std::call_once

From Cppreference

Executes the function f exactly once, even if called from several threads.

Each group of call_once invocations, which receives the same std::once_flag object, follows 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 call_once invocation it was passed to.

• No invocation in the group returns before the abovementioned 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 selected and executed.

[edit] Parameters

[edit] Return value

(none)

[edit] Exceptions

[edit] Example

#include <iostream>
#include <thread>
#include <mutex>
 
std::once_flag flag;
 
void do_once()
{
    std::call_once(flag, [](){ std::cout << "Called once" << std::endl; });
}
 
int main()
{
    std::thread t1(do_once);
    std::thread t2(do_once);
    std::thread t3(do_once);
    std::thread t4(do_once);
 
    t1.join();
    t2.join();
    t3.join();
    t4.join();
}

Output:

Called once

[edit] See also