redirect stdout/stderr to file under unix c++ - again

Question

What I want to do

redirect stdout and stderr to one or more files from inside c++

Why I need it

I am using an external, pr

Answer 1

For C++ iostreams, you can use the non-const overload of rdbuf to set std::cout to a std::filebuf. (This is best done by means of an RAII class, since you have to restore it before leaving main.) For C FILE*, you can use freopen, but I don't think you'll be able to restore it.

FWIW: both of these solutions use only standard C++ or C, so should be portable.

Answer 2

I was inspired by @POW and @James Kanze 's answers and put together a little RAII class for redirecting std::cout to a file. It is intended to demonstrate the principle.

Code:

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

// RAII for redirection
class Redirect {
    public:
    
    explicit Redirect(const std::string& filenm):
        _coutbuf{ std::cout.rdbuf() },   // save original rdbuf
        _outf{ filenm }
    {
        // replace cout's rdbuf with the file's rdbuf
        std::cout.rdbuf(_outf.rdbuf());
    }
    
    ~Redirect() {
        // restore cout's rdbuf to the original
        std::cout << std::flush;
        _outf.close();    ///< not really necessary
        std::cout.rdbuf(_coutbuf); 
    }
    
    private:
    
    std::streambuf* _coutbuf;
    std::ofstream _outf;
};

// == MAIN ==

int main(int argc, char* argv[]) {
    std::cout << "This message is printed to the screen" << std::endl;
    {
        // scope for the redirection
        Redirect redirect{ "output.txt" };
        std::cout << "This message goes to the file" << std::endl;
    }
    std::cout << "Printing to the screen again" << std::endl;
}

Output:

This message is printed to the screen

Printing to the screen again

Contents of the file "output.txt":

This message goes to the file

Answer 3

well i u'd better use freopen()

Usage Syntax:

freopen("RedToFile","r",stdout);
or
freopen("/dev/null","a",stdout);

the same goes for "stderr"

Answer 4

Are you looking for something like this :-

int main()
{
    // Save original std::cin, std::cout
    std::streambuf *coutbuf = std::cout.rdbuf();
    std::streambuf *cinbuf = std::cin.rdbuf(); 

    std::ofstream out("outfile.txt");
    std::ifstream in("infile.txt");

    //Read from infile.txt using std::cin
    std::cin.rdbuf(in.rdbuf());

    //Write to outfile.txt through std::cout 
    std::cout.rdbuf(out.rdbuf());   

    std::string test;
    std::cin >> test;           //from infile.txt
    std::cout << test << "  "; //to outfile.txt

    //Restore back.
    std::cin.rdbuf(cinbuf);   
    std::cout.rdbuf(coutbuf); 

}

From my earlier answer

Answer 5

In addition to afr0ck answer of freopen() I want to say that while using freopen() we should be careful. Once a stream like stdout or stdin is reopened with assigning the new destination(here the 'output.txt' file) always it remains for a program unless it has been explicitly change.

freopen("output.txt", "a", stdout);

Here the standard output stream stdout is reopened and assigned with the 'output.txt' file. After that whenever we use printf() or any other stdout stream like - putchar() then every output will goes to the 'output.txt'. To get back the default behavior (that is printing the output in console/terminal) of printf() or putchar() we can use the following line of code -

• for gcc, linux distribution like ubuntu - freopen("/dev/tty", "w", stdout);
• for Mingw C/C++, windows - freopen("CON", "w", stdout);

See the code example below -

#include <stdio.h>

int main() {
    
    printf("No#1. This line goes to terminal/console\n");

    freopen("output.txt", "a", stdout);
    printf("No#2. This line goes to the \"output.txt\" file\n");
    printf("No#3. This line aslo goes to the \"output.txt\" file\n");

    freopen("/dev/tty", "w", stdout); /*for gcc, diffrent linux distro eg. - ubuntu*/
    //freopen("CON", "w", stdout); /*Mingw C++; Windows*/
    printf("No#4. This line again goes to terminal/console\n");        

}

This code generate a 'output.txt' file in your current directory and the No#2 and No#3 will be printed in the 'output.txt' file.

Thanks

Answer 6

If you want to be able to reuse it, don't close stdoutfd in restore_stdout.