View source for cpp/thread/call once

{{cpp/title|call_once}}
{{cpp/thread/navbar}}
{{ddcl|header=mutex|since=c++11|
template< class Callable, class... Args >
void call_once( std::once_flag& flag, Callable&& f, Args&&... args );
}}

Executes the {{named req|Callable}} object {{c|f}} exactly once, even if called concurrently from several threads. 

In detail:

* If, by the time {{tt|std::call_once}} is called, {{c|flag}} indicates that {{c|f}} was already called, {{tt|std::call_once}} returns right away (such a call to {{tt|std::call_once}} is known as ''passive'').

* Otherwise, {{tt|std::call_once}} calls {{box|{{lti|cpp/utility/functional|INVOKE}}{{c/core|(std::forward<Callable>(f), std::forward<Args>(args)...)}}}}. Unlike the {{lc|std::thread}} constructor or {{lc|std::async}}, the arguments are not moved or copied because they do not need to be transferred to another thread of execution (such a call to {{tt|std::call_once}} is known as ''active'').

:* If that invocation throws an exception, it is propagated to the caller of {{tt|std::call_once}}, and {{c|flag}} is not flipped so that another call will be attempted (such a call to {{tt|std::call_once}} is known as ''exceptional''{{sep}}).
:* If that invocation returns normally (such a call to {{tt|std::call_once}} is known as ''returning''), {{c|flag}} is flipped, and all other calls to {{tt|std::call_once}} with the same {{c|flag}} are guaranteed to be ''passive''.

All ''active'' calls on the same {{c|flag}} form a single total order consisting of zero or more ''exceptional'' calls, followed by one ''returning'' call. The end of each ''active'' call synchronizes-with the next ''active'' call in that order.

The return from the ''returning'' call synchronizes-with the returns from all ''passive'' calls on the same {{c|flag}}: this means that all concurrent calls to {{tt|std::call_once}} are guaranteed to observe any side-effects made by the ''active'' call, with no additional synchronization.

===Parameters===
{{par begin}}
{{par|flag|an object, for which exactly one function gets executed}}
{{par|f|{{named req|Callable}} object to invoke}}
{{par|args...|arguments to pass to the function}}
{{par end}}

===Return value===
(none)

===Exceptions===
* {{lc|std::system_error}} if any condition prevents calls to {{tt|std::call_once}} from executing as specified.
* Any exception thrown by {{c|f}}.

===Notes===
If concurrent calls to {{tt|std::call_once}} pass different functions {{c|f}}, it is unspecified which {{c|f}} will be called. The selected function runs in the same thread as the {{tt|std::call_once}} invocation it was passed to.

Initialization of [[cpp/language/storage duration#Static local variables|function-local statics]] is guaranteed to occur only once even when called from multiple threads, and may be more efficient than the equivalent code using {{tt|std::call_once}}.

The POSIX equivalent of this function is [https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html {{tt|pthread_once}}].

===Example===
{{example
|code=
#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 once
        throw std::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();
}
|p=true
|output=
Simple example: called once
Throw: call_once will retry
Throw: call_once will retry
Throw: call_once will retry
Did not throw, call_once will not attempt again
}}

===Defect reports===
{{dr list begin}}
{{dr list item|wg=lwg|dr=2080|std=C++11|before={{lc|std::invalid_argument}} would be thrown if {{c|f}} is invalid,<br>but the scenario where {{c|f}} is invalidated is not specified|after=removed this error condition}}
{{dr list item|wg=lwg|dr=2442|std=C++11|before=the arguments were copied and/or moved before invocation|after=no copying/moving is performed}}
{{dr list end}}

===See also===
{{dsc begin}}
{{dsc inc|cpp/thread/dsc once_flag}}
{{dsc see c|c/thread/call_once}}
{{dsc end}}

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