# 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 from several threads.

Each group of `call_once` invocations that receives the same std::once_flag object will meet the following requirements:

- Exactly one execution of exactly one of the functions (passed as `f` to the invocations in the group) is performed. It is undefined which function will be selected for execution. The selected function runs in the same thread as the `call_once` invocation it was passed to.

- No invocation in the group returns before the above-mentioned execution of the selected function is completed successfully, that is, doesn't exit via an exception.

- If the selected function exits via exception, it is propagated to the caller. Another function is then selected and executed.

### 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

The arguments to the `Callable` object are moved or copied by value. If a reference argument needs to be passed to the `Callable` object, it has to be wrapped (e.g. with std::ref or std::cref). | (until C++17)
The arguments to the `Callable` object are perfect forwarded (as if by std::forward<Callable>(f) and std::forward<Args>(args))...), which is different from the uses of `Callable`s in the thread constructor or std::async, because `call_once` does not have to transfer its arguments to another thread of execution, and therefore does not need to move or copy. | (since C++17)

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`.

### 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)
C documentation for call_once

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