std::call_once

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Thread support library

Threads
thread (C++11)
hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutual exclusion
mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
Generic lock management
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
Condition variables
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
Futures
promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

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.

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.