# std::call_once

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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

### See also

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

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