std::call_once - cppreference.com (original) (raw)

| | | | | -------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | 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 concurrently from several threads.

In detail:

• If, by the time std::call_once is called, flag indicates that f was already called, std::call_once returns right away (such a call to std::call_once is known as passive).

• Otherwise, std::call_once calls INVOKE(std::forward<Callable>(f), std::forward<Args>(args)...). Unlike the std::thread constructor or std::async, the arguments are not moved or copied because they do not need to be transferred to another thread of execution (such a call to std::call_once is known as active).

• If that invocation throws an exception, it is propagated to the caller of std::call_once, and flag is not flipped so that another call will be attempted (such a call to std::call_once is known as exceptional ).

• If that invocation returns normally (such a call to std::call_once is known as returning), flag is flipped, and all other calls to std::call_once with the same flag are guaranteed to be passive.

All active calls on the same flag form a single total order consisting of zero or more exceptional calls, followed by one returning call. The end of each active call synchronizes-with the next active call in that order.

The return from the returning call synchronizes-with the returns from all passive calls on the same flag: this means that all concurrent calls to std::call_once are guaranteed to observe any side-effects made by the active call, with no additional synchronization.

Contents

• 1 Parameters
• 2 Return value
• 3 Exceptions
• 4 Notes
• 5 Example
• 6 Defect reports
• 7 See also

[edit] Parameters

[edit] Return value

(none)

[edit] Exceptions

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

[edit] Notes

If concurrent calls to std::call_once pass different functions f, it is unspecified which f will be called. The selected function runs in the same thread as the std::call_once invocation it was passed to.

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.

The POSIX equivalent of this function is pthread_once.

[edit] Example

#include #include #include   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 << "Did not 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 Throw: call_once will retry Did not throw, call_once will not attempt again

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also