bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order, memory_order) noexcept; bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_weak(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_strong(floating-point-type&, floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr floating-point-type fetch_add(floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) volatile noexcept; constexpr floating-point-type fetch_sub(floating-point-type, memory_order = memory_order::seq_cst) noexcept; floating-point-type operator+=(floating-point-type) volatile noexcept; constexpr floating-point-type operator+=(floating-point-type) noexcept; floating-point-type operator-=(floating-point-type) volatile noexcept; constexpr floating-point-type operator-=(floating-point-type) noexcept; void wait(floating-point-type, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; }; }

The atomic floating-point specializations are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic addition and subtraction computations.

The correspondence among key, operator, and computation is specified in Table 150 .

🔗

T fetch_key(T operand, memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr T fetch_key(T operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: If the result is not a representable value for its type ([expr.pre]) the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on floating-point-type should conform to the std::numeric_limits<floating-point-type> traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-point-type may be different than the calling thread's floating-point environment.

🔗

T operator op=(T operand) volatile noexcept;
constexpr T operator op=(T operand) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_key(operand) op operand;

Remarks: If the result is not a representable value for its type ([expr.pre]) the result is unspecified, but the operations otherwise have no undefined behavior.

Atomic arithmetic operations on floating-point-type should conform to the std::numeric_limits<floating-point-type> traits associated with the floating-point type ([limits.syn]).

The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-point-type may be different than the calling thread's floating-point environment.

32.5.8.5 Partial specialization for pointers [atomics.types.pointer]

namespace std { template<class T> struct atomic<T*> { using value_type = T*; using difference_type = ptrdiff_t; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const volatile noexcept; bool is_lock_free() const noexcept; constexpr atomic() noexcept; constexpr atomic(T*) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; void store(T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr void store(T*, memory_order = memory_order::seq_cst) noexcept; T* operator=(T*) volatile noexcept; constexpr T* operator=(T*) noexcept; T* load(memory_order = memory_order::seq_cst) const volatile noexcept; constexpr T* load(memory_order = memory_order::seq_cst) const noexcept; operator T*() const volatile noexcept; constexpr operator T*() const noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T* exchange(T*, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_weak(T*&, T*, memory_order, memory_order) noexcept; bool compare_exchange_strong(T*&, T*, memory_order, memory_order) volatile noexcept; constexpr bool compare_exchange_strong(T*&, T*, memory_order, memory_order) noexcept; bool compare_exchange_weak(T*&, T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_weak(T*&, T*, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(T*&, T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool compare_exchange_strong(T*&, T*, memory_order = memory_order::seq_cst) noexcept; T* fetch_add(ptrdiff_t, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T* fetch_add(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; T* fetch_max(T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T* fetch_max(T*, memory_order = memory_order::seq_cst) noexcept; T* fetch_min(T*, memory_order = memory_order::seq_cst) volatile noexcept; constexpr T* fetch_min(T*, memory_order = memory_order::seq_cst) noexcept; T* operator++(int) volatile noexcept; constexpr T* operator++(int) noexcept; T* operator--(int) volatile noexcept; constexpr T* operator--(int) noexcept; T* operator++() volatile noexcept; constexpr T* operator++() noexcept; T* operator--() volatile noexcept; constexpr T* operator--() noexcept; T* operator+=(ptrdiff_t) volatile noexcept; constexpr T* operator+=(ptrdiff_t) noexcept; T* operator-=(ptrdiff_t) volatile noexcept; constexpr T* operator-=(ptrdiff_t) noexcept; void wait(T*, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(T*, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; }; }

There is a partial specialization of the atomic class template for pointers.

Specializations of this partial specialization are standard-layout structs.

They each have a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform pointer arithmetic.

The correspondence among key, operator, and computation is specified in Table 151 .

Table 151: Atomic pointer computations [tab:atomic.types.pointer.comp]

🔗	*key*	Op	Computation	*key*	Op	Computation
🔗	add	+	addition	sub	-	subtraction
🔗	max		maximum	min		minimum

🔗

T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Mandates: T is a complete object type.

[Note 1:

Pointer arithmetic on void* or function pointers is ill-formed.

— end note]

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

[Note 2:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— end note]

🔗

T* operator op=(ptrdiff_t operand) volatile noexcept;
constexpr T* operator op=(ptrdiff_t operand) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_key(operand) op operand;

32.5.8.6 Member operators common to integers and pointers to objects [atomics.types.memop]

🔗

value_type operator++(int) volatile noexcept;
constexpr value_type operator++(int) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_add(1);

🔗

value_type operator--(int) volatile noexcept;
constexpr value_type operator--(int) noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_sub(1);

🔗

value_type operator++() volatile noexcept;
constexpr value_type operator++() noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_add(1) + 1;

🔗

value_type operator--() volatile noexcept;
constexpr value_type operator--() noexcept;

Constraints: For the volatile overload of this function, is_always_lock_free is true.

Effects: Equivalent to: return fetch_sub(1) - 1;

32.5.8.7 Partial specializations for smart pointers [util.smartptr.atomic]

32.5.8.7.1 General [util.smartptr.atomic.general]

The library provides partial specializations of the atomic template for shared-ownership smart pointers ([util.sharedptr]).

[Note 1:

The partial specializations are declared in header <memory>.

— end note]

The behavior of all operations is as specified in [atomics.types.generic], unless specified otherwise.

The template parameter T of these partial specializations may be an incomplete type.

All changes to an atomic smart pointer in [util.smartptr.atomic], and all associated use_count increments, are guaranteed to be performed atomically.

Associated use_count decrements are sequenced after the atomic operation, but are not required to be part of it.

Any associated deletion and deallocation are sequenced after the atomic update step and are not part of the atomic operation.

[Note 2:

If the atomic operation uses locks, locks acquired by the implementation will be held when any use_count adjustments are performed, and will not be held when any destruction or deallocation resulting from this is performed.

— end note]

[Example 1: template<typename T> class atomic_list { struct node { T t; shared_ptr<node> next; }; atomic<shared_ptr<node>> head; public: shared_ptr<node> find(T t) const { auto p = head.load(); while (p && p->t != t) p = p->next; return p; } void push_front(T t) { auto p = make_shared<node>(); p->t = t; p->next = head; while (!head.compare_exchange_weak(p->next, p)) {} } }; — end example]

32.5.8.7.2 Partial specialization for shared_ptr [util.smartptr.atomic.shared]

🔗

namespace std { template<class T> struct atomic<shared_ptr<T>> { using value_type = shared_ptr<T>; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr atomic() noexcept; constexpr atomic(nullptr_t) noexcept : atomic() { } atomic(shared_ptr<T> desired) noexcept; atomic(const atomic&) = delete; void operator=(const atomic&) = delete; shared_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept; operator shared_ptr<T>() const noexcept; void store(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; void operator=(shared_ptr<T> desired) noexcept; void operator=(nullptr_t) noexcept; shared_ptr<T> exchange(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired, memory_order success, memory_order failure) noexcept; bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired, memory_order success, memory_order failure) noexcept; bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; void wait(shared_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept; void notify_one() noexcept; void notify_all() noexcept; private: shared_ptr<T> p; // exposition only }; }

🔗

constexpr atomic() noexcept;

Effects: Initializes p{}.

🔗

atomic(shared_ptr<T> desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[Note 1:

It is possible to have an access to an atomic object A race with its construction, for example, by communicating the address of the just-constructed object A to another thread via memory_order::relaxed operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread.

This results in undefined behavior.

— end note]

🔗

void store(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically replaces the value pointed to by this with the value of desired as if by p.swap(desired).

Memory is affected according to the value of order.

🔗

void operator=(shared_ptr<T> desired) noexcept;

Effects: Equivalent to store(desired).

🔗

void operator=(nullptr_t) noexcept;

Effects: Equivalent to store(nullptr).

🔗

shared_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns p.

🔗

operator shared_ptr<T>() const noexcept;

Effects: Equivalent to: return load();

🔗

shared_ptr<T> exchange(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically replaces p with desired as if by p.swap(desired).

Memory is affected according to the value of order.

This is an atomic read-modify-write operation ([intro.races]).

Returns: Atomically returns the value of p immediately before the effects.

🔗

bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                           memory_order success, memory_order failure) noexcept;
bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                             memory_order success, memory_order failure) noexcept;

Preconditions: failure is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

Effects: If p is equivalent to expected, assigns desired to p and has synchronization semantics corresponding to the value of success, otherwise assigns p to expected and has synchronization semantics corresponding to the value of failure.

Returns: true if p was equivalent to expected, false otherwise.

Remarks: Two shared_ptr objects are equivalent if they store the same pointer value and either share ownership or are both empty.

The weak form may fail spuriously.

See [atomics.types.operations].

If the operation returns true, expected is not accessed after the atomic update and the operation is an atomic read-modify-write operation ([intro.multithread]) on the memory pointed to by this.

Otherwise, the operation is an atomic load operation on that memory, and expected is updated with the existing value read from the atomic object in the attempted atomic update.

The use_count update corresponding to the write to expected is part of the atomic operation.

The write to expected itself is not required to be part of the atomic operation.

🔗

bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                           memory_order order = memory_order::seq_cst) noexcept;

Effects: Equivalent to: return compare_exchange_weak(expected, desired, order, fail_order); where fail_order is the same as order except that a value of memory_order::acq_rel shall be replaced by the value memory_order::acquire and a value of memory_order::release shall be replaced by the value memory_order::relaxed.

🔗

bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                             memory_order order = memory_order::seq_cst) noexcept;

Effects: Equivalent to: return compare_exchange_strong(expected, desired, order, fail_order); where fail_order is the same as order except that a value of memory_order::acq_rel shall be replaced by the value memory_order::acquire and a value of memory_order::release shall be replaced by the value memory_order::relaxed.

🔗

void wait(shared_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(21.1)
Evaluates load(order) and compares it to old.
(21.2)
If the two are not equivalent, returns.
(21.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: Two shared_ptr objects are equivalent if they store the same pointer and either share ownership or are both empty.

This function is an atomic waiting operation ([atomics.wait]).

🔗

void notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

🔗

void notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

32.5.8.7.3 Partial specialization for weak_ptr [util.smartptr.atomic.weak]

🔗

namespace std { template<class T> struct atomic<weak_ptr<T>> { using value_type = weak_ptr<T>; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr atomic() noexcept; atomic(weak_ptr<T> desired) noexcept; atomic(const atomic&) = delete; void operator=(const atomic&) = delete; weak_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept; operator weak_ptr<T>() const noexcept; void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; void operator=(weak_ptr<T> desired) noexcept; weak_ptr<T> exchange(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept; bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order success, memory_order failure) noexcept; bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept; void wait(weak_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept; void notify_one() noexcept; void notify_all() noexcept; private: weak_ptr<T> p; // exposition only }; }

🔗

constexpr atomic() noexcept;

Effects: Initializes p{}.

🔗

atomic(weak_ptr<T> desired) noexcept;

Effects: Initializes the object with the value desired.

Initialization is not an atomic operation ([intro.multithread]).

[Note 1:

This results in undefined behavior.

— end note]

🔗

void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically replaces the value pointed to by this with the value of desired as if by p.swap(desired).

Memory is affected according to the value of order.

🔗

void operator=(weak_ptr<T> desired) noexcept;

Effects: Equivalent to store(desired).

🔗

weak_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns p.

🔗

operator weak_ptr<T>() const noexcept;

Effects: Equivalent to: return load();

🔗

weak_ptr<T> exchange(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically replaces p with desired as if by p.swap(desired).

Memory is affected according to the value of order.

This is an atomic read-modify-write operation ([intro.races]).

Returns: Atomically returns the value of p immediately before the effects.

🔗

bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired,
                           memory_order success, memory_order failure) noexcept;
bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired,
                             memory_order success, memory_order failure) noexcept;

Preconditions: failure is memory_order::relaxed, memory_order::consume, memory_order::acquire, or memory_order::seq_cst.

Returns: true if p was equivalent to expected, false otherwise.

Remarks: Two weak_ptr objects are equivalent if they store the same pointer value and either share ownership or are both empty.

The weak form may fail spuriously.

See [atomics.types.operations].

If the operation returns true, expected is not accessed after the atomic update and the operation is an atomic read-modify-write operation ([intro.multithread]) on the memory pointed to by this.

Otherwise, the operation is an atomic load operation on that memory, and expected is updated with the existing value read from the atomic object in the attempted atomic update.

The use_count update corresponding to the write to expected is part of the atomic operation.

The write to expected itself is not required to be part of the atomic operation.

🔗

bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired,
                           memory_order order = memory_order::seq_cst) noexcept;

🔗

bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired,
                             memory_order order = memory_order::seq_cst) noexcept;

🔗

void wait(weak_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(20.1)
Evaluates load(order) and compares it to old.
(20.2)
If the two are not equivalent, returns.
(20.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: Two weak_ptr objects are equivalent if they store the same pointer and either share ownership or are both empty.

This function is an atomic waiting operation ([atomics.wait]).

🔗

void notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

🔗

void notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

32.5.9 Non-member functions [atomics.nonmembers]

A non-member function template whose name matches the pattern atomic_f or the pattern atomic_f_explicit invokes the member function f, with the value of the first parameter as the object expression and the values of the remaining parameters (if any) as the arguments of the member function call, in order.

An argument for a parameter of type atomic<T>::value_type* is dereferenced when passed to the member function call.

If no such member function exists, the program is ill-formed.

[Note 1:

The non-member functions enable programmers to write code that can be compiled as either C or C++, for example in a shared header file.

— end note]

32.5.10 Flag type and operations [atomics.flag]

namespace std { struct atomic_flag { constexpr atomic_flag() noexcept; atomic_flag(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) volatile = delete; bool test(memory_order = memory_order::seq_cst) const volatile noexcept; constexpr bool test(memory_order = memory_order::seq_cst) const noexcept; bool test_and_set(memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool test_and_set(memory_order = memory_order::seq_cst) noexcept; void clear(memory_order = memory_order::seq_cst) volatile noexcept; constexpr void clear(memory_order = memory_order::seq_cst) noexcept; void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(bool, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; }; }

The atomic_flag type provides the classic test-and-set functionality.

It has two states, set and clear.

Operations on an object of type atomic_flag shall be lock-free.

The operations should also be address-free.

The atomic_flag type is a standard-layout struct.

It has a trivial destructor.

🔗

constexpr atomic_flag::atomic_flag() noexcept;

Effects: Initializes *this to the clear state.

🔗

bool atomic_flag_test(const volatile atomic_flag* object) noexcept;
constexpr bool atomic_flag_test(const atomic_flag* object) noexcept;
bool atomic_flag_test_explicit(const volatile atomic_flag* object,
                               memory_order order) noexcept;
constexpr bool atomic_flag_test_explicit(const atomic_flag* object,
                               memory_order order) noexcept;
bool atomic_flag::test(memory_order order = memory_order::seq_cst) const volatile noexcept;
constexpr bool atomic_flag::test(memory_order order = memory_order::seq_cst) const noexcept;

For atomic_flag_test, let order be memory_order::seq_cst.

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by object or this.

🔗

bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept;
constexpr bool atomic_flag_test_and_set(atomic_flag* object) noexcept;
bool atomic_flag_test_and_set_explicit(volatile atomic_flag* object, memory_order order) noexcept;
constexpr bool atomic_flag_test_and_set_explicit(atomic_flag* object, memory_order order) noexcept;
bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) noexcept;

Effects: Atomically sets the value pointed to by object or by this to true.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value of the object immediately before the effects.

🔗

void atomic_flag_clear(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_clear(atomic_flag* object) noexcept;
void atomic_flag_clear_explicit(volatile atomic_flag* object, memory_order order) noexcept;
constexpr void atomic_flag_clear_explicit(atomic_flag* object, memory_order order) noexcept;
void atomic_flag::clear(memory_order order = memory_order::seq_cst) volatile noexcept;
constexpr void atomic_flag::clear(memory_order order = memory_order::seq_cst) noexcept;

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically sets the value pointed to by object or by this to false.

Memory is affected according to the value of order.

