NAME

std::call_once - std::call_once

Synopsis

Defined in header <mutex>
template< class Callable, class... Args > (since C++11)
void call_once( std::once_flag& flag, Callable&& f, Args&&... args );

Executes the Callable object f exactly once, even if called concurrently, from
several threads.

In detail:

* If, by the time call_once is called, flag indicates that f was already called,
call_once returns right away (such a call to call_once is known as passive).

* Otherwise, call_once invokes std::forward<Callable>(f) with the arguments
std::forward<Args>(args)... (as if by std::invoke). Unlike the std::thread
constructor or std::async, the arguments are not moved or copied because they
don't need to be transferred to another thread of execution. (such a call to
call_once is known as active).

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

All active calls on the same 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 flag: this means that all concurrent calls to call_once are
guaranteed to observe any side-effects made by the active call, with no additional
synchronization.

Parameters

flag - an object, for which exactly one function gets executed
f - Callable object to invoke
args... - arguments to pass to the function

Return value

(none)

Exceptions

* std::system_error if any condition prevents calls to call_once from executing as
specified
* any exception thrown by f

Notes

If concurrent calls to call_once pass different functions f, it is unspecified which
f will be called. The selected function runs in the same thread as the call_once
invocation it was passed to.

Initialization of 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 std::call_once.

The POSIX equivalent of this function is pthread_once.

Example

// Run this code

#include <iostream>
#include <thread>
#include <mutex>

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 << "Didn't 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();
}

Possible output:

Simple example: called once
throw: call_once will retry
throw: call_once will retry
Didn't throw, call_once will not attempt again

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 2442 C++11 the arguments are copied and/or moved no copying/moving is
before invocation performed