Redirect std::cerr to log file.

gistfile1.cpp

namespace {

    // 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 output stream to a string.
    class capture_outputs
    {
        std::ostringstream myContents;
        const redirect myRedirect;
    public:
        capture_outputs ( std::ostream& stream=std::cout )
            : myContents(), myRedirect(myContents, stream)
        {}
        std::string contents () const
        {
            return (myContents.str());
        }
    };

}

int main ( int argc, char ** argv )
{
    // Redirect standard error stream to log file.
    std::ofstream log_file("stde.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);

    // ...
}

Good stuff.

For C++11 compilers adding

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

will clean up a compiler warning (at least for MS Visual Studio--I don't know if gcc can generate an assignment operator automagically or not).