🔗

void atomic_flag_wait(const volatile atomic_flag* object, bool old) noexcept;
constexpr void atomic_flag_wait(const atomic_flag* object, bool old) noexcept;
void atomic_flag_wait_explicit(const volatile atomic_flag* object,
                               bool old, memory_order order) noexcept;
constexpr void atomic_flag_wait_explicit(const atomic_flag* object,
                               bool old, memory_order order) noexcept;
void atomic_flag::wait(bool old, memory_order order =
                                   memory_order::seq_cst) const volatile noexcept;
constexpr void atomic_flag::wait(bool old, memory_order order =
                                   memory_order::seq_cst) const noexcept;

For atomic_flag_wait, let order be memory_order::seq_cst.

Let flag be object for the non-member functions and this for the member functions.

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(15.1)
Evaluates flag->test(order) != old.
(15.2)
If the result of that evaluation is true, returns.
(15.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]).

🔗

void atomic_flag_notify_one(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_notify_one(atomic_flag* object) noexcept;
void atomic_flag::notify_one() volatile noexcept;
constexpr void atomic_flag::notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

🔗

void atomic_flag_notify_all(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_notify_all(atomic_flag* object) noexcept;
void atomic_flag::notify_all() volatile noexcept;
constexpr void atomic_flag::notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

🔗

#define ATOMIC_FLAG_INIT see below

Remarks: The macro ATOMIC_FLAG_INIT is defined in such a way that it can be used to initialize an object of type atomic_flag to the clear state.

The macro can be used in the form: atomic_flag guard = ATOMIC_FLAG_INIT;

It is unspecified whether the macro can be used in other initialization contexts.

For a complete static-duration object, that initialization shall be static.

32.5.11 Fences [atomics.fences]

This subclause introduces synchronization primitives called fences.

Fences can have acquire semantics, release semantics, or both.

A fence with acquire semantics is called an acquire fence.

A fence with release semantics is called a release fence.

A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y, both operating on some atomic object M, such that A is sequenced before X, X modifies M, Y is sequenced before B, and Y reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

A release fence A synchronizes with an atomic operation B that performs an acquire operation on an atomic object M if there exists an atomic operation X such that A is sequenced before X, X modifies M, and B reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

An atomic operation A that is a release operation on an atomic object M synchronizes with an acquire fence B if there exists some atomic operation X on M such that X is sequenced before B and reads the value written by A or a value written by any side effect in the release sequence headed by A.

🔗

extern "C" constexpr void atomic_thread_fence(memory_order order) noexcept;

Effects: Depending on the value of order, this operation:

(5.1)
has no effects, if order == memory_order::relaxed;
(5.2)
is an acquire fence, if order == memory_order::acquire or order == memory_order::consume;
(5.3)
is a release fence, if order == memory_order::release;
(5.4)
is both an acquire fence and a release fence, if order == memory_order::acq_rel;
(5.5)
is a sequentially consistent acquire and release fence, if order == memory_order::seq_cst.

🔗

extern "C" constexpr void atomic_signal_fence(memory_order order) noexcept;

Effects: Equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.

[Note 1:

atomic_signal_fence can be used to specify the order in which actions performed by the thread become visible to the signal handler.

Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not emitted.

— end note]

32.5.12 C compatibility [stdatomic.h.syn]

The header provides the following definitions:

template<class T> using std-atomic = std::atomic<T>; // exposition only #define _Atomic(T) std-atomic<T> #define ATOMIC_BOOL_LOCK_FREE see below #define ATOMIC_CHAR_LOCK_FREE see below #define ATOMIC_CHAR16_T_LOCK_FREE see below #define ATOMIC_CHAR32_T_LOCK_FREE see below #define ATOMIC_WCHAR_T_LOCK_FREE see below #define ATOMIC_SHORT_LOCK_FREE see below #define ATOMIC_INT_LOCK_FREE see below #define ATOMIC_LONG_LOCK_FREE see below #define ATOMIC_LLONG_LOCK_FREE see below #define ATOMIC_POINTER_LOCK_FREE see below using std::memory_order; // see below using std::memory_order_relaxed; // see below using std::memory_order_consume; // see below using std::memory_order_acquire; // see below using std::memory_order_release; // see below using std::memory_order_acq_rel; // see below using std::memory_order_seq_cst; // see below using std::atomic_flag; // see below using std::atomic_bool; // see below using std::atomic_char; // see below using std::atomic_schar; // see below using std::atomic_uchar; // see below using std::atomic_short; // see below using std::atomic_ushort; // see below using std::atomic_int; // see below using std::atomic_uint; // see below using std::atomic_long; // see below using std::atomic_ulong; // see below using std::atomic_llong; // see below using std::atomic_ullong; // see below using std::atomic_char8_t; // see below using std::atomic_char16_t; // see below using std::atomic_char32_t; // see below using std::atomic_wchar_t; // see below using std::atomic_int8_t; // see below using std::atomic_uint8_t; // see below using std::atomic_int16_t; // see below using std::atomic_uint16_t; // see below using std::atomic_int32_t; // see below using std::atomic_uint32_t; // see below using std::atomic_int64_t; // see below using std::atomic_uint64_t; // see below using std::atomic_int_least8_t; // see below using std::atomic_uint_least8_t; // see below using std::atomic_int_least16_t; // see below using std::atomic_uint_least16_t; // see below using std::atomic_int_least32_t; // see below using std::atomic_uint_least32_t; // see below using std::atomic_int_least64_t; // see below using std::atomic_uint_least64_t; // see below using std::atomic_int_fast8_t; // see below using std::atomic_uint_fast8_t; // see below using std::atomic_int_fast16_t; // see below using std::atomic_uint_fast16_t; // see below using std::atomic_int_fast32_t; // see below using std::atomic_uint_fast32_t; // see below using std::atomic_int_fast64_t; // see below using std::atomic_uint_fast64_t; // see below using std::atomic_intptr_t; // see below using std::atomic_uintptr_t; // see below using std::atomic_size_t; // see below using std::atomic_ptrdiff_t; // see below using std::atomic_intmax_t; // see below using std::atomic_uintmax_t; // see below using std::atomic_is_lock_free; // see below using std::atomic_load; // see below using std::atomic_load_explicit; // see below using std::atomic_store; // see below using std::atomic_store_explicit; // see below using std::atomic_exchange; // see below using std::atomic_exchange_explicit; // see below using std::atomic_compare_exchange_strong; // see below using std::atomic_compare_exchange_strong_explicit; // see below using std::atomic_compare_exchange_weak; // see below using std::atomic_compare_exchange_weak_explicit; // see below using std::atomic_fetch_add; // see below using std::atomic_fetch_add_explicit; // see below using std::atomic_fetch_sub; // see below using std::atomic_fetch_sub_explicit; // see below using std::atomic_fetch_and; // see below using std::atomic_fetch_and_explicit; // see below using std::atomic_fetch_or; // see below using std::atomic_fetch_or_explicit; // see below using std::atomic_fetch_xor; // see below using std::atomic_fetch_xor_explicit; // see below using std::atomic_flag_test_and_set; // see below using std::atomic_flag_test_and_set_explicit; // see below using std::atomic_flag_clear; // see below using std::atomic_flag_clear_explicit; // see below #define ATOMIC_FLAG_INIT see below using std::atomic_thread_fence; // see below using std::atomic_signal_fence; // see below

Each using-declaration for some name A in the synopsis above makes available the same entity as std::A declared in <atomic>.

Each macro listed above other than _Atomic(T) is defined as in <atomic>.

It is unspecified whether <stdatomic.h> makes available any declarations in namespace std.

Each of the using-declarations for intN_t, uintN_t, intptr_t, and uintptr_t listed above is defined if and only if the implementation defines the corresponding typedef-name in [atomics.syn].

Neither the _Atomic macro, nor any of the non-macro global namespace declarations, are provided by any C++ standard library header other than <stdatomic.h>.

Recommended practice: Implementations should ensure that C and C++ representations of atomic objects are compatible, so that the same object can be accessed as both an _Atomic(T) from C code and an atomic<T> from C++ code.

The representations should be the same, and the mechanisms used to ensure atomicity and memory ordering should be compatible.

32.6 Mutual exclusion [thread.mutex]

32.6.1 General [thread.mutex.general]

Subclause [thread.mutex] provides mechanisms for mutual exclusion: mutexes, locks, and call once.

These mechanisms ease the production of race-free programs ([intro.multithread]).

32.6.2 Header <mutex> synopsis [mutex.syn]

🔗

namespace std { // [thread.mutex.class], class mutex class mutex; // [thread.mutex.recursive], class recursive_mutex class recursive_mutex; // [thread.timedmutex.class], class timed_mutex class timed_mutex; // [thread.timedmutex.recursive], class recursive_timed_mutex class recursive_timed_mutex; struct defer_lock_t { explicit defer_lock_t() = default; }; struct try_to_lock_t { explicit try_to_lock_t() = default; }; struct adopt_lock_t { explicit adopt_lock_t() = default; }; inline constexpr defer_lock_t defer_lock { }; inline constexpr try_to_lock_t try_to_lock { }; inline constexpr adopt_lock_t adopt_lock { }; // [thread.lock], locks template<class Mutex> class lock_guard; template<class... MutexTypes> class scoped_lock; template<class Mutex> class unique_lock; template<class Mutex> void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept; // [thread.lock.algorithm], generic locking algorithms template<class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...); template<class L1, class L2, class... L3> void lock(L1&, L2&, L3&...); struct once_flag; template<class Callable, class... Args> void call_once(once_flag& flag, Callable&& func, Args&&... args); }

32.6.3 Header <shared_mutex> synopsis [shared.mutex.syn]

32.6.4 Mutex requirements [thread.mutex.requirements]

32.6.4.1 General [thread.mutex.requirements.general]

A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents .

An execution agent owns a mutex from the time it successfully calls one of the lock functions until it calls unlock.

Mutexes can be either recursive or non-recursive, and can grant simultaneous ownership to one or many execution agents.

Both recursive and non-recursive mutexes are supplied.

32.6.4.2 Mutex types [thread.mutex.requirements.mutex]

32.6.4.2.1 General [thread.mutex.requirements.mutex.general]

The mutex types are the standard library types mutex, recursive_mutex, timed_mutex, recursive_timed_mutex, shared_mutex, and shared_timed_mutex.

They meet the requirements set out in [thread.mutex.requirements.mutex].

In this description, m denotes an object of a mutex type.

[Note 1:

The mutex types meet the Cpp17Lockable requirements ([thread.req.lockable.req]).

— end note]

The mutex types meet Cpp17DefaultConstructible and Cpp17Destructible.

If initialization of an object of a mutex type fails, an exception of type system_error is thrown.

The mutex types are neither copyable nor movable.

The error conditions for error codes, if any, reported by member functions of the mutex types are as follows:

(3.1)
resource_unavailable_try_again — if any native handle type manipulated is not available.
(3.2)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(3.3)
invalid_argument — if any native handle type manipulated as part of mutex construction is incorrect.

The implementation provides lock and unlock operations, as described below.

For purposes of determining the existence of a data race, these behave as atomic operations ([intro.multithread]).

The lock and unlock operations on a single mutex appears to occur in a single total order.

[Note 2:

This can be viewed as the modification order of the mutex.

— end note]

[Note 3:

Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization can be used to ensure that mutex objects are initialized and visible to other threads.

— end note]

The expression m.lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.

Effects: Blocks the calling thread until ownership of the mutex can be obtained for the calling thread.

Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Postconditions: The calling thread owns the mutex.

Return type: void.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(12.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(12.2)
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.

The expression m.try_lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_timed_mutex, the calling thread does not own the mutex.

Effects: Attempts to obtain ownership of the mutex for the calling thread without blocking.

If ownership is not obtained, there is no effect and try_lock() immediately returns.

An implementation may fail to obtain the lock even if it is not held by any other thread.

[Note 4:

This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomics]).

— end note]

An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.

Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with this operation.

[Note 5:

Since lock() does not synchronize with a failed subsequent try_lock(), the visibility rules are weak enough that little would be known about the state after a failure, even in the absence of spurious failures.

— end note]

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Nothing.

The expression m.unlock() is well-formed and has the following semantics:

Preconditions: The calling thread owns the mutex.

Effects: Releases the calling thread's ownership of the mutex.

Return type: void.

Synchronization: This operation synchronizes with subsequent lock operations that obtain ownership on the same object.

Throws: Nothing.

32.6.4.2.2 Class mutex [thread.mutex.class]

🔗

namespace std { class mutex { public: constexpr mutex() noexcept; ~mutex(); mutex(const mutex&) = delete; mutex& operator=(const mutex&) = delete; void lock(); bool try_lock(); void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class mutex provides a non-recursive mutex with exclusive ownership semantics.

If one thread owns a mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock()) until the owning thread has released ownership with a call to unlock().

[Note 1:

After a thread A has called unlock(), releasing a mutex, it is possible for another thread B to lock the same mutex, observe that it is no longer in use, unlock it, and destroy it, before thread A appears to have returned from its unlock call.

Conforming implementations handle such scenarios correctly, as long as thread A does not access the mutex after the unlock call returns.

These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count.

— end note]

The class mutex meets all of the mutex requirements ([thread.mutex.requirements]).

It is a standard-layout class ([class.prop]).

[Note 2:

A program can deadlock if the thread that owns a mutex object calls lock() on that object.

If the implementation can detect the deadlock, a resource_deadlock_would_occur error condition might be observed.

— end note]

The behavior of a program is undefined if it destroys a mutex object owned by any thread or a thread terminates while owning a mutex object.

32.6.4.2.3 Class recursive_mutex [thread.mutex.recursive]

🔗

namespace std { class recursive_mutex { public: recursive_mutex(); ~recursive_mutex(); recursive_mutex(const recursive_mutex&) = delete; recursive_mutex& operator=(const recursive_mutex&) = delete; void lock(); bool try_lock() noexcept; void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class recursive_mutex provides a recursive mutex with exclusive ownership semantics.

If one thread owns a recursive_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock()) until the first thread has completely released ownership.

The class recursive_mutex meets all of the mutex requirements ([thread.mutex.requirements]).

It is a standard-layout class ([class.prop]).

A thread that owns a recursive_mutex object may acquire additional levels of ownership by calling lock() or try_lock() on that object.

It is unspecified how many levels of ownership may be acquired by a single thread.

If a thread has already acquired the maximum level of ownership for a recursive_mutex object, additional calls to try_lock() fail, and additional calls to lock() throw an exception of type system_error.

A thread shall call unlock() once for each level of ownership acquired by calls to lock() and try_lock().

Only when all levels of ownership have been released may ownership be acquired by another thread.

The behavior of a program is undefined if

(4.1)
it destroys a recursive_mutex object owned by any thread or
(4.2)
a thread terminates while owning a recursive_mutex object.

32.6.4.3 Timed mutex types [thread.timedmutex.requirements]

32.6.4.3.1 General [thread.timedmutex.requirements.general]

The timed mutex types are the standard library types timed_mutex, recursive_timed_mutex, and shared_timed_mutex.

They meet the requirements set out below.

In this description, m denotes an object of a mutex type, rel_time denotes an object of an instantiation of duration, and abs_time denotes an object of an instantiation of time_point .

[Note 1:

The timed mutex types meet the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

— end note]

The expression m.try_lock_for(rel_time) is well-formed and has the following semantics:

Preconditions: If m is of type timed_mutex or shared_timed_mutex, the calling thread does not own the mutex.

Effects: The function attempts to obtain ownership of the mutex within the relative timeout ([thread.req.timing]) specified by rel_time.

If the time specified by rel_time is less than or equal to rel_time.zero(), the function attempts to obtain ownership without blocking (as if by calling try_lock()).

The function returns within the timeout specified by rel_time only if it has obtained ownership of the mutex object.

[Note 2:

As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.

— end note]

Synchronization: If try_lock_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

The expression m.try_lock_until(abs_time) is well-formed and has the following semantics:

Preconditions: If m is of type timed_mutex or shared_timed_mutex, the calling thread does not own the mutex.

Effects: The function attempts to obtain ownership of the mutex.

If abs_time has already passed, the function attempts to obtain ownership without blocking (as if by calling try_lock()).

The function returns before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained ownership of the mutex object.

[Note 3:

As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.

