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

Example

Run this code

#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

Example

Run this code

#include <iostream>
#include <thread>
#include <mutex>
 
std::once_flag flag;
 
inline void may_throw_function(bool do_throw)
{
  // only one instance of this function can be run simultaneously
  if (do_throw) {
    std::cout << "throw\n"; // this message may be printed from 0 to 3 times
    // if function exits via exception, another function selected
    throw std::exception();
  }
 
  std::cout << "once\n"; // printed exactly once, it's guaranteed that
      // there are no messages after it
}
 
inline void do_once(bool do_throw)
{
  try {
    std::call_once(flag, may_throw_function, do_throw);
  }
  catch (...) {
  }
}
 
int main()
{
    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:

throw
throw
once

See also

once_flag (C++11)

helper object to ensure that call_once invokes the function only once (class)