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.


flag - an object, for which exactly one function gets executed
f - Callable object to invoke
args... - arguments to pass to the function

Return value



  • std::system_error if any condition prevents calls to call_once from executing as specified
  • any exception thrown by f


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.


#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);
    std::thread t1(do_once, true);
    std::thread t2(do_once, true);
    std::thread t3(do_once, false);
    std::thread t4(do_once, true);

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

helper object to ensure that call_once invokes the function only once
C documentation for call_once

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