— end note]

Synchronization: If try_lock_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

32.6.4.3.2 Class timed_mutex [thread.timedmutex.class]

🔗

namespace std { class timed_mutex { public: timed_mutex(); ~timed_mutex(); timed_mutex(const timed_mutex&) = delete; timed_mutex& operator=(const timed_mutex&) = delete; void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class timed_mutex provides a non-recursive mutex with exclusive ownership semantics.

If one thread owns a timed_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock(), try_lock_for(), and try_lock_until()) until the owning thread has released ownership with a call to unlock() or the call to try_lock_for() or try_lock_until() times out (having failed to obtain ownership).

The class timed_mutex meets all of the timed mutex requirements ([thread.timedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if

(3.1)
it destroys a timed_mutex object owned by any thread,
(3.2)
a thread that owns a timed_mutex object calls lock(), try_lock(), try_lock_for(), or try_lock_until() on that object, or
(3.3)
a thread terminates while owning a timed_mutex object.

32.6.4.3.3 Class recursive_timed_mutex [thread.timedmutex.recursive]

🔗

namespace std { class recursive_timed_mutex { public: recursive_timed_mutex(); ~recursive_timed_mutex(); recursive_timed_mutex(const recursive_timed_mutex&) = delete; recursive_timed_mutex& operator=(const recursive_timed_mutex&) = delete; void lock(); // blocking bool try_lock() noexcept; template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class recursive_timed_mutex provides a recursive mutex with exclusive ownership semantics.

If one thread owns a recursive_timed_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock(), try_lock_for(), and try_lock_until()) until the owning thread has completely released ownership or the call to try_lock_for() or try_lock_until() times out (having failed to obtain ownership).

The class recursive_timed_mutex meets all of the timed mutex requirements ([thread.timedmutex.requirements]).

It is a standard-layout class ([class.prop]).

A thread that owns a recursive_timed_mutex object may acquire additional levels of ownership by calling lock(), try_lock(), try_lock_for(), or try_lock_until() on that object.

It is unspecified how many levels of ownership may be acquired by a single thread.

If a thread has already acquired the maximum level of ownership for a recursive_timed_mutex object, additional calls to try_lock(), try_lock_for(), or try_lock_until() fail, and additional calls to lock() throw an exception of type system_error.

A thread shall call unlock() once for each level of ownership acquired by calls to lock(), try_lock(), try_lock_for(), and try_lock_until().

Only when all levels of ownership have been released may ownership of the object be acquired by another thread.

The behavior of a program is undefined if

(4.1)
it destroys a recursive_timed_mutex object owned by any thread, or
(4.2)
a thread terminates while owning a recursive_timed_mutex object.

32.6.4.4 Shared mutex types [thread.sharedmutex.requirements]

32.6.4.4.1 General [thread.sharedmutex.requirements.general]

The standard library types shared_mutex and shared_timed_mutex are shared mutex types.

Shared mutex types meet the requirements of mutex types ([thread.mutex.requirements.mutex]) and additionally meet the requirements set out below.

In this description, m denotes an object of a shared mutex type.

[Note 1:

The shared mutex types meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

— end note]

In addition to the exclusive lock ownership mode specified in [thread.mutex.requirements.mutex], shared mutex types provide a shared lock ownership mode.

Multiple execution agents can simultaneously hold a shared lock ownership of a shared mutex type.

But no execution agent holds a shared lock while another execution agent holds an exclusive lock on the same shared mutex type, and vice-versa.

The maximum number of execution agents which can share a shared lock on a single shared mutex type is unspecified, but is at least 10000.

If more than the maximum number of execution agents attempt to obtain a shared lock, the excess execution agents block until the number of shared locks are reduced below the maximum amount by other execution agents releasing their shared lock.

The expression m.lock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread.

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Postconditions: The calling thread has a shared lock on the mutex.

Return type: void.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(10.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(10.2)
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.

The expression m.unlock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread holds a shared lock on the mutex.

Effects: Releases a shared lock on the mutex held by the calling thread.

Return type: void.

Synchronization: This operation synchronizes with subsequent lock() operations that obtain ownership on the same object.

Throws: Nothing.

The expression m.try_lock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Attempts to obtain shared ownership of the mutex for the calling thread without blocking.

If shared ownership is not obtained, there is no effect and try_lock_shared() immediately returns.

An implementation may fail to obtain the lock even if it is not held by any other thread.

Synchronization: If try_lock_shared() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Nothing.

32.6.4.4.2 Class shared_mutex [thread.sharedmutex.class]

🔗

namespace std { class shared_mutex { public: shared_mutex(); ~shared_mutex(); shared_mutex(const shared_mutex&) = delete; shared_mutex& operator=(const shared_mutex&) = delete; // exclusive ownership void lock(); // blocking bool try_lock(); void unlock(); // shared ownership void lock_shared(); // blocking bool try_lock_shared(); void unlock_shared(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class shared_mutex provides a non-recursive mutex with shared ownership semantics.

The class shared_mutex meets all of the shared mutex requirements ([thread.sharedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if

(3.1)
it destroys a shared_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_mutex.

shared_mutex may be a synonym for shared_timed_mutex.

32.6.4.5 Shared timed mutex types [thread.sharedtimedmutex.requirements]

32.6.4.5.1 General [thread.sharedtimedmutex.requirements.general]

The standard library type shared_timed_mutex is a shared timed mutex type.

Shared timed mutex types meet the requirements of timed mutex types ([thread.timedmutex.requirements]), shared mutex types ([thread.sharedmutex.requirements]), and additionally meet the requirements set out below.

In this description, m denotes an object of a shared timed mutex type, rel_time denotes an object of an instantiation of duration ([time.duration]), and abs_time denotes an object of an instantiation of time_point .

[Note 1:

The shared timed mutex types meet the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

— end note]

The expression m.try_lock_shared_for(rel_time) is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Attempts to obtain shared lock ownership for the calling thread within the relative timeout ([thread.req.timing]) specified by rel_time.

If the time specified by rel_time is less than or equal to rel_time.zero(), the function attempts to obtain ownership without blocking (as if by calling try_lock_shared()).

The function returns within the timeout specified by rel_time only if it has obtained shared ownership of the mutex object.

[Note 2:

As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.

— end note]

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: If try_lock_shared_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

The expression m.try_lock_shared_until(abs_time) is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: The function attempts to obtain shared ownership of the mutex.

If abs_time has already passed, the function attempts to obtain shared ownership without blocking (as if by calling try_lock_shared()).

The function returns before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained shared ownership of the mutex object.

[Note 3:

As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so.

— end note]

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: If try_lock_shared_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

32.6.4.5.2 Class shared_timed_mutex [thread.sharedtimedmutex.class]

🔗

namespace std { class shared_timed_mutex { public: shared_timed_mutex(); ~shared_timed_mutex(); shared_timed_mutex(const shared_timed_mutex&) = delete; shared_timed_mutex& operator=(const shared_timed_mutex&) = delete; // exclusive ownership void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); // shared ownership void lock_shared(); // blocking bool try_lock_shared(); template<class Rep, class Period> bool try_lock_shared_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_shared_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock_shared(); }; }

The class shared_timed_mutex provides a non-recursive mutex with shared ownership semantics.

The class shared_timed_mutex meets all of the shared timed mutex requirements ([thread.sharedtimedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if

(3.1)
it destroys a shared_timed_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_timed_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_timed_mutex.

32.6.5 Locks [thread.lock]

32.6.5.1 General [thread.lock.general]

A lock is an object that holds a reference to a lockable object and may unlock the lockable object during the lock's destruction (such as when leaving block scope).

An execution agent may use a lock to aid in managing ownership of a lockable object in an exception safe manner.

A lock is said to own a lockable object if it is currently managing the ownership of that lockable object for an execution agent.

A lock does not manage the lifetime of the lockable object it references.

[Note 1:

Locks are intended to ease the burden of unlocking the lockable object under both normal and exceptional circumstances.

— end note]

Some lock constructors take tag types which describe what should be done with the lockable object during the lock's construction.

🔗

namespace std { struct defer_lock_t { }; // do not acquire ownership of the mutex struct try_to_lock_t { }; // try to acquire ownership of the mutex // without blocking struct adopt_lock_t { }; // assume the calling thread has already // obtained mutex ownership and manage it inline constexpr defer_lock_t defer_lock { }; inline constexpr try_to_lock_t try_to_lock { }; inline constexpr adopt_lock_t adopt_lock { }; }

32.6.5.2 Class template lock_guard [thread.lock.guard]

🔗

namespace std { template<class Mutex> class lock_guard { public: using mutex_type = Mutex; explicit lock_guard(mutex_type& m); lock_guard(mutex_type& m, adopt_lock_t); ~lock_guard(); lock_guard(const lock_guard&) = delete; lock_guard& operator=(const lock_guard&) = delete; private: mutex_type& pm; // exposition only }; }

An object of type lock_guard controls the ownership of a lockable object within a scope.

A lock_guard object maintains ownership of a lockable object throughout the lock_guard object's lifetime .

The behavior of a program is undefined if the lockable object referenced by pm does not exist for the entire lifetime of the lock_guard object.

The supplied Mutex type shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

🔗

explicit lock_guard(mutex_type& m);

Effects: Initializes pm with m.

Calls m.lock().

🔗

lock_guard(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a non-shared lock on m.

Effects: Initializes pm with m.

Throws: Nothing.

🔗

~lock_guard();

Effects: Equivalent to: pm.unlock()

32.6.5.3 Class template scoped_lock [thread.lock.scoped]

🔗

namespace std { template<class... MutexTypes> class scoped_lock { public: using mutex_type = see below; // Only if sizeof...(MutexTypes) == 1 is true explicit scoped_lock(MutexTypes&... m); explicit scoped_lock(adopt_lock_t, MutexTypes&... m); ~scoped_lock(); scoped_lock(const scoped_lock&) = delete; scoped_lock& operator=(const scoped_lock&) = delete; private: tuple<MutexTypes&...> pm; // exposition only }; }

An object of type scoped_lock controls the ownership of lockable objects within a scope.

A scoped_lock object maintains ownership of lockable objects throughout the scoped_lock object's lifetime .

The behavior of a program is undefined if the lockable objects referenced by pm do not exist for the entire lifetime of the scoped_lock object.

(1.1)
If sizeof...(MutexTypes) is one, let Mutex denote the sole type constituting the pack MutexTypes.

Mutex shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

The member typedef-name mutex_type denotes the same type as Mutex.
(1.2)
Otherwise, all types in the template parameter pack MutexTypes shall meet the Cpp17Lockable requirements ([thread.req.lockable.req]) and there is no member mutex_type.

🔗

explicit scoped_lock(MutexTypes&... m);

Effects: Initializes pm with tie(m...).

Then if sizeof...(MutexTypes) is 0, no effects.

Otherwise if sizeof...(MutexTypes) is 1, then m.lock().

Otherwise, lock(m...).

🔗

explicit scoped_lock(adopt_lock_t, MutexTypes&... m);

Preconditions: The calling thread holds a non-shared lock on each element of m.

Effects: Initializes pm with tie(m...).

Throws: Nothing.

🔗

~scoped_lock();

Effects: For all i in [0, sizeof...(MutexTypes)), get<i>(pm).unlock().

32.6.5.4 Class template unique_lock [thread.lock.unique]

32.6.5.4.1 General [thread.lock.unique.general]

🔗

namespace std { template<class Mutex> class unique_lock { public: using mutex_type = Mutex; // [thread.lock.unique.cons], construct/copy/destroy unique_lock() noexcept; explicit unique_lock(mutex_type& m); unique_lock(mutex_type& m, defer_lock_t) noexcept; unique_lock(mutex_type& m, try_to_lock_t); unique_lock(mutex_type& m, adopt_lock_t); template<class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time); template<class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time); ~unique_lock(); unique_lock(const unique_lock&) = delete; unique_lock& operator=(const unique_lock&) = delete; unique_lock(unique_lock&& u) noexcept; unique_lock& operator=(unique_lock&& u); // [thread.lock.unique.locking], locking void lock(); bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); // [thread.lock.unique.mod], modifiers void swap(unique_lock& u) noexcept; mutex_type* release() noexcept; // [thread.lock.unique.obs], observers bool owns_lock() const noexcept; explicit operator bool() const noexcept; mutex_type* mutex() const noexcept; private: mutex_type* pm; // exposition only bool owns; // exposition only }; }

An object of type unique_lock controls the ownership of a lockable object within a scope.

Ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another unique_lock object.

Objects of type unique_lock are not copyable but are movable.

The behavior of a program is undefined if the contained pointer pm is not null and the lockable object pointed to by pm does not exist for the entire remaining lifetime ([basic.life]) of the unique_lock object.

The supplied Mutex type shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

[Note 1:

unique_lock<Mutex> meets the Cpp17BasicLockable requirements.

If Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]), unique_lock<Mutex> also meets the Cpp17Lockable requirements; if Mutex meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]), unique_lock<Mutex> also meets the Cpp17TimedLockable requirements.

— end note]

32.6.5.4.2 Constructors, destructor, and assignment [thread.lock.unique.cons]

🔗

unique_lock() noexcept;

Postconditions: pm == nullptr and owns == false.

🔗

explicit unique_lock(mutex_type& m);

Effects: Calls m.lock().

Postconditions: pm == addressof(m) and owns == true.

🔗

unique_lock(mutex_type& m, defer_lock_t) noexcept;

Postconditions: pm == addressof(m) and owns == false.

🔗

unique_lock(mutex_type& m, try_to_lock_t);

Preconditions: The supplied Mutex type meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

Effects: Calls m.try_lock().

Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_lock().

🔗

unique_lock(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a non-shared lock on m.

Postconditions: pm == addressof(m) and owns == true.

Throws: Nothing.

🔗

template<class Clock, class Duration>
  unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: Calls m.try_lock_until(abs_time).

Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_lock_until(abs_time).

🔗

template<class Rep, class Period>
  unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: Calls m.try_lock_for(rel_time).

Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_lock_for(rel_time).

🔗

unique_lock(unique_lock&& u) noexcept;

Postconditions: pm == u_p.pm and owns == u_p.owns (where u_p is the state of u just prior to this construction), u.pm == 0 and u.owns == false.

🔗

unique_lock& operator=(unique_lock&& u);

Effects: If owns calls pm->unlock().

Postconditions: pm == u_p.pm and owns == u_p.owns (where u_p is the state of u just prior to this construction), u.pm == 0 and u.owns == false.

[Note 1:

With a recursive mutex it is possible for both *this and u to own the same mutex before the assignment.

In this case, *this will own the mutex after the assignment and u will not.

— end note]

Throws: Nothing.

🔗

~unique_lock();

Effects: If owns calls pm->unlock().

32.6.5.4.3 Locking [thread.lock.unique.locking]

🔗

void lock();

Effects: As if by pm->lock().

Postconditions: owns == true.

Throws: Any exception thrown by pm->lock().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(4.1)
operation_not_permitted — if pm is nullptr.
(4.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

bool try_lock();

Preconditions: The supplied Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

Effects: As if by pm->try_lock().

Postconditions: owns == res, where res is the value returned by pm->try_lock().

Returns: The value returned by pm->try_lock().

Throws: Any exception thrown by pm->try_lock().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(10.1)
operation_not_permitted — if pm is nullptr.
(10.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

template<class Clock, class Duration>
  bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: As if by pm->try_lock_until(abs_time).

Postconditions: owns == res, where res is the value returned by pm->try_lock_until(abs_time).

Returns: The value returned by pm->try_lock_until(abs_time).

Throws: Any exception thrown by pm->try_lock_until(abstime).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(16.1)
operation_not_permitted — if pm is nullptr.
(16.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

template<class Rep, class Period>
  bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: As if by pm->try_lock_for(rel_time).

Postconditions: owns == res, where res is the value returned by pm->try_lock_for(rel_time).

Returns: The value returned by pm->try_lock_for(rel_time).

Throws: Any exception thrown by pm->try_lock_for(rel_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(22.1)
operation_not_permitted — if pm is nullptr.
(22.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

void unlock();

Effects: As if by pm->unlock().

Postconditions: owns == false.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(26.1)
operation_not_permitted — if on entry owns is false.

