CODEWITHSUNDEEP
c++stdout
Unapiedra
Unapiedra

Reputation: 16207

Redirecting function output to /dev/null

I am using a library that is printing a warning message to cout or cerr. I don't want this warning message to reach the output of my program. How can I catch this output and put it into /dev/null or similar?

MWE:

#include <iostream>

void foo()
{
    std::cout << "Boring message. " << std::endl;
};

int main()
{
    foo();
    std::cout << "Interesting message." << std::endl;
    return 0;
}

The output should be: 

Interesting message.

How should I modify main to get the desired output? (foo must not be changed.)

I tried using freopen() and fclose(stdout) as suggested in this question How can I redirect stdout to some visible display in a Windows Application?. The result is that nothing is printed.

Upvotes: 23

Views: 20469

Answers (3)

soulcheck
soulcheck

Reputation: 36777

Use ios::rdbuf:

#include <iostream>

void foo()
{
    std::cout << "Boring message. " << std::endl;
}

int main()
{
    ofstream file("/dev/null");

    //save cout stream buffer
    streambuf* strm_buffer = cout.rdbuf();

    // redirect cout to /dev/null
    cout.rdbuf(file.rdbuf());

    foo();

    // restore cout stream buffer
    cout.rdbuf (strm_buffer);

    std::cout << "Interesting message." << std::endl;
    return 0;
}

Upvotes: 8

wreckgar23
wreckgar23

Reputation: 1045

May I suggest a hack? Set a bad/fail bit on the relevant stream before use of a library function.

#include <iostream>

void foo()
{
    std::cout << "Boring message. " << std::endl;
}

int main()
{
    std::cout.setstate(std::ios::failbit) ;
    foo();
    std::cout.clear() ;
    std::cout << "Interesting message." << std::endl;
    return 0;
}

Upvotes: 25

Sebastian Mach
Sebastian Mach

Reputation: 39109

If you are sure that thing does not redirect output (e.g. to /dev/tty/, which would be standard-out again) (which I don't think), you could redirect before calling them.

#include <iostream>
#include <sstream>

void foobar() { std::cout << "foobar!\nfrob!!"; }

int main () {    
    using namespace std;

    streambuf *old = cout.rdbuf(); // <-- save        
    stringstream ss;

    cout.rdbuf (ss.rdbuf());       // <-- redirect
    foobar();                      // <-- call
    cout.rdbuf (old);              // <-- restore

    // test
    cout << "[[" << ss.str() << "]]" << endl;
}

Upvotes: 20

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