C++11特性之std:call_once介绍

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 as passive).
• 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 don't need to be transferred to another thread of execution. (such a call to call_once is known as active).
• 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 as exceptional).
• If that invocation returns normally (such a call to call_once is known as returning), the flag is flipped, and all other calls to call_once with the same flag are guaranteed to be passive.

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.

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.