32 Concurrency support library [thread]

32.6 Mutual exclusion [thread.mutex]

32.6.7 Call once [thread.once]

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]