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std::call_once

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Defined in header <mutex>
template< class Function, class... Args >
void call_once( std::once_flag& flag, Function&& f, Args&& args... );
(seit C++11)
Führt die Funktion f genau einmal, auch wenn von mehreren Threads aufgerufen .
Original:
Executes the function f exactly once, even if called from several threads.
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Jede Gruppe von call_once Anrufungen, die den gleichen std::once_flag Objekt empfängt, wird die folgenden Anforderungen erfüllen:
Original:
Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:
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  • Genau eine Ausführung genau eine der Funktionen (übergeben als f den Anrufungen in der Gruppe) durchgeführt. Es ist nicht definiert, welche Funktion für die Ausführung ausgewählt werden. Die ausgewählte Funktion läuft im selben Thread wie die call_once Anrufung ging sie an wurde .
    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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  • Kein Aufruf in der Gruppe kehrt vor dem genannten Ausführung der ausgewählten Funktion erfolgreich abgeschlossen ist, das heißt, nicht über eine Ausnahme zu verlassen .
    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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  • Wenn die ausgewählte Funktion tritt über Ausnahme, wird es dem Anrufer propagiert. Eine weitere Funktion wird dann ausgewählt und ausgeführt .
    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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Inhaltsverzeichnis

[Bearbeiten] Parameter

flag -
Eine Aufgabe, für die genau eine Funktion ausgeführt
Original:
an object, for which exactly one function gets executed
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f -
funktionieren zu nennen
Original:
function to call
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args... -
Argumente, die an die Funktion übergeben
Original:
arguments to pass to the function
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[Bearbeiten] Rückgabewert

(None)
Original:
(none)
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[Bearbeiten] Ausnahmen

  • std::system_error, wenn Ihr Zustand Anrufe verhindert call_once von der Ausführung, wie angegeben
    Original:
    std::system_error if any condition prevents calls to call_once from executing as specified
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  • jede Ausnahme von f geworfen
    Original:
    any exception thrown by f
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[Bearbeiten] Beispiel

#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

[Bearbeiten] Siehe auch

(C++11)
Hilfsobjekt um sicherzustellen, dass call_once die Funktion aufruft nur einmal
Original:
helper object to ensure that call_once invokes the function only once
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(Klasse) [edit]