std::call_once

 

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Espressioni regolari(C++11)

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Multithreading(C++11)

 

Discussione libreria di supporto

Threads Original: Threads The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
thread (C++11)
this_thread spazio dei nomi Original: this_thread namespace The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
Mutua esclusione Original: Mutual exclusion The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
mutex (C++11) timed_mutex (C++11)	recursive_mutex (C++11) recursive_timed_mutex (C++11)
Blocco di gestione generico Original: Generic lock management The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
lock_guard (C++11) unique_lock (C++11) defer_lock_t try_to_lock_t adopt_lock_t (C++11) (C++11) (C++11)	lock (C++11) try_lock (C++11) defer_lock try_to_lock adopt_lock (C++11) (C++11) (C++11)
Condizioni variabili Original: Condition variables The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
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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)

 

Elemento definito nell'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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Parametri

flag	-	un oggetto, per cui esattamente una funzione viene eseguita 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	-	funzione da chiamare 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...	-	argomenti da passare alla funzione 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.

Valore di ritorno

(Nessuno)

Original:

(none)

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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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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

Vedi anche

once_flag (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 The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe)