C++11: std::call_once和std::one_flag

有时候咱们须要的变量(variable)容器(container)只须要初始化一次，以容器(containter)为例子，可能只是须要向其中注入一次元素.

只打C++11开始标准库提供了std::call_once()和std::once_flag的组合能够帮助咱们作到这一点.

 

demo1:

#include <iostream>
#include <thread>
#include <mutex>
#include <vector>

std::vector<int> vec;
std::mutex mutex;
std::once_flag flag;

static int val = 0;

void initialized_vec(std::vector<int>& vec)
{
	for (int i = 0; i < 10; ++i) {
		vec.push_back(val++);
	}
}

void print_vec()
{
	std::call_once(flag, initialized_vec, vec);
	std::lock_guard<std::mutex> auto_lock(mutex);
	for (const int& number : vec) {
		std::cout << number << " ";
	}
	std::cout << std::endl;
}

int main()
{
	std::thread threads[5];
	for (int i = 0; i < 5; ++i) {
		threads[i] = std::thread(print_vec);
	}

	for (std::thread& ref_thread : threads) {
		ref_thread.join();
	}

	return 0;
}

 

demo2:

#include <iostream>
#include <thread>
#include <mutex>

#include "Singleton.h"


void function(int& number)
{
	number = 20;
}

int main()
{
	int number{ 0 };
	std::once_flag onceFlag;

	std::call_once(onceFlag, function, number);
	std::cout << number << std::endl;  //20! ref number

	return 0;
}