This Clause describes components for fine-grained atomic access.

This access is
provided via operations on atomic objects.

The following subclauses describe atomics requirements and components for types
and operations, as summarized below.

Table 131 — Atomics library summary

Subclause | Header(s) | |

Type aliases | ||

Order and consistency | ||

Lock-free property | ||

Class template atomic_ref | <atomic> | |

Class template atomic | <atomic> | |

Non-member functions | <atomic> | |

Flag type and operations | <atomic> | |

Fences | <atomic> |

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

namespace std { enum class memory_order : unspecified { relaxed, consume, acquire, release, acq_rel, seq_cst }; inline constexpr memory_order memory_order_relaxed = memory_order::relaxed; inline constexpr memory_order memory_order_consume = memory_order::consume; inline constexpr memory_order memory_order_acquire = memory_order::acquire; inline constexpr memory_order memory_order_release = memory_order::release; inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel; inline constexpr memory_order memory_order_seq_cst = memory_order::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:

- memory_order::relaxed: no operation orders memory.
- memory_order::release, memory_order::acq_rel, and memory_order::seq_cst: a store operation performs a release operation on the affected memory location.
- memory_order::consume: a load operation performs a consume operation on the affected memory location.
- memory_order::acquire, memory_order::acq_rel, and memory_order::seq_cst: a load operation performs an acquire operation on the affected memory location.

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

- A is a modification, and B reads the value stored by A, or
- A precedes B in the modification order of M, or
- 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
- 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:

- if A and B are both memory_order::seq_cst operations, then A precedes B in S; and
- 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
- 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
- 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

: *end note*

]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.

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

[ Note

: *end note*

]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);should not produce 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.

— [ Note

: *end note*

]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);—

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.

Implementations should make atomic stores visible to atomic loads within a reasonable
amount of time.

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

#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.

The function atomic_is_lock_free
indicates whether the object is lock-free.

In any given program execution, the
result of the lock-free query shall be consistent for all pointers of the same
type.

[ Note

: *end note*

]Operations that are lock-free should also be address-free.

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

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

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

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

```
static constexpr bool is_always_lock_free;
```

```
static constexpr size_t required_alignment;
```

[ Note

: *end note*

]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).

— ```
atomic_ref(T& obj);
```

```
atomic_ref(const atomic_ref& ref) noexcept;
```

```
T operator=(T desired) const noexcept;
```

```
operator T() const noexcept;
```

```
bool is_lock_free() const noexcept;
```

```
void store(T desired, memory_order order = memory_order_seq_cst) const noexcept;
```

```
T load(memory_order order = memory_order_seq_cst) const noexcept;
```

```
T exchange(T desired, memory_order order = memory_order_seq_cst) const noexcept;
```

Memory is affected according to the value of order.

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

```
bool compare_exchange_weak(T& expected, T desired,
memory_order success, memory_order failure) const noexcept;
bool compare_exchange_strong(T& expected, T desired,
memory_order success, memory_order failure) const noexcept;
bool compare_exchange_weak(T& expected, T desired,
memory_order order = memory_order_seq_cst) const noexcept;
bool compare_exchange_strong(T& expected, T desired,
memory_order order = memory_order_seq_cst) const noexcept;
```

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.

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

: *end note*

]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.

— There are specializations of the atomic_ref 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,
the specialization atomic_ref<integral> provides
additional atomic operations appropriate to integral types.

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

```
integral fetch_key(integral operand, memory_order order = memory_order_seq_cst) const noexcept;
```

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]).

Remarks: 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.

```
integral operator op=(integral operand) const noexcept;
```

There are specializations of the atomic_ref class template
for the floating-point types
float,
double, and
long double.

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

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

```
floating-point fetch_key(floating-point operand,
memory_order order = memory_order_seq_cst) const noexcept;
```

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]).

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

```
floating-point operator op=(floating-point operand) const noexcept;
```

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

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

```
T* fetch_key(difference_type operand, memory_order order = memory_order_seq_cst) const noexcept;
```

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]).

