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 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 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 Callables 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