C++ std::cerr redirect to file.

cerr2file.cpp

#define __TEST__

#ifdef __TEST__
#include <sstream>

// redirect outputs to another output stream.
class redirect_outputs {
    std::ostream &myStream;
    std::streambuf *const myBuffer;

  public:
    redirect_outputs(std::ostream &lhs, std::ostream &rhs = std::cout) : myStream(rhs), myBuffer(myStream.rdbuf())
    {
        myStream.rdbuf(lhs.rdbuf());
    }

    ~redirect_outputs()
    {
        myStream.rdbuf(myBuffer);
    }

    redirect_outputs &operator=(const redirect_outputs &) = delete;
};

// redirect output stream to a string.
class capture_outputs {
    std::ostringstream myContents;
    const redirect_outputs myRedirect;

  public:
    capture_outputs(std::ostream &stream = std::cout) : myContents(), myRedirect(myContents, stream)
    {
    }
    std::string contents() const
    {
        return (myContents.str());
    }
};
#endif !__TEST__

/*-----------------------------------------------------------------------------------------------------*/

int main ( int argc, char ** argv )
{
#ifdef __TEST__
    // Redirect standard error stream to log file.
    std::ofstream log_file("stderr.log", std::ios::binary);
    if (!log_file.is_open()) {
        std::cerr
            << "Could not open log file for writing."
            << std::endl;
        return (EXIT_FAILURE);
    }
    const redirect_outputs _(log_file, std::cerr);
#endif __TEST__

    // ...
}