```
T* operator op=(difference_type operand) const noexcept;
```

```
T* operator++(int) const noexcept;
```

```
T* operator--(int) const noexcept;
```

```
T* operator++() const noexcept;
```

```
T* operator--(int) const noexcept;
```

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; void store(T, memory_order = memory_order::seq_cst) volatile noexcept; void store(T, memory_order = memory_order::seq_cst) noexcept; T load(memory_order = memory_order::seq_cst) const volatile noexcept; T load(memory_order = memory_order::seq_cst) const noexcept; operator T() const volatile noexcept; operator T() const noexcept; T exchange(T, memory_order = memory_order::seq_cst) volatile noexcept; T exchange(T, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) noexcept; bool compare_exchange_strong(T&, T, memory_order, memory_order) volatile noexcept; 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; 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; bool compare_exchange_strong(T&, T, memory_order = memory_order::seq_cst) noexcept; atomic() noexcept = default; constexpr atomic(T) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T operator=(T) volatile noexcept; T operator=(T) noexcept; }; }

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

The program is ill-formed if any of

- is_trivially_copyable_v<T>,
- is_copy_constructible_v<T>,
- is_move_constructible_v<T>,
- is_copy_assignable_v<T>, or
- is_move_assignable_v<T>

[ Note

: *end note*

]Many operations are volatile-qualified.

The “volatile as device register”
semantics have not changed in the standard.

This qualification means that volatility is
preserved when applying these operations to volatile objects.

It does not mean that
operations on non-volatile objects become volatile.

— ```
atomic() noexcept = default;
```

```
constexpr atomic(T desired) noexcept;
```

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

[ Note

: *end note*

]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.

