How to capture stdout/stderr with googletest?

Question

Is it possible to capture the stdout and stderr when using the googletest framework?

For example, I would like to call a function that writes errors to the console (stderr). Now, when calling the function in the tests, I want to assert that no output appears there.

Or, maybe I want to test the error behaviour and want to assert that a certain string gets printed when I (deliberately) produce an error.

Answer 1

Googletest offers functions for this:

testing::internal::CaptureStdout();
std::cout << "My test";
std::string output = testing::internal::GetCapturedStdout();

Answer 2

Avoiding having to do this is always a good design idea. If you really want to do it the following works:

#include <cstdio>
#include <cassert>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <iostream>

int main() {
   int fd = open("my_file.log", O_WRONLY|O_CREAT|O_TRUNC, 0660);
   assert(fd >= 0);
   int ret = dup2(fd, 1);
   assert(ret >= 0);
   printf("This is stdout now!\n");
   std::cout << "This is C++ iostream cout now!" << std::endl;
   close(fd);
}

To use stderr instead of stdout change the second argument to dup2 to be 2. For capturing without going via a file you could use a pipe pair instead.

Answer 3

Rather than do this, use dependency injection to remove the direct use of std::cout. In your test code use a mock object of class std:ostringstream as a mock object instead of the real std::cout.

So instead of this:

 void func() {
    ...
    std::cout << "message";
    ...
 }

 int main (int argc, char **argv) {
    ...
    func();
    ...
 }

have this:

 void func(std::ostream &out) {
    ...
    out << "message";
    ...
 }

 int main(int argc, char **argv) {
    ...
    func(std::cout);
    ...
 }

Answer 4

I have used this snippet before to redirect cout calls to a stringstream when testing output. Hopefully it might spark some ideas. I've never used googletest before.

// This can be an ofstream as well or any other ostream
std::stringstream buffer;

// Save cout's buffer here
std::streambuf *sbuf = std::cout.rdbuf();

// Redirect cout to our stringstream buffer or any other ostream
std::cout.rdbuf(buffer.rdbuf());

// Use cout as usual
std::cout << "Hello World";

// When done redirect cout to its old self
std::cout.rdbuf(sbuf);

Before redirecting back to the original output use your google test to check the output in buffer.