32 Concurrency support library [thread]

32.4 Threads [thread.threads]

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:
  • is_constructible_v<decay_t<F>, F>,
  • (is_constructible_v<decay_t<Args>, Args> && ...), and
  • 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:
  • 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:
  • resource_deadlock_would_occur — if deadlock is detected or get_id() == this_thread​::​​get_id().
  • no_such_process — if the thread is not valid.
  • 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:
  • no_such_process — if the thread is not valid.
  • 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().