— ```
#define ATOMIC_VAR_INIT(value) see below
```

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

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

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

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

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

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

Memory is affected according to the value of order.

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

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

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.

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
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.

[ Note

: *end note*

]For example, the effect of
compare_exchange_strong
on objects without padding bits ([basic.types]) is

— if (memcmp(this, &expected, sizeof(*this)) == 0) memcpy(this, &desired, sizeof(*this)); else memcpy(expected, this, sizeof(*this));

[ Example

: *end example*

]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));—

[ Example

: *end example*

]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

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

: *end note*

]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.

— [ Note

: *end note*

]Under cases where the memcpy and memcmp semantics of the compare-and-exchange
operations apply, the outcome might be failed comparisons 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/IEEE 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==.

— [ Note

: *end note*

]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 = ATOMIC_VAR_INIT({}); bool zap() { padded expected, desired{0, 0}; return pad.compare_exchange_strong(expected, desired); }

[ Note

: *end note*

]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 indeteminate 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 = ATOMIC_VAR_INIT({}); bool party(pony desired) { pony expected; return princesses.compare_exchange_strong(expected, desired); }

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, the specialization
atomic<integral> provides additional atomic operations appropriate to integral types.

namespace std { template<> struct atomic<integral> { using value_type = integral; 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; void store(integral, memory_order = memory_order::seq_cst) volatile noexcept; void store(integral, memory_order = memory_order::seq_cst) noexcept; integral load(memory_order = memory_order::seq_cst) const volatile noexcept; integral load(memory_order = memory_order::seq_cst) const noexcept; operator integral() const volatile noexcept; operator integral() const noexcept; integral exchange(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral exchange(integral, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(integral&, integral, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(integral&, integral, memory_order, memory_order) noexcept; bool compare_exchange_strong(integral&, integral, memory_order, memory_order) volatile noexcept; bool compare_exchange_strong(integral&, integral, memory_order, memory_order) noexcept; bool compare_exchange_weak(integral&, integral, memory_order = memory_order::seq_cst) volatile noexcept; bool compare_exchange_weak(integral&, integral, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_strong(integral&, integral, memory_order = memory_order::seq_cst) volatile noexcept; bool compare_exchange_strong(integral&, integral, memory_order = memory_order::seq_cst) noexcept; integral fetch_add(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral fetch_add(integral, memory_order = memory_order::seq_cst) noexcept; integral fetch_sub(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral fetch_sub(integral, memory_order = memory_order::seq_cst) noexcept; integral fetch_and(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral fetch_and(integral, memory_order = memory_order::seq_cst) noexcept; integral fetch_or(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral fetch_or(integral, memory_order = memory_order::seq_cst) noexcept; integral fetch_xor(integral, memory_order = memory_order::seq_cst) volatile noexcept; integral fetch_xor(integral, memory_order = memory_order::seq_cst) noexcept; atomic() noexcept = default; constexpr atomic(integral) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; integral operator=(integral) volatile noexcept; integral operator=(integral) noexcept; integral operator++(int) volatile noexcept; integral operator++(int) noexcept; integral operator--(int) volatile noexcept; integral operator--(int) noexcept; integral operator++() volatile noexcept; integral operator++() noexcept; integral operator--() volatile noexcept; integral operator--() noexcept; integral operator+=(integral) volatile noexcept; integral operator+=(integral) noexcept; integral operator-=(integral) volatile noexcept; integral operator-=(integral) noexcept; integral operator&=(integral) volatile noexcept; integral operator&=(integral) noexcept; integral operator|=(integral) volatile noexcept; integral operator|=(integral) noexcept; integral operator^=(integral) volatile noexcept; integral operator^=(integral) noexcept; }; }

The atomic integral specializations
are standard-layout structs.

They each have a trivial default constructor
and a trivial destructor.

The following operations perform arithmetic computations.

The key, operator, and computation correspondence is:

Table 132 — Atomic arithmetic computations

key | Op | Computation | key | Op | Computation |

add | + | addition | sub | - | subtraction |

or | | | bitwise inclusive or | xor | ^ | bitwise exclusive or |

and | & | bitwise and |

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

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]).

Remarks: 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.

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

There are specializations of the atomic
class template for the floating-point types
float,
double, and
long double.

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

namespace std { template<> struct atomic<floating-point> { using value_type = floating-point; 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; void store(floating-point, memory_order = memory_order_seq_cst) volatile noexcept; void store(floating-point, memory_order = memory_order_seq_cst) noexcept; floating-point load(memory_order = memory_order_seq_cst) volatile noexcept; floating-point load(memory_order = memory_order_seq_cst) noexcept; operator floating-point() volatile noexcept; operator floating-point() noexcept; floating-point exchange(floating-point, memory_order = memory_order_seq_cst) volatile noexcept; floating-point exchange(floating-point, memory_order = memory_order_seq_cst) noexcept; bool compare_exchange_weak(floating-point&, floating-point, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(floating-point&, floating-point, memory_order, memory_order) noexcept; bool compare_exchange_strong(floating-point&, floating-point, memory_order, memory_order) volatile noexcept; bool compare_exchange_strong(floating-point&, floating-point, memory_order, memory_order) noexcept; bool compare_exchange_weak(floating-point&, floating-point, memory_order = memory_order_seq_cst) volatile noexcept; bool compare_exchange_weak(floating-point&, floating-point, memory_order = memory_order_seq_cst) noexcept; bool compare_exchange_strong(floating-point&, floating-point, memory_order = memory_order_seq_cst) volatile noexcept; bool compare_exchange_strong(floating-point&, floating-point, memory_order = memory_order_seq_cst) noexcept; floating-point fetch_add(floating-point, memory_order = memory_order_seq_cst) volatile noexcept; floating-point fetch_add(floating-point, memory_order = memory_order_seq_cst) noexcept; floating-point fetch_sub(floating-point, memory_order = memory_order_seq_cst) volatile noexcept; floating-point fetch_sub(floating-point, memory_order = memory_order_seq_cst) noexcept; atomic() noexcept = default; constexpr atomic(floating-point) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; floating-point operator=(floating-point) volatile noexcept; floating-point operator=(floating-point) noexcept; floating-point operator+=(floating-point) volatile noexcept; floating-point operator+=(floating-point) noexcept; floating-point operator-=(floating-point) volatile noexcept; floating-point operator-=(floating-point) noexcept; }; }

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

They each have a trivial default constructor
and a trivial destructor.

