std::call_once - cppreference.com
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      std::call_once

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      Threads
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      this_thread namespace
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      this_thread namespace
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      Exclusão mútua
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      Mutual exclusion
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      Gestão de bloqueio genérico
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      Generic lock management
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      Variáveis ​​de condição
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      Condition variables
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      Definido no cabeçalho <mutex>
      template< class Function, class... Args > void call_once( std::once_flag& flag, Function&& f, Args&& args... );
      		 (desde C++11)
      Executa a função f exatamente uma vez, mesmo se chamado de vários segmentos.
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      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:
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      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.
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        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.
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        If the selected function exits via exception, it is propagated to the caller. Another function is then selected and executed.
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      Parâmetros

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

      (Nenhum)
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      (none)
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      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
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        any exception thrown by f
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      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();
}

      Saída: 

      Called once

      Veja também