32.6.5.4.4 Modifiers [thread.lock.unique.mod]

🔗

void swap(unique_lock& u) noexcept;

Effects: Swaps the data members of *this and u.

🔗

mutex_type* release() noexcept;

Postconditions: pm == 0 and owns == false.

Returns: The previous value of pm.

🔗

template<class Mutex>
  void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;

Effects: As if by x.swap(y).

32.6.5.4.5 Observers [thread.lock.unique.obs]

🔗

bool owns_lock() const noexcept;

Returns: owns.

🔗

explicit operator bool() const noexcept;

Returns: owns.

🔗

mutex_type *mutex() const noexcept;

Returns: pm.

32.6.5.5 Class template shared_lock [thread.lock.shared]

32.6.5.5.1 General [thread.lock.shared.general]

🔗

namespace std { template<class Mutex> class shared_lock { public: using mutex_type = Mutex; // [thread.lock.shared.cons], construct/copy/destroy shared_lock() noexcept; explicit shared_lock(mutex_type& m); // blocking shared_lock(mutex_type& m, defer_lock_t) noexcept; shared_lock(mutex_type& m, try_to_lock_t); shared_lock(mutex_type& m, adopt_lock_t); template<class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time); template<class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time); ~shared_lock(); shared_lock(const shared_lock&) = delete; shared_lock& operator=(const shared_lock&) = delete; shared_lock(shared_lock&& u) noexcept; shared_lock& operator=(shared_lock&& u) noexcept; // [thread.lock.shared.locking], locking void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); // [thread.lock.shared.mod], modifiers void swap(shared_lock& u) noexcept; mutex_type* release() noexcept; // [thread.lock.shared.obs], observers bool owns_lock() const noexcept; explicit operator bool() const noexcept; mutex_type* mutex() const noexcept; private: mutex_type* pm; // exposition only bool owns; // exposition only }; }

An object of type shared_lock controls the shared ownership of a lockable object within a scope.

Shared ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another shared_lock object.

Objects of type shared_lock are not copyable but are movable.

The supplied Mutex type shall meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

[Note 1:

shared_lock<Mutex> meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

If Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]), shared_lock<Mutex> also meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

— end note]

32.6.5.5.2 Constructors, destructor, and assignment [thread.lock.shared.cons]

🔗

shared_lock() noexcept;

Postconditions: pm == nullptr and owns == false.

🔗

explicit shared_lock(mutex_type& m);

Effects: Calls m.lock_shared().

Postconditions: pm == addressof(m) and owns == true.

🔗

shared_lock(mutex_type& m, defer_lock_t) noexcept;

Postconditions: pm == addressof(m) and owns == false.

🔗

shared_lock(mutex_type& m, try_to_lock_t);

Effects: Calls m.try_lock_shared().

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared().

🔗

shared_lock(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a shared lock on m.

Postconditions: pm == addressof(m) and owns == true.

🔗

template<class Clock, class Duration>
  shared_lock(mutex_type& m,
              const chrono::time_point<Clock, Duration>& abs_time);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: Calls m.try_lock_shared_until(abs_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_until(abs_time).

🔗

template<class Rep, class Period>
  shared_lock(mutex_type& m,
              const chrono::duration<Rep, Period>& rel_time);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: Calls m.try_lock_shared_for(rel_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_for(rel_time).

🔗

~shared_lock();

Effects: If owns calls pm->unlock_shared().

🔗

shared_lock(shared_lock&& sl) noexcept;

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this construction), sl.pm == nullptr and sl.owns == false.

🔗

shared_lock& operator=(shared_lock&& sl) noexcept;

Effects: If owns calls pm->unlock_shared().

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this assignment), sl.pm == nullptr and sl.owns == false.

32.6.5.5.3 Locking [thread.lock.shared.locking]

🔗

void lock();

Effects: As if by pm->lock_shared().

Postconditions: owns == true.

Throws: Any exception thrown by pm->lock_shared().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(4.1)
operation_not_permitted — if pm is nullptr.
(4.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

bool try_lock();

Effects: As if by pm->try_lock_shared().

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared().

Returns: The value returned by the call to pm->try_lock_shared().

Throws: Any exception thrown by pm->try_lock_shared().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(9.1)
operation_not_permitted — if pm is nullptr.
(9.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

template<class Clock, class Duration>
  bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by pm->try_lock_shared_until(abs_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_until(abs_time).

Returns: The value returned by the call to pm->try_lock_shared_until(abs_time).

Throws: Any exception thrown by pm->try_lock_shared_until(abs_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(15.1)
operation_not_permitted — if pm is nullptr.
(15.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

template<class Rep, class Period>
  bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by pm->try_lock_shared_for(rel_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_for(rel_time).

Returns: The value returned by the call to pm->try_lock_shared_for(rel_time).

Throws: Any exception thrown by pm->try_lock_shared_for(rel_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(21.1)
operation_not_permitted — if pm is nullptr.
(21.2)
resource_deadlock_would_occur — if on entry owns is true.

🔗

void unlock();

Effects: As if by pm->unlock_shared().

Postconditions: owns == false.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(25.1)
operation_not_permitted — if on entry owns is false.

32.6.5.5.4 Modifiers [thread.lock.shared.mod]

🔗

void swap(shared_lock& sl) noexcept;

Effects: Swaps the data members of *this and sl.

🔗

mutex_type* release() noexcept;

Postconditions: pm == nullptr and owns == false.

Returns: The previous value of pm.

🔗

template<class Mutex>
  void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;

Effects: As if by x.swap(y).

32.6.5.5.5 Observers [thread.lock.shared.obs]

🔗

bool owns_lock() const noexcept;

Returns: owns.

🔗

explicit operator bool() const noexcept;

Returns: owns.

🔗

mutex_type* mutex() const noexcept;

Returns: pm.

32.6.6 Generic locking algorithms [thread.lock.algorithm]

🔗

template<class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

Preconditions: Each template parameter type meets the Cpp17Lockable requirements.

[Note 1:

The unique_lock class template meets these requirements when suitably instantiated.

— end note]

Effects: Calls try_lock() for each argument in order beginning with the first until all arguments have been processed or a call to try_lock() fails, either by returning false or by throwing an exception.

If a call to try_lock() fails, unlock() is called for all prior arguments with no further calls to try_lock().

Returns: -1 if all calls to try_lock() returned true, otherwise a zero-based index value that indicates the argument for which try_lock() returned false.

🔗

template<class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

Preconditions: Each template parameter type meets the Cpp17Lockable requirements.

[Note 2:

The unique_lock class template meets these requirements when suitably instantiated.

— end note]

Effects: All arguments are locked via a sequence of calls to lock(), try_lock(), or unlock() on each argument.

The sequence of calls does not result in deadlock, but is otherwise unspecified.

[Note 3:

A deadlock avoidance algorithm such as try-and-back-off can be used, but the specific algorithm is not specified to avoid over-constraining implementations.

— end note]

If a call to lock() or try_lock() throws an exception, unlock() is called for any argument that had been locked by a call to lock() or try_lock().

32.6.7 Call once [thread.once]

32.6.7.1 Struct once_flag [thread.once.onceflag]

🔗

namespace std { struct once_flag { constexpr once_flag() noexcept; once_flag(const once_flag&) = delete; once_flag& operator=(const once_flag&) = delete; }; }

The class once_flag is an opaque data structure that call_once uses to initialize data without causing a data race or deadlock.

🔗

constexpr once_flag() noexcept;

Synchronization: The construction of a once_flag object is not synchronized.

Postconditions: The object's internal state is set to indicate to an invocation of call_once with the object as its initial argument that no function has been called.

32.6.7.2 Function call_once [thread.once.callonce]

🔗

template<class Callable, class... Args>
  void call_once(once_flag& flag, Callable&& func, Args&&... args);

Mandates: is_invocable_v<Callable, Args...> is true.

Effects: An execution of call_once that does not call its func is a passive execution.

An execution of call_once that calls its func is an active execution.

An active execution evaluates INVOKE(std::forward<Callable>(func), std::forward<Args>(args)...) ([func.require]).

If such a call to func throws an exception the execution is exceptional, otherwise it is returning.

An exceptional execution propagates the exception to the caller of call_once.

Among all executions of call_once for any given once_flag: at most one is a returning execution; if there is a returning execution, it is the last active execution; and there are passive executions only if there is a returning execution.

[Note 1:

Passive executions allow other threads to reliably observe the results produced by the earlier returning execution.

— end note]

Synchronization: For any given once_flag: all active executions occur in a total order; completion of an active execution synchronizes with the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.

Throws: system_error when an exception is required ([thread.req.exception]), or any exception thrown by func.

[Example 1: // global flag, regular function void init(); std::once_flag flag; void f() { std::call_once(flag, init); } // function static flag, function object struct initializer { void operator()(); }; void g() { static std::once_flag flag2; std::call_once(flag2, initializer()); } // object flag, member function class information { std::once_flag verified; void verifier(); public: void verify() { std::call_once(verified, &information::verifier, *this); } }; — end example]

32.7 Condition variables [thread.condition]

32.7.1 General [thread.condition.general]

Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached.

Class condition_variable provides a condition variable that can only wait on an object of type unique_lock<mutex>, allowing the implementation to be more efficient.

Class condition_variable_any provides a general condition variable that can wait on objects of user-supplied lock types.

Condition variables permit concurrent invocation of the wait, wait_for, wait_until, notify_one and notify_all member functions.

The executions of notify_one and notify_all are atomic.

The executions of wait, wait_for, and wait_until are performed in three atomic parts:

1.	the release of the mutex and entry into the waiting state;
2.	the unblocking of the wait; and
3.	the reacquisition of the lock.

The implementation behaves as if all executions of notify_one, notify_all, and each part of the wait, wait_for, and wait_until executions are executed in a single unspecified total order consistent with the “happens before” order.

Condition variable construction and destruction need not be synchronized.

32.7.2 Header <condition_variable> synopsis [condition.variable.syn]

32.7.3 Non-member functions [thread.condition.nonmember]

🔗

void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

Preconditions: lk is locked by the calling thread and either

(1.1)
no other thread is waiting on cond, or
(1.2)
lk.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects: Transfers ownership of the lock associated with lk into internal storage and schedules cond to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

This notification is equivalent to: lk.unlock(); cond.notify_all();

Synchronization: The implied lk.unlock() call is sequenced after the destruction of all objects with thread storage duration associated with the current thread.

[Note 1:

The supplied lock is held until the thread exits, which might cause deadlock due to lock ordering issues.

— end note]

[Note 2:

It is the user's responsibility to ensure that waiting threads do not incorrectly assume that the thread has finished if they experience spurious wakeups.

This typically requires that the condition being waited for is satisfied while holding the lock on lk, and that this lock is not released and reacquired prior to calling notify_all_at_thread_exit.

— end note]

32.7.4 Class condition_variable [thread.condition.condvar]

🔗

namespace std { class condition_variable { public: condition_variable(); ~condition_variable(); condition_variable(const condition_variable&) = delete; condition_variable& operator=(const condition_variable&) = delete; void notify_one() noexcept; void notify_all() noexcept; void wait(unique_lock<mutex>& lock); template<class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred); template<class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time); template<class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template<class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time); template<class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class condition_variable is a standard-layout class ([class.prop]).

🔗

condition_variable();

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(3.1)
resource_unavailable_try_again — if some non-memory resource limitation prevents initialization.

🔗

~condition_variable();

Preconditions: There is no thread blocked on *this.

[Note 1:

That is, all threads have been notified; they can subsequently block on the lock specified in the wait.

This relaxes the usual rules, which would have required all wait calls to happen before destruction.

Only the notification to unblock the wait needs to happen before destruction.

Undefined behavior ensues if a thread waits on *this once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads of wait, wait_for, or wait_until that take a predicate.

— end note]

🔗

void notify_one() noexcept;

Effects: If any threads are blocked waiting for *this, unblocks one of those threads.

🔗

void notify_all() noexcept;

Effects: Unblocks all threads that are blocked waiting for *this.

🔗

void wait(unique_lock<mutex>& lock);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(7.1)
no other thread is waiting on this condition_variable object or
(7.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects:

(8.1)
Atomically calls lock.unlock() and blocks on *this.
(8.2)
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
(8.3)
The function will unblock when signaled by a call to notify_one() or a call to notify_all(), or spuriously.

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

Throws: Nothing.

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 2:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Predicate>
  void wait(unique_lock<mutex>& lock, Predicate pred);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(12.1)
no other thread is waiting on this condition_variable object or
(12.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects: Equivalent to: while (!pred()) wait(lock);

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

Throws: Any exception thrown by pred.

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 3:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Clock, class Duration>
  cv_status wait_until(unique_lock<mutex>& lock,
                       const chrono::time_point<Clock, Duration>& abs_time);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(17.1)
no other thread is waiting on this condition_variable object or
(17.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects:

(18.1)
Atomically calls lock.unlock() and blocks on *this.
(18.2)
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
(18.3)
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), expiration of the absolute timeout ([thread.req.timing]) specified by abs_time, or spuriously.
(18.4)
If the function exits via an exception, lock.lock() is called prior to exiting the function.

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

Returns: cv_status::timeout if the absolute timeout ([thread.req.timing]) specified by abs_time expired, otherwise cv_status::no_timeout.

Throws: Timeout-related exceptions ([thread.req.timing]).

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 4:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Rep, class Period>
  cv_status wait_for(unique_lock<mutex>& lock,
                     const chrono::duration<Rep, Period>& rel_time);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(23.1)
no other thread is waiting on this condition_variable object or
(23.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time);

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

Returns: cv_status::timeout if the relative timeout ([thread.req.timing]) specified by rel_time expired, otherwise cv_status::no_timeout.

Throws: Timeout-related exceptions ([thread.req.timing]).

Remarks: If the function fails to meet the postcondition, terminate is invoked ([except.terminate]).

[Note 5:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Clock, class Duration, class Predicate>
  bool wait_until(unique_lock<mutex>& lock,
                  const chrono::time_point<Clock, Duration>& abs_time,
                  Predicate pred);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(29.1)
no other thread is waiting on this condition_variable object or
(29.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects: Equivalent to: while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

[Note 6:

The returned value indicates whether the predicate evaluated to true regardless of whether the timeout was triggered.

— end note]

Throws: Timeout-related exceptions ([thread.req.timing]) or any exception thrown by pred.

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 7:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Rep, class Period, class Predicate>
  bool wait_for(unique_lock<mutex>& lock,
                const chrono::duration<Rep, Period>& rel_time,
                Predicate pred);

Preconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either

(35.1)
no other thread is waiting on this condition_variable object or
(35.2)
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.

Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

[Note 8:

There is no blocking if pred() is initially true, even if the timeout has already expired.

— end note]

Postconditions: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.

[Note 9:

The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered.

— end note]

Throws: Timeout-related exceptions ([thread.req.timing]) or any exception thrown by pred.

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 10:

This can happen if the re-locking of the mutex throws an exception.

— end note]

32.7.5 Class condition_variable_any [thread.condition.condvarany]

32.7.5.1 General [thread.condition.condvarany.general]

In [thread.condition.condvarany], template arguments for template parameters named Lock shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

[Note 1:

All of the standard mutex types meet this requirement.

If a type other than one of the standard mutex types or a unique_lock wrapper for a standard mutex type is used with condition_variable_any, any necessary synchronization is assumed to be in place with respect to the predicate associated with the condition_variable_any instance.

— end note]

🔗

namespace std { class condition_variable_any { public: condition_variable_any(); ~condition_variable_any(); condition_variable_any(const condition_variable_any&) = delete; condition_variable_any& operator=(const condition_variable_any&) = delete; void notify_one() noexcept; void notify_all() noexcept; // [thread.condvarany.wait], noninterruptible waits template<class Lock> void wait(Lock& lock); template<class Lock, class Predicate> void wait(Lock& lock, Predicate pred); template<class Lock, class Clock, class Duration> cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time); template<class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template<class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time); template<class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); // [thread.condvarany.intwait], interruptible waits template<class Lock, class Predicate> bool wait(Lock& lock, stop_token stoken, Predicate pred); template<class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, stop_token stoken, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template<class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, stop_token stoken, const chrono::duration<Rep, Period>& rel_time, Predicate pred); }; }

🔗

condition_variable_any();

Throws: bad_alloc or system_error when an exception is required ([thread.req.exception]).