```
T A::fetch_key(T operand, memory_order order = memory_order_seq_cst) volatile noexcept;
T A::fetch_key(T operand, memory_order order = memory_order_seq_cst) noexcept;
```

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]).

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

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

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

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; void store(T*, memory_order = memory_order::seq_cst) volatile noexcept; void store(T*, memory_order = memory_order::seq_cst) noexcept; T* load(memory_order = memory_order::seq_cst) const volatile noexcept; T* load(memory_order = memory_order::seq_cst) const noexcept; operator T*() const volatile noexcept; operator T*() const noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) volatile noexcept; T* exchange(T*, memory_order = memory_order::seq_cst) noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) volatile noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) noexcept; bool compare_exchange_strong(T*&, T*, memory_order, memory_order) volatile noexcept; 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; 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; 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; 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; T* fetch_sub(ptrdiff_t, memory_order = memory_order::seq_cst) noexcept; atomic() noexcept = default; constexpr atomic(T*) noexcept; atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; T* operator=(T*) volatile noexcept; T* operator=(T*) noexcept; T* operator++(int) volatile noexcept; T* operator++(int) noexcept; T* operator--(int) volatile noexcept; T* operator--(int) noexcept; T* operator++() volatile noexcept; T* operator++() noexcept; T* operator--() volatile noexcept; T* operator--() noexcept; T* operator+=(ptrdiff_t) volatile noexcept; T* operator+=(ptrdiff_t) noexcept; T* operator-=(ptrdiff_t) volatile noexcept; T* operator-=(ptrdiff_t) 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 default constructor and a trivial destructor.

The following operations perform pointer arithmetic.

The key, operator,
and computation correspondence is:

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

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]).

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

```
T operator++(int) volatile noexcept;
T operator++(int) noexcept;
```

```
T operator--(int) volatile noexcept;
T operator--(int) noexcept;
```

```
T operator++() volatile noexcept;
T operator++() noexcept;
```

```
T operator--() volatile noexcept;
T operator--() noexcept;
```

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.

```
template<class T>
void atomic_init(volatile atomic<T>* object, typename atomic<T>::value_type desired) noexcept;
template<class T>
void atomic_init(atomic<T>* object, typename atomic<T>::value_type desired) noexcept;
```

This function shall only be applied
to objects that have been default constructed, and then only once.

namespace std { struct atomic_flag { bool test_and_set(memory_order = memory_order::seq_cst) volatile noexcept; bool test_and_set(memory_order = memory_order::seq_cst) noexcept; void clear(memory_order = memory_order::seq_cst) volatile noexcept; void clear(memory_order = memory_order::seq_cst) noexcept; atomic_flag() noexcept = default; atomic_flag(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) volatile = delete; }; bool atomic_flag_test_and_set(volatile atomic_flag*) noexcept; bool atomic_flag_test_and_set(atomic_flag*) noexcept; bool atomic_flag_test_and_set_explicit(volatile atomic_flag*, memory_order) noexcept; bool atomic_flag_test_and_set_explicit(atomic_flag*, memory_order) noexcept; void atomic_flag_clear(volatile atomic_flag*) noexcept; void atomic_flag_clear(atomic_flag*) noexcept; void atomic_flag_clear_explicit(volatile atomic_flag*, memory_order) noexcept; void atomic_flag_clear_explicit(atomic_flag*, memory_order) noexcept; #define ATOMIC_FLAG_INIT see below }

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 atomic_flag type is a standard-layout struct.

It has a trivial default constructor and a trivial destructor.

The macro ATOMIC_FLAG_INIT shall be 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.

Unless initialized with ATOMIC_FLAG_INIT, it is unspecified whether an
atomic_flag object has an initial state of set or clear.

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

Memory is affected according to the value of
order.

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

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

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" void atomic_thread_fence(memory_order order) noexcept;
```

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

- has no effects, if order == memory_order::relaxed;
- is an acquire fence, if order == memory_order::acquire or order == memory_order::consume;
- is a release fence, if order == memory_order::release;
- is both an acquire fence and a release fence, if order == memory_order::acq_rel;
- is a sequentially consistent acquire and release fence, if order == memory_order::seq_cst.

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

[ Note

: *end note*

]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.

—