Using freopen() to print to file and screen

Question

I am trying to use freopen() to print to a text file and the screen, but I am only achieving the printing to a file.

I was wondering if there was an easy to save the

Answer 1

The easy way in a UNIX-like environment is to use the shell command tee:

$ my-program | tee output.txt

will copy stdout to the terminal, and also to the file output.txt.

---

If you have to do it in code, you could use your own output stream instead of cout, which forwards every operator<< to two (or more) ostreams. This feels nicer (to me) than mucking around with the C output file underlying the C++ ostream cout.

#include 

class Tee {
    std::ostream &first, &second;

    template friend Tee& operator<< (Tee&, T);

public:
    Tee(std::ostream &f, std::ostream &s) : first(f), second(s) {}
};

template 
Tee& operator<< (Tee &t, T val)
{
    t.first << val;
    t.second << val;
    return t;
}

Then, if you replace your freopen line with:

std::ofstream outfile("file.txt");
Tee tee(std::cout, outfile);

you can just use tee << instead of cout <<.

Note that you'll either need to pass tee into your functions, or make it a global for that to work.