Executes theCallable objectf exactly once, even if called concurrently from several threads.

In detail:

• If, by the timestd::call_once is called,flag indicates thatf was already called,std::call_once returns right away (such a call tostd::call_once is known aspassive).

• Otherwise,std::call_once callsINVOKE(std::forward<Callable>(f),std::forward<Args>(args)...). Unlike thestd::thread constructor orstd::async, the arguments are not moved or copied because they do not need to be transferred to another thread of execution (such a call tostd::call_once is known asactive).

• If that invocation throws an exception, it is propagated to the caller ofstd::call_once, andflag is not flipped so that another call will be attempted (such a call tostd::call_once is known asexceptional ).
• If that invocation returns normally (such a call tostd::call_once is known asreturning),flag is flipped, and all other calls tostd::call_once with the sameflag are guaranteed to bepassive.

Allactive calls on the sameflag form a single total order consisting of zero or moreexceptional calls, followed by onereturning call. The end of eachactive call synchronizes-with the nextactive call in that order.

The return from thereturning call synchronizes-with the returns from allpassive calls on the sameflag: this means that all concurrent calls tostd::call_once are guaranteed to observe any side-effects made by theactive call, with no additional synchronization.

[edit]Parameters

[edit]Return value

(none)

[edit]Exceptions

• std::system_error if any condition prevents calls tostd::call_once from executing as specified.
• Any exception thrown byf.

[edit]Notes

If concurrent calls tostd::call_once pass different functionsf, it is unspecified whichf will be called. The selected function runs in the same thread as thestd::call_once invocation it was passed to.

Initialization offunction-local statics is guaranteed to occur only once even when called from multiple threads, and may be more efficient than the equivalent code usingstd::call_once.

The POSIX equivalent of this function ispthread_once.

[edit]Example

#include <iostream>#include <mutex>#include <thread> std::once_flag flag1, flag2; void simple_do_once(){    std::call_once(flag1,[](){std::cout<<"Simple example: called once\n";});} void may_throw_function(bool do_throw){if(do_throw){std::cout<<"Throw: call_once will retry\n";// this may appear more than oncethrowstd::exception();}std::cout<<"Did not throw, call_once will not attempt again\n";// guaranteed once} void do_once(bool do_throw){try{        std::call_once(flag2, may_throw_function, do_throw);}catch(...){}} int main(){std::thread st1(simple_do_once);std::thread st2(simple_do_once);std::thread st3(simple_do_once);std::thread st4(simple_do_once);    st1.join();    st2.join();    st3.join();    st4.join(); std::thread t1(do_once,true);std::thread t2(do_once,true);std::thread t3(do_once,false);std::thread t4(do_once,true);    t1.join();    t2.join();    t3.join();    t4.join();}

Simple example: called onceThrow: call_once will retryThrow: call_once will retryThrow: call_once will retryDid not throw, call_once will not attempt again

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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