How to redirect cin and cout to files?

Question

How can I redirect cin to in.txt and cout to out.txt?

Answer 1

Here is an working example of what you want to do. Read the comments to know what each line in the code does. I've tested it on my pc with gcc 4.6.1; it works fine.

#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 } 

You could save and redirect in just one line as:

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

Here std::cin.rdbuf(in.rdbuf()) sets std::cin's buffer to in.rdbuf() and then returns the old buffer associated with std::cin. The very same can be done with std::cout — or any stream for that matter.

Hope that helps.

Answer 2

Just write

#include <cstdio> #include <iostream> using namespace std;  int main() {     freopen("output.txt","w",stdout);     cout<<"write in file";     return 0; }