多线程编程实例

多线程的好处通过一个程序示例来展示

单线程

#include <iostream>
#include <vector>
#include <algorithm>
#include<numeric>
#include <string>
using namespace std;
 
int GetSum(vector<int>::iterator first,vector<int>::iterator last)
{
    return accumulate(first,last,0);//调用C++标准库算法
}
 static string getCurrentTime()
{
	time_t t = time(NULL);
	char ch[64] ={0};
	strftime(ch,sizeof(ch) -1,"%Y-%m-%d %H:%M:%S",localtime(&t));
	return ch;
}

int main()
{
 cout << getCurrentTime() << endl;
    vector<int> largeArrays;
    for(int i=0;i<100000000;i++)
    {
        if(i%2==0)
           largeArrays.push_back(i);
        else
            largeArrays.push_back(-1*i);
    }
    int res = GetSum(largeArrays.begin(),largeArrays.end());
 cout << getCurrentTime() << endl;
    return 0;
}

2020-09-22 14:36:36
2020-09-22 14:36:39

编译输出计算时间可以看到大概需要3s。

多线程

#include <iostream>
#include <vector>
#include <algorithm>
#include <thread>
#include <numeric>
#include <string>
using namespace std;
 
//线程要做的事情就写在这个线程函数中
void GetSumT(vector<int>::iterator first,vector<int>::iterator last,int &result)
{
    result = accumulate(first,last,0); //调用C++标准库算法
}
 

static string getCurrentTime()
{
	time_t t = time(NULL);
	char ch[64] ={0};
	strftime(ch,sizeof(ch) -1,"%Y-%m-%d %H:%M:%S",localtime(&t));
	return ch;
}

int main() //主线程
{
    cout << getCurrentTime() << endl;
    int result1,result2,result3,result4,result5;
    vector<int> largeArrays;
    for(int i=0;i<100000000;i++)
    {
        if(i%2==0)
            largeArrays.push_back(i);
        else
            largeArrays.push_back(-1*i);
    }
    thread first(GetSumT,largeArrays.begin(),
        largeArrays.begin()+20000000,std::ref(result1)); //子线程1
    thread second(GetSumT,largeArrays.begin()+20000000,
        largeArrays.begin()+40000000,std::ref(result2)); //子线程2
    thread third(GetSumT,largeArrays.begin()+40000000,
        largeArrays.begin()+60000000,std::ref(result3)); //子线程3
    thread fouth(GetSumT,largeArrays.begin()+60000000,
        largeArrays.begin()+80000000,std::ref(result4)); //子线程4
    thread fifth(GetSumT,largeArrays.begin()+80000000,
        largeArrays.end(),std::ref(result5)); //子线程5
 
    first.join(); //主线程要等待子线程执行完毕
    second.join();
    third.join();
    fouth.join();
    fifth.join();
 
    int resultSum = result1+result2+result3+result4+result5; //汇总各个子线程的结果
    cout << getCurrentTime() << endl;
    return 0;
}

2020-09-22 14:37:02
2020-09-22 14:37:04

大概需要2s时间

两个程序对比可以看出多线程节约时间。很好理解多个cpu同时计算嘛。

多线程和多进程的关系

火车和火车厢的关系：进程好比火车，火车车厢好比线程。