freopen()重定向

freopen函数：

        使用不同的文件或模式重新打开流，即重定向

C++

   //把一个字符串写入到文件中去：
#include<iostream>
using namespace std;

int main(void)
{
    /* redirect standard output to a file */
    if(freopen("D:\out.txt", "w", stdout) == NULL)
        fprintf(stderr,"error redirecting stdout
");
    /* this output will go to a file */
    printf("This will go into a file.
");
    string s = "12345";
    cout<<s<<endl; 
    int a,b;
    while(cin>>a>>b){
        cout<<a<<b<<endl; 
    }
    /*close the standard output stream*/
    fclose(stdout);
    return 0;
}

#include <iostream>
using namespace std;

int main(void)
{
    FILE *fp;
    fp = freopen("D:\in.txt","r",stdin);   //freopen() 的返回值，如果与文件连接建立失败，返回NULL
    
    if(fp!=NULL){            //与文件连接建立成功
        int c;
        cin>>c;             //从文件中读入数据
        cout<<c<<endl;      //在屏幕上显示数据
    }else{
        cout<<"Open Error"<<endl;
    }
    
    //输出到控制台“CON”
    fp = freopen("CON","r",stdin);
    int a, b;
    cin>>a>>b;
    cout<<a<<b<<endl<<endl;  
    return 0;
}

#include<iostream>
using namespace std;

int main()
{
    
    freopen("D:\in.txt", "r", stdin);
    freopen("D:\out.txt", "w", stdout);
    
    int a, b;
    while (cin >> a >> b)
        cout << a + b << endl;
    fclose(stdin);
    
    fclose(stdout);

    return 0;
}

#include<iostream>
using namespace std;

int main()
{
    int a, b;
    freopen("D:\in.txt", "r", stdin);
    freopen("D:\out.txt", "w", stdout);
    while (scanf("%d%d", &a, &b) == 2)
        printf("%d
", a + b);
    fclose(stdin);
    fclose(stdout);

    return 0;
}

#include<iostream>
using namespace std;

int main()
{
    int a, b;
    freopen("D:\in.txt", "r", stdin);
    freopen("D:\out.txt", "w", stdout);
    while (scanf("%d%d", &a, &b) == 2)
        printf("%d
", a + b);
    //fclose(stdin);
    //fclose(stdout);
    freopen("CON", "r", stdin);
    freopen("CON", "w", stdout);
    printf("Hello World
");
    scanf("%d%d", &a,&b);

    return 0;
}

#include<iostream>
using namespace std;

int main()
{
    int a, b;
    freopen("D:\in.txt", "r", stdin);
    freopen("D:\out.txt", "w", stdout);
    while (scanf("%d%d", &a, &b) == 2)
        printf("%d
", a + b);
    //fclose(stdin);
    //fclose(stdout);
    freopen("CON", "r", stdin);
    freopen("CON", "w", stdout);
    printf("Hello World
");
    scanf("%d%d", &a,&b);

    return 0;
}

需要注意，这里其实没有真正关闭，只是再次重定向，回到控制台。

在windows/DOS，读文件后用freopen("CON", "r", stdin)，写文件后  freopen("CON", "w", stdout)。

在linux中，控制台设备是 /dev/console：freopen("/dev/console", "r", stdin)。