std::call_once (3) - Linux Manuals

std::call_once: std::call_once

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 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