C++11特性之std:call_once介绍 _C++11

笛里谁知壮士心，沙头空照征人骨。这篇文章主要讲述C++11特性之std:call_once介绍相关的知识，希望能为你提供帮助。
std:call_once是C++11引入的新特性，如需使用，只需要#include < mutex> 即可，简单来说std:call_once的作用，确保函数或代码片段在多线程环境下，只需要执行一次，常用的场景如Init()操作或一些系统参数的获取等。
相对来说，std::call_once用法比较简单，配合std::once_flag即可实现，废话不多说，上代码：

#include "stdafx.h"
#include < iostream>
#include < thread>
#include < mutex>

std::once_flag flag;

void Initialize()

std::cout < < "Run into Initialize.." < < std::endl;

void Init()

std::call_once(flag, Initialize);

int main()

std::thread t1(Init);
std::thread t2(Init);
std::thread t3(Init);
std::thread t4(Init);
t1.join();
t2.join();
t3.join();
t4.join();

执行结果如下：

可以看到，Initialize()只调用了一次。
需要了解更多的话，可以参考??https://en.cppreference.com/w/cpp/thread/call_once??关于std::call_once的描述：

Executes the ??Callable??? object ??f?? exactly once, even if called concurrently, from several threads.
In detail:

If, by the time??call_once??? is called,??flag??? indicates that??f??? was already called,??call_once??? returns right away (such a call to??call_once?? is known aspassive).
Otherwise, call_once invokes ??std::forward??< Callable> (f)with the arguments ??std::forward??< Args> (args)... (as if by??std::invoke???). Unlike the??std::thread??? constructor or??std::async???, the arguments are not moved or copied because they dont need to be transferred to another thread of execution. (such a call to??call_once?? is known asactive).
If that invocation throws an exception, it is propagated to the caller of??call_once???, and the flag is not flipped so that another call will be attempted (such a call to??call_once?? is known asexceptional).
If that invocation returns normally (such a call to??call_once?? is known asreturning), the flag is flipped, and all other calls to??call_once?? with the same flag are guaranteed to bepassive.
All active calls on the same ??flag?? form a single total order consisting of zero or more exceptional calls, followed by one returning call. The end of each active call synchronizes-with the next active call in that order.
【C++11特性之std:call_once介绍】The return from the returning call synchronizes-with the returns from all passive calls on the same ??flag???: this means that all concurrent calls to ??call_once?? are guaranteed to observe any side-effects made by the active call, with no additional synchronization.