std::call_once

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

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 call to call_once is known as exceptional).
If that invocation returns normally (such 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.

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 before invocation	no copying/moving is performed

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

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