std::call_once

 

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スレッドのサポートライブラリ

スレッド 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空間 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)
相互排除 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)
汎用のロック管理 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)
条件変数 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)
先物 Original: Futures The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
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)

 

Defined in header <mutex>
template< class Function, class... Args > void call_once( std::once_flag& flag, Function&& f, Args&& args... );		(C + + 11以来)

複数のスレッドから呼び出された場合であっても、一度だけ関数fを実行.

Original:

Executes the function f exactly once, even if called from several threads.

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同じcall_once·オブジェクトを受け取りstd::once_flag呼び出しの各グループは、次の要件を満たします

Original:

Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:

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• まさに機能のちょうど1つの（グループ内の呼び出しにfとして渡される）のいずれかの実行が行われます。これは、実行のために選択される関数は未定義です。選択した機能は、それが可決されたcall_onceの呼び出しと同じスレッドで実行され.

  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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• 選択した機能の実行前に上記のグループに戻りなしで呼び出しが正常に完了されていない、つまり、例外によって終了しません.

  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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• 例外によって選択された関数が終了する場合は、呼び出し元に伝播されます。もう一つの機能は、次に選択され、実行され.

  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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パラメータ

flag	-	正確に1つの関数が実行されているオブジェクト、 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	-	呼び出される関数です 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...	-	引数は、関数に渡す 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.

値を返します

（なし）

Original:

(none)

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例外

• std::system_errorいずれかの条件が指定されるように実行されるのcall_onceへの呼び出しを防ぎます

  Original:

  std::system_error if any condition prevents calls to call_once from executing as specified

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• fによってスローされた例外

  Original:

  any exception thrown by f

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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

も参照してください

once_flag (C++11)	call_onceは一度だけ関数を呼び出していることを確認するためのヘルパーオブジェクト 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. (クラス)