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std::call_once

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Threads
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thread(C++11)
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this_thread namespace
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Generic lock management
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Defined in header <mutex>
template< class Function, class... Args >
void call_once( std::once_flag& flag, Function&& f, Args&& args... );
(dal C++11)
Esegue la funzione f esattamente una volta, anche se chiamato da più thread.
Original:
Executes the function f exactly once, even if called from several threads.
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Ogni gruppo di invocazioni call_once che riceve lo stesso oggetto std::once_flag soddisferà i seguenti requisiti:
Original:
Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:
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  • Esattamente una esecuzione di esattamente una delle funzioni (passato come f alle invocazioni del gruppo) viene eseguita. Esso è definito quale funzione verrà selezionato per l'esecuzione. La funzione selezionata viene eseguito nello stesso thread come invocazione call_once è stato passato a.
    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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  • No invocazione nei rendimenti gruppo prima dell'esecuzione della suddetta funzione selezionata viene completato correttamente, cioè non esce con un'eccezione.
    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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  • Se le uscite funzione selezionate attraverso eccezione, si propaga al chiamante. Un'altra funzione è quindi selezionato ed eseguito.
    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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Indice

[modifica] Parametri

flag -
un oggetto, per cui esattamente una funzione viene eseguita
Original:
an object, for which exactly one function gets executed
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f -
funzione da chiamare
Original:
function to call
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args... -
argomenti da passare alla funzione
Original:
arguments to pass to the function
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[modifica] Valore di ritorno

(Nessuno)
Original:
(none)
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[modifica] Eccezioni

  • std::system_error eventuale condizione impedisce le chiamate a call_once dall'esecuzione come specificato
    Original:
    std::system_error if any condition prevents calls to call_once from executing as specified
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  • qualsiasi eccezione generata dal f
    Original:
    any exception thrown by f
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[modifica] Esempio

#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

[modifica] Vedi anche

(C++11)
oggetto di supporto per garantire che call_once richiama la funzione una sola volta
Original:
helper object to ensure that call_once invokes the function only once
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(classe) [edit]