std::call_once

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Threads
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Defined in header <mutex>
template< class Function, class... Args >
void call_once( std::once_flag& flag, Function&& f, Args&& args... );
 （C++11 起）
。執行功能f一次，即使從多個線程調用.
Original:
Executes the function f exactly once, even if called from several threads.
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。每個組的call_once調用接收相同的std::once_flag對象的將符合下列要求：。
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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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  • 。
整整一個執行完全相同的功能（通過f該組中的調用）。將被選擇用於執行的函數，它是未定義。所選擇的功能call_once調用傳遞給運行在同一個線程中.
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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  • 。沒有調用成功完成之前，上述執行所選擇的功能組中的回報，也就是說，不通過異常退出.
    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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  • 。如果所選的功能異常退出，它會傳播給調用者。然後，選擇和執行的另一項功能.
    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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目錄

[编辑] 。參數。

flag -
。一個對象，其中一個功能被執行。
Original:
an object, for which exactly one function gets executed
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f -
。要調用的函數。
Original:
function to call
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args... -
。傳遞給函數的參數。
Original:
arguments to pass to the function
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===。 返回值。===

。 （無）。
Original:
(none)
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[编辑] 。例外。

  • 。 std::system_error任何條件阻止call_once執行所指定的調用。
    Original:
    std::system_error if any condition prevents calls to call_once from executing as specified
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  • 。 f拋出的任何異常。
    Original:
    any exception thrown by f
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[编辑] 。為例。

#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

[编辑] 。另請參閱。

（C++11）
call_once的輔助對象，以確保調用該函數一次
Original:
helper object to ensure that call_once invokes the function only once
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（類） [edit]
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/thread/call_once&oldid=28671”