Error conditions:

(3.1)
resource_unavailable_try_again — if some non-memory resource limitation prevents initialization.
(3.2)
operation_not_permitted — if the thread does not have the privilege to perform the operation.

🔗

~condition_variable_any();

Preconditions: There is no thread blocked on *this.

[Note 2:

That is, all threads have been notified; they can subsequently block on the lock specified in the wait.

This relaxes the usual rules, which would have required all wait calls to happen before destruction.

Only the notification to unblock the wait needs to happen before destruction.

— end note]

🔗

void notify_one() noexcept;

Effects: If any threads are blocked waiting for *this, unblocks one of those threads.

🔗

void notify_all() noexcept;

Effects: Unblocks all threads that are blocked waiting for *this.

32.7.5.2 Noninterruptible waits [thread.condvarany.wait]

🔗

template<class Lock>
  void wait(Lock& lock);

Effects:

(1.1)
Atomically calls lock.unlock() and blocks on *this.
(1.2)
When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
(1.3)
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), or spuriously.

Postconditions: lock is locked by the calling thread.

Throws: Nothing.

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 1:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Lock, class Predicate>
  void wait(Lock& lock, Predicate pred);

Effects: Equivalent to: while (!pred()) wait(lock);

🔗

template<class Lock, class Clock, class Duration>
  cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

Effects:

(6.1)
Atomically calls lock.unlock() and blocks on *this.
(6.2)
When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
(6.3)
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), expiration of the absolute timeout ([thread.req.timing]) specified by abs_time, or spuriously.
(6.4)
If the function exits via an exception, lock.lock() is called prior to exiting the function.

Postconditions: lock is locked by the calling thread.

Returns: cv_status::timeout if the absolute timeout ([thread.req.timing]) specified by abs_time expired, otherwise cv_status::no_timeout.

Throws: Timeout-related exceptions ([thread.req.timing]).

Remarks: If the function fails to meet the postcondition, terminate() is invoked ([except.terminate]).

[Note 2:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Lock, class Rep, class Period>
  cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time);

Postconditions: lock is locked by the calling thread.

Returns: cv_status::timeout if the relative timeout ([thread.req.timing]) specified by rel_time expired, otherwise cv_status::no_timeout.

Throws: Timeout-related exceptions ([thread.req.timing]).

Remarks: If the function fails to meet the postcondition, terminate is invoked ([except.terminate]).

[Note 3:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Lock, class Clock, class Duration, class Predicate>
  bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);

Effects: Equivalent to: while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;

[Note 4:

There is no blocking if pred() is initially true, or if the timeout has already expired.

— end note]

[Note 5:

The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered.

— end note]

🔗

template<class Lock, class Rep, class Period, class Predicate>
  bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

32.7.5.3 Interruptible waits [thread.condvarany.intwait]

The following wait functions will be notified when there is a stop request on the passed stop_token.

In that case the functions return immediately, returning false if the predicate evaluates to false.

🔗

template<class Lock, class Predicate>
  bool wait(Lock& lock, stop_token stoken, Predicate pred);

Effects: Registers for the duration of this call *this to get notified on a stop request on stoken during this call and then equivalent to: while (!stoken.stop_requested()) { if (pred()) return true; wait(lock); } return pred();

[Note 1:

The returned value indicates whether the predicate evaluated to true regardless of whether there was a stop request.

— end note]

Postconditions: lock is locked by the calling thread.

Throws: Any exception thrown by pred.

Remarks: If the function fails to meet the postcondition, terminate is called ([except.terminate]).

[Note 2:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Lock, class Clock, class Duration, class Predicate>
  bool wait_until(Lock& lock, stop_token stoken,
                  const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);

[Note 3:

There is no blocking if pred() is initially true, stoken.stop_requested() was already true or the timeout has already expired.

— end note]

[Note 4:

The returned value indicates whether the predicate evaluated to true regardless of whether the timeout was triggered or a stop request was made.

— end note]

Postconditions: lock is locked by the calling thread.

Throws: Timeout-related exceptions ([thread.req.timing]), or any exception thrown by pred.

Remarks: If the function fails to meet the postcondition, terminate is called ([except.terminate]).

[Note 5:

This can happen if the re-locking of the mutex throws an exception.

— end note]

🔗

template<class Lock, class Rep, class Period, class Predicate>
  bool wait_for(Lock& lock, stop_token stoken,
                const chrono::duration<Rep, Period>& rel_time, Predicate pred);

Effects: Equivalent to: return wait_until(lock, std::move(stoken), chrono::steady_clock::now() + rel_time, std::move(pred));

32.8 Semaphore [thread.sema]

32.8.1 General [thread.sema.general]

Semaphores are lightweight synchronization primitives used to constrain concurrent access to a shared resource.

They are widely used to implement other synchronization primitives and, whenever both are applicable, can be more efficient than condition variables.

A counting semaphore is a semaphore object that models a non-negative resource count.

A binary semaphore is a semaphore object that has only two states.

A binary semaphore should be more efficient than the default implementation of a counting semaphore with a unit resource count.

32.8.2 Header <semaphore> synopsis [semaphore.syn]

32.8.3 Class template counting_semaphore [thread.sema.cnt]

namespace std { template<ptrdiff_t least_max_value = implementation-defined> class counting_semaphore { public: static constexpr ptrdiff_t max() noexcept; constexpr explicit counting_semaphore(ptrdiff_t desired); ~counting_semaphore(); counting_semaphore(const counting_semaphore&) = delete; counting_semaphore& operator=(const counting_semaphore&) = delete; void release(ptrdiff_t update = 1); void acquire(); bool try_acquire() noexcept; template<class Rep, class Period> bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time); private: ptrdiff_t counter; // exposition only }; }

Class template counting_semaphore maintains an internal counter that is initialized when the semaphore is created.

The counter is decremented when a thread acquires the semaphore, and is incremented when a thread releases the semaphore.

If a thread tries to acquire the semaphore when the counter is zero, the thread will block until another thread increments the counter by releasing the semaphore.

least_max_value shall be non-negative; otherwise the program is ill-formed.

Concurrent invocations of the member functions of counting_semaphore, other than its destructor, do not introduce data races.

🔗

static constexpr ptrdiff_t max() noexcept;

Returns: The maximum value of counter.

This value is greater than or equal to least_max_value.

🔗

constexpr explicit counting_semaphore(ptrdiff_t desired);

Preconditions: desired >= 0 is true, and desired <= max() is true.

Effects: Initializes counter with desired.

Throws: Nothing.

🔗

void release(ptrdiff_t update = 1);

Preconditions: update >= 0 is true, and update <= max() - counter is true.

Effects: Atomically execute counter += update.

Then, unblocks any threads that are waiting for counter to be greater than zero.

Synchronization: Strongly happens before invocations of try_acquire that observe the result of the effects.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

🔗

bool try_acquire() noexcept;

Effects: Attempts to atomically decrement counter if it is positive, without blocking.

If counter is not decremented, there is no effect and try_acquire immediately returns.

An implementation may fail to decrement counter even if it is positive.

[Note 1:

This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomics]).

— end note]

An implementation should ensure that try_acquire does not consistently return false in the absence of contending semaphore operations.

Returns: true if counter was decremented, otherwise false.

🔗

void acquire();

Effects: Repeatedly performs the following steps, in order:

(15.1)
Evaluates try_acquire().

If the result is true, returns.
(15.2)
Blocks on *this until counter is greater than zero.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

🔗

template<class Rep, class Period>
  bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
template<class Clock, class Duration>
  bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);

Effects: Repeatedly performs the following steps, in order:

(18.1)
Evaluates try_acquire().

If the result is true, returns true.
(18.2)
Blocks on *this until counter is greater than zero or until the timeout expires.

If it is unblocked by the timeout expiring, returns false.

The timeout expires ([thread.req.timing]) when the current time is after abs_time (for try_acquire_until) or when at least rel_time has passed from the start of the function (for try_acquire_for).

Throws: Timeout-related exceptions ([thread.req.timing]), or system_error when a non-timeout-related exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

32.9 Coordination types [thread.coord]

32.9.1 General [thread.coord.general]

Subclause [thread.coord] describes various concepts related to thread coordination, and defines the coordination types latch and barrier.

These types facilitate concurrent computation performed by a number of threads.

32.9.2 Latches [thread.latch]

32.9.2.1 General [thread.latch.general]

A latch is a thread coordination mechanism that allows any number of threads to block until an expected number of threads arrive at the latch (via the count_down function).

The expected count is set when the latch is created.

An individual latch is a single-use object; once the expected count has been reached, the latch cannot be reused.

32.9.2.2 Header <latch> synopsis [latch.syn]

32.9.2.3 Class latch [thread.latch.class]

namespace std { class latch { public: static constexpr ptrdiff_t max() noexcept; constexpr explicit latch(ptrdiff_t expected); ~latch(); latch(const latch&) = delete; latch& operator=(const latch&) = delete; void count_down(ptrdiff_t update = 1); bool try_wait() const noexcept; void wait() const; void arrive_and_wait(ptrdiff_t update = 1); private: ptrdiff_t counter; // exposition only }; }

A latch maintains an internal counter that is initialized when the latch is created.

Threads can block on the latch object, waiting for counter to be decremented to zero.

Concurrent invocations of the member functions of latch, other than its destructor, do not introduce data races.

🔗

static constexpr ptrdiff_t max() noexcept;

Returns: The maximum value of counter that the implementation supports.

🔗

constexpr explicit latch(ptrdiff_t expected);

Preconditions: expected >= 0 is true and expected <= max() is true.

Effects: Initializes counter with expected.

Throws: Nothing.

🔗

void count_down(ptrdiff_t update = 1);

Preconditions: update >= 0 is true, and update <= counter is true.

Effects: Atomically decrements counter by update.

If counter is equal to zero, unblocks all threads blocked on *this.

Synchronization: Strongly happens before the returns from all calls that are unblocked.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

🔗

bool try_wait() const noexcept;

Returns: With very low probability false.

Otherwise counter == 0.

🔗

void wait() const;

Effects: If counter equals zero, returns immediately.

Otherwise, blocks on *this until a call to count_down that decrements counter to zero.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

🔗

void arrive_and_wait(ptrdiff_t update = 1);

Effects: Equivalent to: count_down(update); wait();

32.9.3 Barriers [thread.barrier]

32.9.3.1 General [thread.barrier.general]

A barrier is a thread coordination mechanism whose lifetime consists of a sequence of barrier phases, where each phase allows at most an expected number of threads to block until the expected number of threads arrive at the barrier.

[Note 1:

A barrier is useful for managing repeated tasks that are handled by multiple threads.

— end note]

32.9.3.2 Header <barrier> synopsis [barrier.syn]

32.9.3.3 Class template barrier [thread.barrier.class]

namespace std { template<class CompletionFunction = see below> class barrier { public: using arrival_token = see below; static constexpr ptrdiff_t max() noexcept; constexpr explicit barrier(ptrdiff_t expected, CompletionFunction f = CompletionFunction()); ~barrier(); barrier(const barrier&) = delete; barrier& operator=(const barrier&) = delete; arrival_token arrive(ptrdiff_t update = 1); void wait(arrival_token&& arrival) const; void arrive_and_wait(); void arrive_and_drop(); private: CompletionFunction completion; // exposition only }; }

Each barrier phase consists of the following steps:

(1.1)
The expected count is decremented by each call to arrive or arrive_and_drop.
(1.2)
Exactly once after the expected count reaches zero, a thread executes the completion step during its call to arrive, arrive_and_drop, or wait, except that it is implementation-defined whether the step executes if no thread calls wait.
(1.3)
When the completion step finishes, the expected count is reset to what was specified by the expected argument to the constructor, possibly adjusted by calls to arrive_and_drop, and the next phase starts.

Each phase defines a phase synchronization point.

Threads that arrive at the barrier during the phase can block on the phase synchronization point by calling wait, and will remain blocked until the phase completion step is run.

The phase completion step that is executed at the end of each phase has the following effects:

(3.1)
Invokes the completion function, equivalent to completion().
(3.2)
Unblocks all threads that are blocked on the phase synchronization point.

The end of the completion step strongly happens before the returns from all calls that were unblocked by the completion step.

For specializations that do not have the default value of the CompletionFunction template parameter, the behavior is undefined if any of the barrier object's member functions other than wait are called while the completion step is in progress.

Concurrent invocations of the member functions of barrier, other than its destructor, do not introduce data races.

The member functions arrive and arrive_and_drop execute atomically.

CompletionFunction shall meet the Cpp17MoveConstructible (Table 31) and Cpp17Destructible (Table 35) requirements.

is_nothrow_invocable_v<CompletionFunction&> shall be true.

The default value of the CompletionFunction template parameter is an unspecified type, such that, in addition to satisfying the requirements of CompletionFunction, it meets the Cpp17DefaultConstructible requirements (Table 30) and completion() has no effects.

barrier::arrival_token is an unspecified type, such that it meets the Cpp17MoveConstructible (Table 31), Cpp17MoveAssignable (Table 33), and Cpp17Destructible (Table 35) requirements.

🔗

static constexpr ptrdiff_t max() noexcept;

Returns: The maximum expected count that the implementation supports.

🔗

constexpr explicit barrier(ptrdiff_t expected,
                           CompletionFunction f = CompletionFunction());

Preconditions: expected >= 0 is true and expected <= max() is true.

Effects: Sets both the initial expected count for each barrier phase and the current expected count for the first phase to expected.

Initializes completion with std::move(f).

Starts the first phase.

[Note 1:

If expected is 0 this object can only be destroyed.

— end note]

Throws: Any exception thrown by CompletionFunction's move constructor.

🔗

arrival_token arrive(ptrdiff_t update = 1);

Preconditions: update > 0 is true, and update is less than or equal to the expected count for the current barrier phase.

Effects: Constructs an object of type arrival_token that is associated with the phase synchronization point for the current phase.

Then, decrements the expected count by update.

Synchronization: The call to arrive strongly happens before the start of the phase completion step for the current phase.

Returns: The constructed arrival_token object.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

[Note 2:

This call can cause the completion step for the current phase to start.

— end note]

🔗

void wait(arrival_token&& arrival) const;

Preconditions: arrival is associated with the phase synchronization point for the current phase or the immediately preceding phase of the same barrier object.

Effects: Blocks at the synchronization point associated with std::move(arrival) until the phase completion step of the synchronization point's phase is run.

[Note 3:

If arrival is associated with the synchronization point for a previous phase, the call returns immediately.

— end note]

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

🔗

void arrive_and_wait();

Effects: Equivalent to: wait(arrive()).

🔗

void arrive_and_drop();

Preconditions: The expected count for the current barrier phase is greater than zero.

Effects: Decrements the initial expected count for all subsequent phases by one.

Then decrements the expected count for the current phase by one.

Synchronization: The call to arrive_and_drop strongly happens before the start of the phase completion step for the current phase.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions: Any of the error conditions allowed for mutex types ([thread.mutex.requirements.mutex]).

[Note 4:

This call can cause the completion step for the current phase to start.

— end note]

32.10 Futures [futures]

32.10.1 Overview [futures.overview]

[futures] describes components that a C++ program can use to retrieve in one thread the result (value or exception) from a function that has run in the same thread or another thread.

[Note 1:

These components are not restricted to multi-threaded programs but can be useful in single-threaded programs as well.

— end note]

32.10.2 Header <future> synopsis [future.syn]

🔗

namespace std { enum class future_errc { broken_promise = implementation-defined, future_already_retrieved = implementation-defined, promise_already_satisfied = implementation-defined, no_state = implementation-defined }; enum class launch : unspecified { async = unspecified, deferred = unspecified, implementation-defined }; enum class future_status { ready, timeout, deferred }; // [futures.errors], error handling template<> struct is_error_code_enum<future_errc> : public true_type { }; error_code make_error_code(future_errc e) noexcept; error_condition make_error_condition(future_errc e) noexcept; const error_category& future_category() noexcept; // [futures.future.error], class future_error class future_error; // [futures.promise], class template promise template<class R> class promise; template<class R> class promise<R&>; template<> class promise<void>; template<class R> void swap(promise<R>& x, promise<R>& y) noexcept; template<class R, class Alloc> struct uses_allocator<promise<R>, Alloc>; // [futures.unique.future], class template future template<class R> class future; template<class R> class future<R&>; template<> class future<void>; // [futures.shared.future], class template shared_future template<class R> class shared_future; template<class R> class shared_future<R&>; template<> class shared_future<void>; // [futures.task], class template packaged_task template<class> class packaged_task; // not defined template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)>; template<class R, class... ArgTypes> void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&) noexcept; // [futures.async], function template async template<class F, class... Args> future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(F&& f, Args&&... args); template<class F, class... Args> future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(launch policy, F&& f, Args&&... args); }

The enum type launch is a bitmask type ([bitmask.types]) with elements launch::async and launch::deferred.

