How to capture stdout/stderr with googletest?

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.

Googletest offers functions for this:

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

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.

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.

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);
    ...
 }

Putting Wgaffa's suggestion (which I like) to a Google Test fixture, one might write:

namespace {

    class MyTestFixture : public ::testing::Test {
    protected:
        MyTestFixture() : sbuf{nullptr} {
            // intentionally empty
        }

        ~MyTestFixture() override = default;

        // Called before each unit test
        void SetUp() override {
            // Save cout's buffer...
            sbuf = std::cout.rdbuf();
            // Redirect cout to our stringstream buffer or any other ostream
            std::cout.rdbuf(buffer.rdbuf());
        }

        // Called after each unit test
        void TearDown() override {
            // When done redirect cout to its old self
            std::cout.rdbuf(sbuf);
            sbuf = nullptr;
        }

        // The following objects can be reused in each unit test
    
        // This can be an ofstream as well or any other ostream
        std::stringstream buffer{};
        // Save cout's buffer here
        std::streambuf *sbuf;
    };

    TEST_F(MyTestFixture, StackOverflowTest) {
        std::string expected{"Hello"};
        // Use cout as usual
        std::cout << expected;
        std::string actual{buffer.str()};
        EXPECT_EQ(expected, actual);
    }
} // end namespace

We are doing exactly what you are referring to.

First we created some macros:

    #define CAPTURE_STDOUT StdoutRedirect::instance().redirect();
    #define RELEASE_STDOUT StdoutRedirect::instance().reset();
    #define ASSERT_INFO( COUNT, TARGET )   \
      ASSERT_PRED_FORMAT2(OurTestPredicates::AssertInfoMsgOutput, TARGET, COUNT );

See this answer for capturing stdout and stderr: https://stackoverflow.com/a/5419409/9796918 Just use their BeginCapture(), EndCapture() in place of our redirect() and reset().

In the AssertInfoMsgOutput method:

    AssertionResult OurTestPredicates::AssertInfoMsgOutput( const char* TARGET,
        const char* d1,
        const char* d2,
        int         COUNT )
    {
      int count = 0;
      bool match = false;
      std::string StdOutMessagge = GetCapture();
      // Here is where you process the stdout/stderr info for the TARGET, and for
      // COUNT instances of that TARGET message, and set count and match
      // appropriately
      ...
      if (( count == COUNT ) && match )
      {
        return ::testing::AssertionSuccess();
      }
      return :: testing::AssertionFailure() << "not found";
    }

Now in your unit test just wrap your calls that you want to capture stdout/stderr with:

    CAPTURE_STDOUT
    // Make your call to your code to test / capture here
    ASSERT_INFO( 1, "Foo bar" );
    RELEASE_STDOUT

Based on the answer of Wgaffa I made this helper class which can be constructed with either std::cout or std::cerr:

class CaptureHelper
{
public:
  CaptureHelper(std::ostream& ioStream)
    : mStream(ioStream),
    mIsCapturing(false)
  { }

  ~CaptureHelper()
  {
    release();
  }

  void capture()
  {
    if (!mIsCapturing)
    {
      mOriginalBuffer = mStream.rdbuf();
      mStream.rdbuf(mRedirectStream.rdbuf());
      mIsCapturing = true;
    }
  }

  std::string release()
  {
    if (mIsCapturing)
    {
      std::string wOutput = mRedirectStream.str();
      mStream.rdbuf(mOriginalBuffer);
      mIsCapturing = false;
      return wOutput;
    }
  }

private:
  std::ostream& mStream;
  bool mIsCapturing;
  std::stringstream mRedirectStream;
  std::streambuf* mOriginalBuffer;

};