How to Redirect C Standard Input and Output to External Files?

Redirecting Input and Output to External Files

Redirecting input from standard input (cin) and output from standard output (cout) to external files is a useful technique for testing, debugging, or analyzing program behavior.

To redirect cin to a specified file (e.g., in.txt):

• Open an ifstream object for the input file.
• Save the old buffer associated with cin using cin.rdbuf().
• Redirect cin to the input file's buffer using cin.rdbuf(in.rdbuf()).

To redirect cout to a specified file (e.g., out.txt):

• Follow the same steps as for redirecting cin, but use ofstream object and cout.rdbuf() instead.

Example Code:

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

int main() {
    // Redirect cin to in.txt
    std::ifstream in("in.txt");
    std::streambuf *cinbuf = std::cin.rdbuf();
    std::cin.rdbuf(in.rdbuf());

    // Redirect cout to out.txt
    std::ofstream out("out.txt");
    std::streambuf *coutbuf = std::cout.rdbuf();
    std::cout.rdbuf(out.rdbuf());

    // Read and write to the redirected streams
    std::string line;
    while (std::getline(std::cin, line)) {
        std::cout << line << "\n";
    }

    // Reset to standard input and output
    std::cin.rdbuf(cinbuf);
    std::cout.rdbuf(coutbuf);
}

Note: You can also redirect both input and output in a single line using the following syntax:

auto cinbuf = std::cin.rdbuf(in.rdbuf());
auto coutbuf = std::cout.rdbuf(out.rdbuf());

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