如何将 cin 和 cout 重定向到文件？

我怎样才能重定向cin to in.txt and cout to out.txt?

这是您想要做的事情的一个工作示例。阅读注释以了解代码中每一行的作用。我在我的电脑上用 gcc 4.6.1 测试了它；效果很好。

#include <iostream>
#include <fstream>
#include <string>

void f()
{
    std::string line;
    while(std::getline(std::cin, line))  //input from the file in.txt
    {
        std::cout << line << "\n";   //output to the file out.txt
    }
}
int main()
{
    std::ifstream in("in.txt");
    std::streambuf *cinbuf = std::cin.rdbuf(); //save old buf
    std::cin.rdbuf(in.rdbuf()); //redirect std::cin to in.txt!
    
    std::ofstream out("out.txt");
    std::streambuf *coutbuf = std::cout.rdbuf(); //save old buf
    std::cout.rdbuf(out.rdbuf()); //redirect std::cout to out.txt!

    std::string word;
    std::cin >> word;           //input from the file in.txt
    std::cout << word << "  ";  //output to the file out.txt
    
    f(); //call function


    std::cin.rdbuf(cinbuf);   //reset to standard input again
    std::cout.rdbuf(coutbuf); //reset to standard output again

    std::cin >> word;   //input from the standard input
    std::cout << word;  //output to the standard input
}

你可以save and redirect仅一行：

auto cinbuf = std::cin.rdbuf(in.rdbuf()); //save and redirect

Here std::cin.rdbuf(in.rdbuf()) sets std::cin's缓冲到in.rdbuf()然后返回与关联的旧缓冲区std::cin。同样可以用std::cout— 或任何stream对于这个问题。