std::call_once(3)

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

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