[Note 1:

Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by async() applicable to a corresponding subset of available launch policies.

Implementations can extend the behavior of the first overload of async() by adding their extensions to the launch policy under the “as if” rule.

— end note]

The enum values of future_errc are distinct and not zero.

32.10.3 Error handling [futures.errors]

🔗

const error_category& future_category() noexcept;

Returns: A reference to an object of a type derived from class error_category.

The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category.

The object's name virtual function returns a pointer to the string "future".

🔗

error_code make_error_code(future_errc e) noexcept;

Returns: error_code(static_cast<int>(e), future_category()).

🔗

error_condition make_error_condition(future_errc e) noexcept;

Returns: error_condition(static_cast<int>(e), future_category()).

32.10.4 Class future_error [futures.future.error]

🔗

namespace std { class future_error : public logic_error { public: explicit future_error(future_errc e); const error_code& code() const noexcept; const char* what() const noexcept; private: error_code ec_; // exposition only }; }

🔗

explicit future_error(future_errc e);

Effects: Initializes ec_ with make_error_code(e).

🔗

const error_code& code() const noexcept;

Returns: ec_.

🔗

const char* what() const noexcept;

Returns: An ntbs incorporating code().message().

32.10.5 Shared state [futures.state]

Many of the classes introduced in subclause [futures] use some state to communicate results.

This shared state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly void) value or an exception.

[Note 1:

Futures, promises, and tasks defined in this Clause reference such shared state.

— end note]

[Note 2:

The result can be any kind of object including a function to compute that result, as used by async when policy is launch::deferred.

— end note]

An asynchronous return object is an object that reads results from a shared state.

A waiting function of an asynchronous return object is one that potentially blocks to wait for the shared state to be made ready.

If a waiting function can return before the state is made ready because of a timeout ([thread.req.lockable]), then it is a timed waiting function, otherwise it is a non-timed waiting function.

An asynchronous provider is an object that provides a result to a shared state.

The result of a shared state is set by respective functions on the asynchronous provider.

[Example 1:

Promises and tasks are examples of asynchronous providers.

— end example]

The means of setting the result of a shared state is specified in the description of those classes and functions that create such a state object.

When an asynchronous return object or an asynchronous provider is said to release its shared state, it means:

(5.1)
if the return object or provider holds the last reference to its shared state, the shared state is destroyed; and
(5.2)
the return object or provider gives up its reference to its shared state; and
(5.3)
these actions will not block for the shared state to become ready, except that it may block if all of the following are true: the shared state was created by a call to std::async, the shared state is not yet ready, and this was the last reference to the shared state.

When an asynchronous provider is said to make its shared state ready, it means:

(6.1)
first, the provider marks its shared state as ready; and
(6.2)
second, the provider unblocks any execution agents waiting for its shared state to become ready.

When an asynchronous provider is said to abandon its shared state, it means:

(7.1)
first, if that state is not ready, the provider
- (7.1.1)
  stores an exception object of type future_error with an error condition of broken_promise within its shared state; and then
- (7.1.2)
  makes its shared state ready;
(7.2)
second, the provider releases its shared state.

A shared state is ready only if it holds a value or an exception ready for retrieval.

Waiting for a shared state to become ready may invoke code to compute the result on the waiting thread if so specified in the description of the class or function that creates the state object.

Calls to functions that successfully set the stored result of a shared state synchronize with calls to functions successfully detecting the ready state resulting from that setting.

The storage of the result (whether normal or exceptional) into the shared state synchronizes with the successful return from a call to a waiting function on the shared state.

Some functions (e.g., promise::set_value_at_thread_exit) delay making the shared state ready until the calling thread exits.

The destruction of each of that thread's objects with thread storage duration is sequenced before making that shared state ready.

Access to the result of the same shared state may conflict .

[Note 3:

This explicitly specifies that the result of the shared state is visible in the objects that reference this state in the sense of data race avoidance ([res.on.data.races]).

For example, concurrent accesses through references returned by shared_future::get() ([futures.shared.future]) must either use read-only operations or provide additional synchronization.

— end note]

32.10.6 Class template promise [futures.promise]

🔗

namespace std { template<class R> class promise { public: promise(); template<class Allocator> promise(allocator_arg_t, const Allocator& a); promise(promise&& rhs) noexcept; promise(const promise&) = delete; ~promise(); // assignment promise& operator=(promise&& rhs) noexcept; promise& operator=(const promise&) = delete; void swap(promise& other) noexcept; // retrieving the result future<R> get_future(); // setting the result void set_value(see below); void set_exception(exception_ptr p); // setting the result with deferred notification void set_value_at_thread_exit(see below); void set_exception_at_thread_exit(exception_ptr p); }; template<class R, class Alloc> struct uses_allocator<promise<R>, Alloc>; }

For the primary template, R shall be an object type that meets the Cpp17Destructible requirements.

The implementation provides the template promise and two specializations, promise<R&> and promise<void>.

These differ only in the argument type of the member functions set_value and set_value_at_thread_exit, as set out in their descriptions, below.

The set_value, set_exception, set_value_at_thread_exit, and set_exception_at_thread_exit member functions behave as though they acquire a single mutex associated with the promise object while updating the promise object.

🔗

template<class R, class Alloc>
  struct uses_allocator<promise<R>, Alloc>
    : true_type { };

Preconditions: Alloc meets the Cpp17Allocator requirements ([allocator.requirements.general]).

🔗

promise();
template<class Allocator>
  promise(allocator_arg_t, const Allocator& a);

Effects: Creates a shared state.

The second constructor uses the allocator a to allocate memory for the shared state.

🔗

promise(promise&& rhs) noexcept;

Effects: Transfers ownership of the shared state of rhs (if any) to the newly-constructed object.

Postconditions: rhs has no shared state.

🔗

~promise();

Effects: Abandons any shared state ([futures.state]).

🔗

promise& operator=(promise&& rhs) noexcept;

Effects: Abandons any shared state ([futures.state]) and then as if promise(std::move(rhs)).swap(*this).

Returns: *this.

🔗

void swap(promise& other) noexcept;

Effects: Exchanges the shared state of *this and other.

Postconditions: *this has the shared state (if any) that other had prior to the call to swap.

other has the shared state (if any) that *this had prior to the call to swap.

🔗

future<R> get_future();

Synchronization: Calls to this function do not introduce data races ([intro.multithread]) with calls to set_value, set_exception, set_value_at_thread_exit, or set_exception_at_thread_exit.

[Note 1:

Such calls need not synchronize with each other.

— end note]

Returns: A future<R> object with the same shared state as *this.

Throws: future_error if *this has no shared state or if get_future has already been called on a promise with the same shared state as *this.

Error conditions:

(16.1)
future_already_retrieved if get_future has already been called on a promise with the same shared state as *this.
(16.2)
no_state if *this has no shared state.

🔗

void promise::set_value(const R& r);
void promise::set_value(R&& r);
void promise<R&>::set_value(R& r);
void promise<void>::set_value();

Effects: Atomically stores the value r in the shared state and makes that state ready ([futures.state]).

Throws:

(18.1)
future_error if its shared state already has a stored value or exception, or
(18.2)
for the first version, any exception thrown by the constructor selected to copy an object of R, or
(18.3)
for the second version, any exception thrown by the constructor selected to move an object of R.

Error conditions:

(19.1)
promise_already_satisfied if its shared state already has a stored value or exception.
(19.2)
no_state if *this has no shared state.

🔗

void set_exception(exception_ptr p);

Preconditions: p is not null.

Effects: Atomically stores the exception pointer p in the shared state and makes that state ready ([futures.state]).

Throws: future_error if its shared state already has a stored value or exception.

Error conditions:

(23.1)
promise_already_satisfied if its shared state already has a stored value or exception.
(23.2)
no_state if *this has no shared state.

🔗

void promise::set_value_at_thread_exit(const R& r);
void promise::set_value_at_thread_exit(R&& r);
void promise<R&>::set_value_at_thread_exit(R& r);
void promise<void>::set_value_at_thread_exit();

Effects: Stores the value r in the shared state without making that state ready immediately.

Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

Throws:

(25.1)
future_error if its shared state already has a stored value or exception, or
(25.2)
for the first version, any exception thrown by the constructor selected to copy an object of R, or
(25.3)
for the second version, any exception thrown by the constructor selected to move an object of R.

Error conditions:

(26.1)
promise_already_satisfied if its shared state already has a stored value or exception.
(26.2)
no_state if *this has no shared state.

🔗

void set_exception_at_thread_exit(exception_ptr p);

Preconditions: p is not null.

Effects: Stores the exception pointer p in the shared state without making that state ready immediately.

Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

Throws: future_error if an error condition occurs.

Error conditions:

(30.1)
promise_already_satisfied if its shared state already has a stored value or exception.
(30.2)
no_state if *this has no shared state.

🔗

template<class R>
  void swap(promise<R>& x, promise<R>& y) noexcept;

Effects: As if by x.swap(y).

32.10.7 Class template future [futures.unique.future]

The class template future defines a type for asynchronous return objects which do not share their shared state with other asynchronous return objects.

A default-constructed future object has no shared state.

A future object with shared state can be created by functions on asynchronous providers or by the move constructor and shares its shared state with the original asynchronous provider.

The result (value or exception) of a future object can be set by calling a respective function on an object that shares the same shared state.

[Note 1:

Member functions of future do not synchronize with themselves or with member functions of shared_future.

— end note]

The effect of calling any member function other than the destructor, the move assignment operator, share, or valid on a future object for which valid() == false is undefined.

[Note 2:

It is valid to move from a future object for which valid() == false.

— end note]

Recommended practice: Implementations should detect this case and throw an object of type future_error with an error condition of future_errc::no_state.

🔗

namespace std { template<class R> class future { public: future() noexcept; future(future&&) noexcept; future(const future&) = delete; ~future(); future& operator=(const future&) = delete; future& operator=(future&&) noexcept; shared_future<R> share() noexcept; // retrieving the value see below get(); // functions to check state bool valid() const noexcept; void wait() const; template<class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const; template<class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const; }; }

For the primary template, R shall be an object type that meets the Cpp17Destructible requirements.

The implementation provides the template future and two specializations, future<R&> and future<void>.

These differ only in the return type and return value of the member function get, as set out in its description, below.

🔗

future() noexcept;

Effects: The object does not refer to a shared state.

Postconditions: valid() == false.

🔗

future(future&& rhs) noexcept;

Effects: Move constructs a future object that refers to the shared state that was originally referred to by rhs (if any).

Postconditions:

(9.1)
valid() returns the same value as rhs.valid() prior to the constructor invocation.
(9.2)
rhs.valid() == false.

🔗

~future();

Effects:

(10.1)
Releases any shared state ([futures.state]);
(10.2)
destroys *this.

🔗

future& operator=(future&& rhs) noexcept;

Effects: If addressof(rhs) == this is true, there are no effects.

Otherwise:

(11.1)
Releases any shared state ([futures.state]).
(11.2)
move assigns the contents of rhs to *this.

Postconditions:

(12.1)
valid() returns the same value as rhs.valid() prior to the assignment.
(12.2)
If addressof(rhs) == this is false, rhs.valid() == false.

Postconditions: valid() == false.

Returns: shared_future<R>(std::move(*this)).

🔗

R future::get();
R& future<R&>::get();
void future<void>::get();

[Note 3:

As described above, the template and its two required specializations differ only in the return type and return value of the member function get.

— end note]

Effects:

(16.1)
wait()s until the shared state is ready, then retrieves the value stored in the shared state;
(16.2)
releases any shared state ([futures.state]).

Postconditions: valid() == false.

Returns:

(18.1)
future::get() returns the value v stored in the object's shared state as std::move(v).
(18.2)
future<R&>::get() returns the reference stored as value in the object's shared state.
(18.3)
future<void>::get() returns nothing.

Throws: The stored exception, if an exception was stored in the shared state.

🔗

bool valid() const noexcept;

Returns: true only if *this refers to a shared state.

🔗

void wait() const;

Effects: Blocks until the shared state is ready.

🔗

template<class Rep, class Period>
  future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

Effects: None if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the relative timeout ([thread.req.timing]) specified by rel_time has expired.

Returns:

(23.1)
future_status::deferred if the shared state contains a deferred function.
(23.2)
future_status::ready if the shared state is ready.
(23.3)
future_status::timeout if the function is returning because the relative timeout ([thread.req.timing]) specified by rel_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).

🔗

template<class Clock, class Duration>
  future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

Effects: None if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout ([thread.req.timing]) specified by abs_time has expired.

Returns:

(26.1)
future_status::deferred if the shared state contains a deferred function.
(26.2)
future_status::ready if the shared state is ready.
(26.3)
future_status::timeout if the function is returning because the absolute timeout ([thread.req.timing]) specified by abs_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).

32.10.8 Class template shared_future [futures.shared.future]

The class template shared_future defines a type for asynchronous return objects which may share their shared state with other asynchronous return objects.

A default-constructed shared_future object has no shared state.

A shared_future object with shared state can be created by conversion from a future object and shares its shared state with the original asynchronous provider of the shared state.

The result (value or exception) of a shared_future object can be set by calling a respective function on an object that shares the same shared state.

[Note 1:

Member functions of shared_future do not synchronize with themselves, but they synchronize with the shared state.

— end note]

The effect of calling any member function other than the destructor, the move assignment operator, the copy assignment operator, or valid() on a shared_future object for which valid() == false is undefined.

[Note 2:

It is valid to copy or move from a shared_future object for which valid() is false.

— end note]

Recommended practice: Implementations should detect this case and throw an object of type future_error with an error condition of future_errc::no_state.

🔗

namespace std { template<class R> class shared_future { public: shared_future() noexcept; shared_future(const shared_future& rhs) noexcept; shared_future(future<R>&&) noexcept; shared_future(shared_future&& rhs) noexcept; ~shared_future(); shared_future& operator=(const shared_future& rhs) noexcept; shared_future& operator=(shared_future&& rhs) noexcept; // retrieving the value see below get() const; // functions to check state bool valid() const noexcept; void wait() const; template<class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const; template<class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const; }; }

For the primary template, R shall be an object type that meets the Cpp17Destructible requirements.

The implementation provides the template shared_future and two specializations, shared_future<R&> and shared_future<void>.

These differ only in the return type and return value of the member function get, as set out in its description, below.

🔗

shared_future() noexcept;

Effects: The object does not refer to a shared state.

Postconditions: valid() == false.

🔗

shared_future(const shared_future& rhs) noexcept;

Effects: The object refers to the same shared state as rhs (if any).

Postconditions: valid() returns the same value as rhs.valid().

🔗

shared_future(future<R>&& rhs) noexcept;
shared_future(shared_future&& rhs) noexcept;

Effects: Move constructs a shared_future object that refers to the shared state that was originally referred to by rhs (if any).

Postconditions:

(11.1)
valid() returns the same value as rhs.valid() returned prior to the constructor invocation.
(11.2)
rhs.valid() == false.

🔗

~shared_future();

Effects:

(12.1)
Releases any shared state ([futures.state]);
(12.2)
destroys *this.

🔗

shared_future& operator=(shared_future&& rhs) noexcept;

Effects: If addressof(rhs) == this is true, there are no effects.

Otherwise:

(13.1)
Releases any shared state ([futures.state]);
(13.2)
move assigns the contents of rhs to *this.

Postconditions:

(14.1)
valid() returns the same value as rhs.valid() returned prior to the assignment.
(14.2)
If addressof(rhs) == this is false, rhs.valid() == false.

