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 std::call_once is called, flag indicates that f was already
called, std::call_once returns right away (such a call to std::call_once is
known as passive).
* Otherwise, std::call_once calls INVOKE(std::forward<Callable>(f),
std::forward<Args>(args)...). Unlike the std::thread constructor or 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 std::call_once is known as
active).

* If that invocation throws an exception, it is propagated to the caller of
std::call_once, and flag is not flipped so that another call will be attempted
(such a call to std::call_once is known as exceptional ).
* If that invocation returns normally (such a call to std::call_once is known as
returning), flag is flipped, and all other calls to std::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 std::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 std::call_once from
executing as specified.
* Any exception thrown by f.

Notes¶

If concurrent calls to std::call_once pass different functions f, it is unspecified
which f will be called. The selected function runs in the same thread as the
std::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 <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();
}

Possible 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

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

DR Applied to Behavior as published Correct behavior
std::invalid_argument would be thrown if f
LWG 2080 C++11 is invalid, removed this error
but the scenario where f is invalidated is condition
not specified
LWG 2442 C++11 the arguments were copied and/or moved no copying/moving is
before invocation performed

See also¶

once_flag helper object to ensure that call_once invokes the function only once
(C++11) (class)
C documentation for
call_once