std::call_once

 

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Заголовочный файл <mutex>
template< class Function, class... Args > void call_once( std::once_flag& flag, Function&& f, Args&& args... );		(начиная с C++11)

Выполняет функции f только один раз, даже если вызывать из нескольких потоков.

Original:

Executes the function f exactly once, even if called from several threads.

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Каждая группа call_once вызова, который получает тот же объект std::once_flag будет отвечать следующим требованиям:

Original:

Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:

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• Ровно исполнения ровно одна из функций (передается как f на вызовы в группе) не выполняется. Не определено, какие функции будут отобраны для исполнения. Выбранная функция выполняется в том же потоке, что и call_once вызов был передан.

  Original:

  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.

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• Нет вызова в группу возвращается до вышеуказанного исполнения Выбранная функция завершилась успешно, то есть не выходить через исключение.

  Original:

  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.

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• Если выбранная функция выходит через исключение, оно распространяется на абонентов. Еще одна функция, затем выбирается и выполняется.

  Original:

  If the selected function exits via exception, it is propagated to the caller. Another function is then selected and executed.

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Параметры

flag	-	Объект, для которого ровно одна функция запускается на выполнение Original: an object, for which exactly one function gets executed The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
f	-	функции для вызова Original: function to call The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
args...	-	Аргументы для передачи в функцию Original: arguments to pass to the function The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

Возвращаемое значение

(Нет)

Original:

(none)

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Исключения

• std::system_error если условие не позволяет звонки на call_once от исполнения, как указано

  Original:

  std::system_error if any condition prevents calls to call_once from executing as specified

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• любое исключение по f

  Original:

  any exception thrown by f

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Пример

#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();
}

Вывод:

Called once

См. также

once_flag (C++11)	вспомогательный объект, чтобы убедиться, что call_once вызывает функцию только один раз Original: helper object to ensure that call_once invokes the function only once The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (класс)