🔗

shared_future& operator=(const shared_future& rhs) noexcept;

Effects: If addressof(rhs) == this is true, there are no effects.

Otherwise:

(15.1)
Releases any shared state ([futures.state]);
(15.2)
assigns the contents of rhs to *this.
[Note 3:
As a result, *this refers to the same shared state as rhs (if any).
— end note]

Postconditions: valid() == rhs.valid().

🔗

const R& shared_future::get() const;
R& shared_future<R&>::get() const;
void shared_future<void>::get() const;

[Note 4:

As described above, the template and its two required specializations differ only in the return type and return value of the member function get.

— end note]

[Note 5:

Access to a value object stored in the shared state is unsynchronized, so operations on R might introduce a data race ([intro.multithread]).

— end note]

Effects: wait()s until the shared state is ready, then retrieves the value stored in the shared state.

Returns:

(20.1)
shared_future::get() returns a const reference to the value stored in the object's shared state.

[Note 6:
Access through that reference after the shared state has been destroyed produces undefined behavior; this can be avoided by not storing the reference in any storage with a greater lifetime than the shared_future object that returned the reference.
— end note]
(20.2)
shared_future<R&>::get() returns the reference stored as value in the object's shared state.
(20.3)
shared_future<void>::get() returns nothing.

Throws: The stored exception, if an exception was stored in the shared state.

🔗

bool valid() const noexcept;

Returns: true only if *this refers to a shared state.

🔗

void wait() const;

Effects: Blocks until the shared state is ready.

🔗

template<class Rep, class Period>
  future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

Returns:

(25.1)
future_status::deferred if the shared state contains a deferred function.
(25.2)
future_status::ready if the shared state is ready.
(25.3)
future_status::timeout if the function is returning because the relative timeout ([thread.req.timing]) specified by rel_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).

🔗

template<class Clock, class Duration>
  future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

Returns:

(28.1)
future_status::deferred if the shared state contains a deferred function.
(28.2)
future_status::ready if the shared state is ready.
(28.3)
future_status::timeout if the function is returning because the absolute timeout ([thread.req.timing]) specified by abs_time has expired.

Throws: timeout-related exceptions ([thread.req.timing]).

32.10.9 Function template async [futures.async]

The function template async provides a mechanism to launch a function potentially in a new thread and provides the result of the function in a future object with which it shares a shared state.

🔗

template<class F, class... Args>
  future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
    async(F&& f, Args&&... args);
template<class F, class... Args>
  future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
    async(launch policy, F&& f, Args&&... args);

Mandates: The following are all true:

(2.1)
is_constructible_v<decay_t<F>, F>,
(2.2)
(is_constructible_v<decay_t<Args>, Args> && ...), and
(2.3)
is_invocable_v<decay_t<F>, decay_t<Args>...>.

Effects: The first function behaves the same as a call to the second function with a policy argument of launch::async | launch::deferred and the same arguments for F and Args.

The second function creates a shared state that is associated with the returned future object.

The further behavior of the second function depends on the policy argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):

(3.1)
If launch::async is set in policy, calls invoke(auto(std::forward<F>(f)), auto(std::forward<Args>(args))...) ([func.invoke], [thread.thread.constr]) as if in a new thread of execution represented by a thread object with the values produced by auto being materialized ([conv.rval]) in the thread that called async.

Any return value is stored as the result in the shared state.

Any exception propagated from the execution of invoke(auto(std::forward<F>(f)), auto(std::forward<Args>(args))...) is stored as the exceptional result in the shared state.

The thread object is stored in the shared state and affects the behavior of any asynchronous return objects that reference that state.
(3.2)
If launch::deferred is set in policy, stores auto(std::forward<F>(f)) and auto(std::forward<Args>(args))... in the shared state.

These copies of f and args constitute a deferred function.

Invocation of the deferred function evaluates invoke(std::move(g), std::move(xyz)) where g is the stored value of auto(std::forward<F>(f)) and xyz is the stored copy of auto(std::forward<Args>(args))....

Any return value is stored as the result in the shared state.

Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state.

The shared state is not made ready until the function has completed.

The first call to a non-timed waiting function ([futures.state]) on an asynchronous return object referring to this shared state invokes the deferred function in the thread that called the waiting function.

Once evaluation of invoke(std::move(g), std::move(xyz)) begins, the function is no longer considered deferred.
Recommended practice: If this policy is specified together with other policies, such as when using a policy value of launch::async | launch::deferred, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively exploited.
(3.3)
If no value is set in the launch policy, or a value is set that is neither specified in this document nor by the implementation, the behavior is undefined.

Synchronization: The invocation of async synchronizes with the invocation of f.

The completion of the function f is sequenced before the shared state is made ready.

[Note 1:

These apply regardless of the provided policy argument, and even if the corresponding future object is moved to another thread.

However, it is possible for f not to be called at all, in which case its completion never happens.

— end note]

If the implementation chooses the launch::async policy,

(4.1)
a call to a waiting function on an asynchronous return object that shares the shared state created by this async call shall block until the associated thread has completed, as if joined, or else time out ([thread.thread.member]);
(4.2)
the associated thread completion synchronizes with the return from the first function that successfully detects the ready status of the shared state or with the return from the last function that releases the shared state, whichever happens first.

Returns: An object of type future<invoke_result_t<decay_t<F>, decay_t<Args>...>> that refers to the shared state created by this call to async.

[Note 2:

If a future obtained from async is moved outside the local scope, the future's destructor can block for the shared state to become ready.

— end note]

Throws: system_error if policy == launch::async and the implementation is unable to start a new thread, or std::bad_alloc if memory for the internal data structures cannot be allocated.

Error conditions:

(7.1)
resource_unavailable_try_again — if policy == launch::async and the system is unable to start a new thread.

[Example 1: int work1(int value); int work2(int value); int work(int value) { auto handle = std::async([=]{ return work2(value); }); int tmp = work1(value); return tmp + handle.get(); // #1 }

[Note 3:

Line #1 might not result in concurrency because the async call uses the default policy, which might use launch::deferred, in which case the lambda might not be invoked until the get() call; in that case, work1 and work2 are called on the same thread and there is no concurrency.

— end note]

— end example]

32.10.10 Class template packaged_task [futures.task]

32.10.10.1 General [futures.task.general]

The class template packaged_task defines a type for wrapping a function or callable object so that the return value of the function or callable object is stored in a future when it is invoked.

When the packaged_task object is invoked, its stored task is invoked and the result (whether normal or exceptional) stored in the shared state.

Any futures that share the shared state will then be able to access the stored result.

🔗

namespace std { template<class> class packaged_task; // not defined template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: // construction and destruction packaged_task() noexcept; template<class F> explicit packaged_task(F&& f); ~packaged_task(); // no copy packaged_task(const packaged_task&) = delete; packaged_task& operator=(const packaged_task&) = delete; // move support packaged_task(packaged_task&& rhs) noexcept; packaged_task& operator=(packaged_task&& rhs) noexcept; void swap(packaged_task& other) noexcept; bool valid() const noexcept; // result retrieval future<R> get_future(); // execution void operator()(ArgTypes... ); void make_ready_at_thread_exit(ArgTypes...); void reset(); }; template<class R, class... ArgTypes> packaged_task(R (*)(ArgTypes...)) -> packaged_task<R(ArgTypes...)>; template<class F> packaged_task(F) -> packaged_task<see below>; }

32.10.10.2 Member functions [futures.task.members]

🔗

packaged_task() noexcept;

Effects: The object has no shared state and no stored task.

🔗

template<class F>
  explicit packaged_task(F&& f);

Constraints: remove_cvref_t<F> is not the same type as packaged_task<R(ArgTypes...)>.

Mandates: is_invocable_r_v<R, decay_t<F>&, ArgTypes...> is true.

Effects: Constructs a new packaged_task object with a stored task of type decay_t<F> and a shared state.

Initializes the object's stored task with std::forward<F>(f).

Throws: Any exceptions thrown by the copy or move constructor of f, or bad_alloc if memory for the internal data structures cannot be allocated.

🔗

template<class F> packaged_task(F) -> packaged_task<see below>;

Constraints: &F::operator() is well-formed when treated as an unevaluated operand ([expr.context]) and either

(6.1)
F::operator() is a non-static member function and decltype(&F::operator()) is either of the form R(G::*)(A...) cv & $_{o p t}$ noexcept $_{o p t}$ or of the form R(*)(G, A...) noexcept $_{o p t}$ for a type G, or
(6.2)
F::operator() is a static member function and decltype(&F::operator()) is of the form R(*)(A...) noexcept $_{o p t}$ .

Remarks: The deduced type is packaged_task<R(A...)>.

🔗

packaged_task(packaged_task&& rhs) noexcept;

Effects: Transfers ownership of rhs's shared state to *this, leaving rhs with no shared state.

Moves the stored task from rhs to *this.

Postconditions: rhs has no shared state.

🔗

packaged_task& operator=(packaged_task&& rhs) noexcept;

Effects:

(10.1)
Releases any shared state ([futures.state]);
(10.2)
calls packaged_task(std::move(rhs)).swap(*this).

🔗

~packaged_task();

Effects: Abandons any shared state ([futures.state]).

🔗

void swap(packaged_task& other) noexcept;

Effects: Exchanges the shared states and stored tasks of *this and other.

Postconditions: *this has the same shared state and stored task (if any) as other prior to the call to swap.

other has the same shared state and stored task (if any) as *this prior to the call to swap.

🔗

bool valid() const noexcept;

Returns: true only if *this has a shared state.

🔗

future<R> get_future();

Synchronization: Calls to this function do not introduce data races ([intro.multithread]) with calls to operator() or make_ready_at_thread_exit.

[Note 1:

Such calls need not synchronize with each other.

— end note]

Returns: A future object that shares the same shared state as *this.

Throws: A future_error object if an error occurs.

Error conditions:

(18.1)
future_already_retrieved if get_future has already been called on a packaged_task object with the same shared state as *this.
(18.2)
no_state if *this has no shared state.

🔗

void operator()(ArgTypes... args);

Effects: As if by INVOKE<R>(f, t

_{1}

, t

_{2}

, …, t

_{N}

) ([func.require]), where f is the stored task of *this and t

_{1}

, t

_{2}

, …, t

_{N}

are the values in args....

If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored.

The shared state of *this is made ready, and any threads blocked in a function waiting for the shared state of *this to become ready are unblocked.

Throws: A future_error exception object if there is no shared state or the stored task has already been invoked.

Error conditions:

(21.1)
promise_already_satisfied if the stored task has already been invoked.
(21.2)
no_state if *this has no shared state.

🔗

void make_ready_at_thread_exit(ArgTypes... args);

Effects: As if by INVOKE<R>(f, t

_{1}

, t

_{2}

, …, t

_{N}

) ([func.require]), where f is the stored task and t

_{1}

, t

_{2}

, …, t

_{N}

are the values in args....

If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored.

In either case, this is done without making that state ready ([futures.state]) immediately.

Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

Throws: future_error if an error condition occurs.

Error conditions:

(24.1)
promise_already_satisfied if the stored task has already been invoked.
(24.2)
no_state if *this has no shared state.

🔗

void reset();

Effects: As if *this = packaged_task(std::move(f)), where f is the task stored in *this.

[Note 2:

This constructs a new shared state for *this.

The old state is abandoned ([futures.state]).

— end note]

Throws:

(26.1)
bad_alloc if memory for the new shared state cannot be allocated.
(26.2)
Any exception thrown by the move constructor of the task stored in the shared state.
(26.3)
future_error with an error condition of no_state if *this has no shared state.

32.10.10.3 Globals [futures.task.nonmembers]

🔗

template<class R, class... ArgTypes>
  void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;

Effects: As if by x.swap(y).

32.11 Safe reclamation [saferecl]

32.11.1 General [saferecl.general]

Subclause [saferecl] contains safe-reclamation techniques, which are most frequently used to straightforwardly resolve access-deletion races.

32.11.2 Read-copy update (RCU) [saferecl.rcu]

32.11.2.1 General [saferecl.rcu.general]

RCU is a synchronization mechanism that can be used for linked data structures that are frequently read, but seldom updated.

RCU does not provide mutual exclusion, but instead allows the user to schedule specified actions such as deletion at some later time.

A class type T is rcu-protectable if it has exactly one base class of type rcu_obj_base<T, D> for some D, and that base is public and non-virtual, and it has no base classes of type rcu_obj_base<X, Y> for any other combination X, Y.

An object is rcu-protectable if it is of rcu-protectable type.

An invocation of unlock U on an rcu_domain dom corresponds to an invocation of lock L on dom if L is sequenced before U and either

(3.1)
no other invocation of lock on dom is sequenced after L and before U, or
(3.2)
every invocation of unlock U2 on dom such that L is sequenced before U2 and U2 is sequenced before U corresponds to an invocation of lock L2 on dom such that L is sequenced before L2 and L2 is sequenced before U2.

[Note 1:

This pairs nested locks and unlocks on a given domain in each thread.

— end note]

A region of RCU protection on a domain dom starts with a lock L on dom and ends with its corresponding unlock U.

Given a region of RCU protection R on a domain dom and given an evaluation E that scheduled another evaluation F in dom, if E does not strongly happen before the start of R, the end of R strongly happens before evaluating F.

The evaluation of a scheduled evaluation is potentially concurrent with any other scheduled evaluation.

Each scheduled evaluation is evaluated at most once.

32.11.2.2 Header <rcu> synopsis [rcu.syn]

🔗

namespace std { // [saferecl.rcu.base], class template rcu_obj_base template<class T, class D = default_delete<T>> class rcu_obj_base; // [saferecl.rcu.domain], class rcu_domain class rcu_domain; // [saferecl.rcu.domain.func], non-member functions rcu_domain& rcu_default_domain() noexcept; void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept; void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept; template<class T, class D = default_delete<T>> void rcu_retire(T* p, D d = D(), rcu_domain& dom = rcu_default_domain()); }

32.11.2.3 Class template rcu_obj_base [saferecl.rcu.base]

Objects of type T to be protected by RCU inherit from a specialization rcu_obj_base<T, D> for some D.

namespace std { template<class T, class D = default_delete<T>> class rcu_obj_base { public: void retire(D d = D(), rcu_domain& dom = rcu_default_domain()) noexcept; protected: rcu_obj_base() = default; rcu_obj_base(const rcu_obj_base&) = default; rcu_obj_base(rcu_obj_base&&) = default; rcu_obj_base& operator=(const rcu_obj_base&) = default; rcu_obj_base& operator=(rcu_obj_base&&) = default; ~rcu_obj_base() = default; private: D deleter; // exposition only }; }

The behavior of a program that adds specializations for rcu_obj_base is undefined.

T may be an incomplete type.

It shall be complete before any member of the resulting specialization of rcu_obj_base is referenced.

D shall be a function object type ([function.objects]) for which, given a value d of type D and a value ptr of type T*, the expression d(ptr) is valid.

D shall meet the requirements for Cpp17DefaultConstructible and Cpp17MoveAssignable.

If D is trivially copyable, all specializations of rcu_obj_base<T, D> are trivially copyable.

🔗

void retire(D d = D(), rcu_domain& dom = rcu_default_domain()) noexcept;

Mandates: T is an rcu-protectable type.

Preconditions: *this is a base class subobject of an object x of type T.

The member function rcu_obj_base<T, D>::retire was not invoked on x before.

The assignment to deleter does not exit via an exception.

Effects: Evaluates deleter = std::move(d) and schedules the evaluation of the expression deleter(
addressof(x)) in the domain dom; the behavior is undefined if that evaluation exits via an exception.

May invoke scheduled evaluations in dom.

[Note 1:

If such evaluations acquire resources held across any invocation of retire on dom, deadlock can occur.

— end note]

32.11.2.4 Class rcu_domain [saferecl.rcu.domain]

32.11.2.4.1 General [saferecl.rcu.domain.general]

namespace std { class rcu_domain { public: rcu_domain(const rcu_domain&) = delete; rcu_domain& operator=(const rcu_domain&) = delete; void lock() noexcept; bool try_lock() noexcept; void unlock() noexcept; }; }

This class meets the requirements of Cpp17Lockable ([thread.req.lockable.req]) and provides regions of RCU protection.

