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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... );
(a partir do C++ 11)
Executa a função f exatamente uma vez, mesmo se chamado de vários segmentos.
Original:
Executes the function f exactly once, even if called from several threads.
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Cada grupo de invocações call_once que recebe o objeto std::once_flag mesmo irá atender os seguintes requisitos:
Original:
Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:
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  • Exatamente uma execução de exatamente uma das funções (passado como f para as invocações do grupo) é realizada. É indefinido cuja função será seleccionada para execução. A função selecionada é executado no mesmo segmento como a invocação call_once foi passado para.
    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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  • Nenhuma invocação nos retornos do grupo antes da execução acima mencionado da função seleccionada é completada com sucesso, isto é, não sai via uma excepção.
    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 a função sai selecionados através de exceção, ela é propagada para o chamador. Outra função é então seleccionado e executado.
    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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Índice

[editar] Parâmetros

flag -
um objeto, por que exatamente uma função é executada
Original:
an object, for which exactly one function gets executed
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f -
função a ser chamada
Original:
function to call
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args... -
argumentos para passar para a função
Original:
arguments to pass to the function
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[editar] Valor de retorno

(Nenhum)
Original:
(none)
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[editar] Exceções

  • std::system_error se qualquer condição impede chamadas para call_once de execução, conforme especificado
    Original:
    std::system_error if any condition prevents calls to call_once from executing as specified
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  • qualquer exceção lançada pelo f
    Original:
    any exception thrown by f
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[editar] Exemplo

#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

[editar] Veja também

(C++11)
objeto auxiliar a assegurar que call_once invoca a função apenas uma vez
Original:
helper object to ensure that call_once invokes the function only once
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(classe) [edit]