[Example 1: std::scoped_lock<rcu_domain> rlock(rcu_default_domain()); — end example]

The functions lock and unlock establish (possibly nested) regions of RCU protection.

32.11.2.4.2 Member functions [saferecl.rcu.domain.members]

🔗

void lock() noexcept;

Effects: Opens a region of RCU protection.

Remarks: Calls to lock do not introduce a data race ([intro.races]) involving *this.

🔗

bool try_lock() noexcept;

Effects: Equivalent to lock().

Returns: true.

🔗

void unlock() noexcept;

Preconditions: A call to lock that opened an unclosed region of RCU protection is sequenced before the call to unlock.

Effects: Closes the unclosed region of RCU protection that was most recently opened.

May invoke scheduled evaluations in *this.

[Note 1:

If such evaluations acquire resources held across any invocation of unlock on *this, deadlock can occur.

— end note]

Remarks: Calls to unlock do not introduce a data race involving *this.

[Note 2:

Evaluation of scheduled evaluations can still cause a data race.

— end note]

32.11.2.4.3 Non-member functions [saferecl.rcu.domain.func]

🔗

rcu_domain& rcu_default_domain() noexcept;

Returns: A reference to a static-duration object of type rcu_domain.

A reference to the same object is returned every time this function is called.

🔗

void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept;

Effects: If the call to rcu_synchronize does not strongly happen before the lock opening an RCU protection region R on dom, blocks until the unlock closing R happens.

Synchronization: The unlock closing R strongly happens before the return from rcu_synchronize.

🔗

void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept;

Effects: May evaluate any scheduled evaluations in dom.

For any evaluation that happens before the call to rcu_barrier and that schedules an evaluation E in dom, blocks until E has been evaluated.

Synchronization: The evaluation of any such E strongly happens before the return from rcu_barrier.

[Note 1:

A call to rcu_barrier does not imply a call to rcu_synchronize and vice versa.

— end note]

🔗

template<class T, class D = default_delete<T>>
void rcu_retire(T* p, D d = D(), rcu_domain& dom = rcu_default_domain());

Mandates: is_move_constructible_v<D> is true and the expression d(p) is well-formed.

Preconditions: D meets the Cpp17MoveConstructible and Cpp17Destructible requirements.

Effects: May allocate memory.

It is unspecified whether the memory allocation is performed by invoking operator new.

Initializes an object d1 of type D from std::move(d).

Schedules the evaluation of d1(p) in the domain dom; the behavior is undefined if that evaluation exits via an exception.

May invoke scheduled evaluations in dom.

[Note 2:

If rcu_retire exits via an exception, no evaluation is scheduled.

— end note]

Throws: bad_alloc or any exception thrown by the initialization of d1.

[Note 3:

If scheduled evaluations acquire resources held across any invocation of rcu_retire on dom, deadlock can occur.

— end note]

32.11.3 Hazard pointers [saferecl.hp]

32.11.3.1 General [saferecl.hp.general]

A hazard pointer is a single-writer multi-reader pointer that can be owned by at most one thread at any time.

Only the owner of the hazard pointer can set its value, while any number of threads may read its value.

The owner thread sets the value of a hazard pointer to point to an object in order to indicate to concurrent threads—which may delete such an object—that the object is not yet safe to delete.

A class type T is hazard-protectable if it has exactly one base class of type hazard_pointer_obj_base<T, D> for some D, that base is public and non-virtual, and it has no base classes of type hazard_pointer_obj_base<T2, D2> for any other combination T2, D2.

An object is hazard-protectable if it is of hazard-protectable type.

The time span between creation and destruction of a hazard pointer h is partitioned into a series of protection epochs; in each protection epoch, h either is associated with a hazard-protectable object, or is unassociated.

Upon creation, a hazard pointer is unassociated.

Changing the association (possibly to the same object) initiates a new protection epoch and ends the preceding one.

An object x of hazard-protectable type T is retired with a deleter of type D when the member function hazard_pointer_obj_base<T, D>::retire is invoked on x.

Any given object x shall be retired at most once.

A retired object x is reclaimed by invoking its deleter with a pointer to x; the behavior is undefined if that invocation exits via an exception.

A hazard-protectable object x is possibly-reclaimable with respect to an evaluation A if

(6.1)
x is not reclaimed; and
(6.2)
x is retired in an evaluation R and A does not happen before R; and
(6.3)
for all hazard pointers h and for every protection epoch E of h during which h is associated with x:
- (6.3.1)
  if the beginning of E happens before R, the end of E strongly happens before A; and
- (6.3.2)
  if E began by an evaluation of try_protect with argument src, label its atomic load operation L.
  If there exists an atomic modification B on src such that L observes a modification that is modification-ordered before B, and B happens before x is retired, the end of E strongly happens before A.
  
  [Note 1:
  In typical use, a store to src sequenced before retiring x will be such an atomic operation B.
  — end note]
[Note 2:
The latter two conditions convey the informal notion that a protection epoch that began before retiring x, as implied either by the happens-before relation or the coherence order of some source, delays the reclamation of x.
— end note]

The number of possibly-reclaimable objects has an unspecified bound.

[Note 3:

The bound can be a function of the number of hazard pointers, the number of threads that retire objects, and the number of threads that use hazard pointers.

— end note]

[Example 1:

The following example shows how hazard pointers allow updates to be carried out in the presence of concurrent readers.

The object of type hazard_pointer in print_name protects the object *ptr from being reclaimed by ptr->retire until the end of the protection epoch.

struct Name : public hazard_pointer_obj_base<Name> { /* details */ }; atomic<Name*> name; // called often and in parallel! void print_name() { hazard_pointer h = make_hazard_pointer(); Name* ptr = h.protect(name); // Protection epoch starts // ... safe to access *ptr } // Protection epoch ends. // called rarely, but possibly concurrently with print_name void update_name(Name* new_name) { Name* ptr = name.exchange(new_name); ptr->retire(); } — end example]

32.11.3.2 Header <hazard_pointer> synopsis [hazard.pointer.syn]

32.11.3.3 Class template hazard_pointer_obj_base [saferecl.hp.base]

namespace std { template<class T, class D = default_delete<T>> class hazard_pointer_obj_base { public: void retire(D d = D()) noexcept; protected: hazard_pointer_obj_base() = default; hazard_pointer_obj_base(const hazard_pointer_obj_base&) = default; hazard_pointer_obj_base(hazard_pointer_obj_base&&) = default; hazard_pointer_obj_base& operator=(const hazard_pointer_obj_base&) = default; hazard_pointer_obj_base& operator=(hazard_pointer_obj_base&&) = default; ~hazard_pointer_obj_base() = default; private: D deleter; // exposition only }; }

D shall be a function object type ([func.require]) for which, given a value d of type D and a value ptr of type T*, the expression d(ptr) is valid.

The behavior of a program that adds specializations for hazard_pointer_obj_base is undefined.

D shall meet the requirements for Cpp17DefaultConstructible and Cpp17MoveAssignable.

T may be an incomplete type.

It shall be complete before any member of the resulting specialization of hazard_pointer_obj_base is referenced.

🔗

void retire(D d = D()) noexcept;

Mandates: T is a hazard-protectable type.

Preconditions: *this is a base class subobject of an object x of type T.

x is not retired.

Move-assigning d to deleter does not exit via an exception.

Effects: Move-assigns d to deleter, thereby setting it as the deleter of x, then retires x.

May reclaim possibly-reclaimable objects.

32.11.3.4 Class hazard_pointer [saferecl.hp.holder]

32.11.3.4.1 General [saferecl.hp.holder.general]

namespace std { class hazard_pointer { public: hazard_pointer() noexcept; hazard_pointer(hazard_pointer&&) noexcept; hazard_pointer& operator=(hazard_pointer&&) noexcept; ~hazard_pointer(); bool empty() const noexcept; template<class T> T* protect(const atomic<T*>& src) noexcept; template<class T> bool try_protect(T*& ptr, const atomic<T*>& src) noexcept; template<class T> void reset_protection(const T* ptr) noexcept; void reset_protection(nullptr_t = nullptr) noexcept; void swap(hazard_pointer&) noexcept; }; }

An object of type hazard_pointer is either empty or owns a hazard pointer.

Each hazard pointer is owned by exactly one object of type hazard_pointer.

[Note 1:

An empty hazard_pointer object is different from a hazard_pointer object that owns an unassociated hazard pointer.

An empty hazard_pointer object does not own any hazard pointers.

— end note]

32.11.3.4.2 Constructors, destructor, and assignment [saferecl.hp.holder.ctor]

🔗

hazard_pointer() noexcept;

Postconditions: *this is empty.

🔗

hazard_pointer(hazard_pointer&& other) noexcept;

Postconditions: If other is empty, *this is empty.

Otherwise, *this owns the hazard pointer originally owned by other; other is empty.

🔗

~hazard_pointer();

Effects: If *this is not empty, destroys the hazard pointer owned by *this, thereby ending its current protection epoch.

🔗

hazard_pointer& operator=(hazard_pointer&& other) noexcept;

Effects: If this == &other is true, no effect.

Otherwise, if *this is not empty, destroys the hazard pointer owned by *this, thereby ending its current protection epoch.

Postconditions: If other was empty, *this is empty.

Otherwise, *this owns the hazard pointer originally owned by other.

If this != &other is true, other is empty.

Returns: *this.

32.11.3.4.3 Member functions [saferecl.hp.holder.mem]

🔗

bool empty() const noexcept;

Returns: true if and only if *this is empty.

🔗

template<class T> T* protect(const atomic<T*>& src) noexcept;

Effects: Equivalent to: T* ptr = src.load(memory_order::relaxed); while (!try_protect(ptr, src)) {} return ptr;

🔗

template<class T> bool try_protect(T*& ptr, const atomic<T*>& src) noexcept;

Mandates: T is a hazard-protectable type.

Preconditions: *this is not empty.

Effects: Performs the following steps in order:

(5.1)
Initializes a variable old of type T* with the value of ptr.
(5.2)
Evaluates reset_protection(old).
(5.3)
Assigns the value of src.load(memory_order::acquire) to ptr.
(5.4)
If old == ptr is false, evaluates reset_protection().

Returns: old == ptr.

🔗

template<class T> void reset_protection(const T* ptr) noexcept;

Mandates: T is a hazard-protectable type.

Preconditions: *this is not empty.

Effects: If ptr is a null pointer value, invokes reset_protection().

Otherwise, associates the hazard pointer owned by *this with *ptr, thereby ending the current protection epoch.

Complexity: Constant.

🔗

void reset_protection(nullptr_t = nullptr) noexcept;

Preconditions: *this is not empty.

Postconditions: The hazard pointer owned by *this is unassociated.

Complexity: Constant.

🔗

void swap(hazard_pointer& other) noexcept;

Effects: Swaps the hazard pointer ownership of this object with that of other.

[Note 1:

The owned hazard pointers, if any, remain unchanged during the swap and continue to be associated with the respective objects that they were protecting before the swap, if any.

No protection epochs are ended or initiated.

— end note]

Complexity: Constant.

32.11.3.4.4 Non-member functions [saferecl.hp.holder.nonmem]

🔗

hazard_pointer make_hazard_pointer();

Effects: Constructs a hazard pointer.

Returns: A hazard_pointer object that owns the newly-constructed hazard pointer.

Throws: May throw bad_alloc if memory for the hazard pointer could not be allocated.

🔗

void swap(hazard_pointer& a, hazard_pointer& b) noexcept;

Effects: Equivalent to a.swap(b).

32 Concurrency support library [thread]

32.1 General [thread.general]

32.2 Requirements [thread.req]

32.2.1 Template parameter names [thread.req.paramname]

32.2.2 Exceptions [thread.req.exception]

32.2.3 Native handles [thread.req.native]

32.2.4 Timing specifications [thread.req.timing]

32.2.5 Requirements for Cpp17Lockable types [thread.req.lockable]

32.2.5.1 General [thread.req.lockable.general]

32.2.5.2 Cpp17BasicLockable requirements [thread.req.lockable.basic]

32.2.5.3 Cpp17Lockable requirements [thread.req.lockable.req]

32.2.5.4 Cpp17TimedLockable requirements [thread.req.lockable.timed]

32.2.5.5 Cpp17SharedLockable requirements [thread.req.lockable.shared]

32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]

32.3 Stop tokens [thread.stoptoken]

32.3.1 Introduction [thread.stoptoken.intro]

32.3.2 Header <stop_token> synopsis [thread.stoptoken.syn]

32.3.3 Stop token concepts [stoptoken.concepts]

32.3.4 Class stop_token [stoptoken]

32.3.4.1 General [stoptoken.general]

32.3.4.2 Member functions [stoptoken.mem]

32.3.5 Class stop_source [stopsource]

32.3.5.1 General [stopsource.general]

32.3.5.2 Constructors, copy, and assignment [stopsource.cons]

32.3.5.3 Member functions [stopsource.mem]

32.3.6 Class template stop_callback [stopcallback]

32.3.6.1 General [stopcallback.general]

32.3.6.2 Constructors and destructor [stopcallback.cons]

32.3.7 Class never_stop_token [stoptoken.never]

32.3.8 Class inplace_stop_token [stoptoken.inplace]

32.3.8.1 General [stoptoken.inplace.general]

32.3.8.2 Member functions [stoptoken.inplace.mem]

32.3.9 Class inplace_stop_source [stopsource.inplace]

32.3.9.1 General [stopsource.inplace.general]

32.3.9.2 Constructors [stopsource.inplace.cons]

32.3.9.3 Member functions [stopsource.inplace.mem]

32.3.10 Class template inplace_stop_callback [stopcallback.inplace]

32.3.10.1 General [stopcallback.inplace.general]

32.3.10.2 Constructors and destructor [stopcallback.inplace.cons]

32.4 Threads [thread.threads]

32.4.1 General [thread.threads.general]

32.4.2 Header <thread> synopsis [thread.syn]

32.4.3 Class thread [thread.thread.class]

32.4.3.1 General [thread.thread.class.general]

32.4.3.2 Class thread​::​id [thread.thread.id]

32.4.3.3 Constructors [thread.thread.constr]

32.4.3.4 Destructor [thread.thread.destr]

32.4.3.5 Assignment [thread.thread.assign]

32.4.3.6 Members [thread.thread.member]

32.4.3.7 Static members [thread.thread.static]

32.4.3.8 Specialized algorithms [thread.thread.algorithm]

32.4.4 Class jthread [thread.jthread.class]

32.4.4.1 General [thread.jthread.class.general]

32.4.4.2 Constructors, move, and assignment [thread.jthread.cons]

32.4.4.3 Members [thread.jthread.mem]

32.4.4.4 Stop token handling [thread.jthread.stop]

32.4.4.5 Specialized algorithms [thread.jthread.special]

32.4.4.6 Static members [thread.jthread.static]

32.4.5 Namespace this_thread [thread.thread.this]

32.5 Atomic operations [atomics]

32.5.1 General [atomics.general]

32.5.2 Header <atomic> synopsis [atomics.syn]

32.5.3 Type aliases [atomics.alias]

32.5.4 Order and consistency [atomics.order]

32.5.5 Lock-free property [atomics.lockfree]

32.5.6 Waiting and notifying [atomics.wait]

32.5.7 Class template atomic_ref [atomics.ref.generic]

32.5.7.1 General [atomics.ref.generic.general]

32.5.7.2 Operations [atomics.ref.ops]

32.5.7.3 Specializations for integral types [atomics.ref.int]

32.5.7.4 Specializations for floating-point types [atomics.ref.float]

32.5.7.5 Partial specialization for pointers [atomics.ref.pointer]

32.5.7.6 Member operators common to integers and pointers to objects [atomics.ref.memop]

32.5.8 Class template atomic [atomics.types.generic]

32.5.8.1 General [atomics.types.generic.general]

32.5.8.2 Operations on atomic types [atomics.types.operations]

32.5.8.3 Specializations for integers [atomics.types.int]

32.5.8.4 Specializations for floating-point types [atomics.types.float]

32.5.8.5 Partial specialization for pointers [atomics.types.pointer]

32.5.8.6 Member operators common to integers and pointers to objects [atomics.types.memop]

32.4.3.2 Class thread::id [thread.thread.id]