How to redirect standard output to output window from Visual Studio

Question

Is it possible to redirect standard output to the output window from Visual Studio?

I use OutputDebugString in my program, but I use some libraries that have output debug messages with printf's or cout's.

Answer 1

From Redirecting cerr and clog to OutputDebugString():

#include <ostream>
#include <Windows.h>

/// \brief This class is derives from basic_stringbuf which will output
/// all the written data using the OutputDebugString function
template<typename TChar, typename TTraits = std::char_traits<TChar>>
class OutputDebugStringBuf : public std::basic_stringbuf<TChar,TTraits> {
public:
    explicit OutputDebugStringBuf() : _buffer(256) {
        setg(nullptr, nullptr, nullptr);
        setp(_buffer.data(), _buffer.data(), _buffer.data() + _buffer.size());
    }

    ~OutputDebugStringBuf() {
    }

    static_assert(std::is_same<TChar,char>::value ||
                    std::is_same<TChar,wchar_t>::value,
                  "OutputDebugStringBuf only supports char and wchar_t types");

    int sync() try {
        MessageOutputer<TChar,TTraits>()(pbase(), pptr());
        setp(_buffer.data(), _buffer.data(), _buffer.data() + _buffer.size());
        return 0;
    }
    catch(...) {
        return -1;
    }

    int_type overflow(int_type c = TTraits::eof()) {
        auto syncRet = sync();
        if (c != TTraits::eof()) {
            _buffer[0] = c;
            setp(_buffer.data(), _buffer.data() + 1, _buffer.data() + _buffer.size());
        }
        return syncRet == -1 ? TTraits::eof() : 0;
    }


private:
    std::vector<TChar> _buffer;

    template<typename TChar, typename TTraits>
    struct MessageOutputer;

    template<>
    struct MessageOutputer<char,std::char_traits<char>> {
        template<typename TIterator>
        void operator()(TIterator begin, TIterator end) const {
            std::string s(begin, end);
            OutputDebugStringA(s.c_str());
        }
    };

    template<>
    struct MessageOutputer<wchar_t,std::char_traits<wchar_t>> {
        template<typename TIterator>
        void operator()(TIterator begin, TIterator end) const {
            std::wstring s(begin, end);
            OutputDebugStringW(s.c_str());
        }
    };
};

Then:

int main() {
    #ifndef NDEBUG
        #ifdef _WIN32
            static OutputDebugStringBuf<char> charDebugOutput;
            std::cerr.rdbuf(&charDebugOutput);
            std::clog.rdbuf(&charDebugOutput);

            static OutputDebugStringBuf<wchar_t> wcharDebugOutput;
            std::wcerr.rdbuf(&wcharDebugOutput);
            std::wclog.rdbuf(&wcharDebugOutput);
        #endif
    #endif

    ...

    // Will be displayed in the debugger
    std::cerr << "Error: something bad happened" << std::endl;

    ...
}

You might want to use it with

IsDebuggerPresent()

so that it still outputs to console when not run from the Visual Studio debugger.

Answer 2

Straightforward standard output redirection will not work, as there is no handle corresponding to OutputDebugString. However, there should be a way:

It could be done by redirecting standard output to a pipe, and then creating a thread which would read the pipe and print anything read from it using OutputDebugString.

Note: I was contemplating for a long ago to implement this, as I am facing exactly the same problem as you do (some libraries using printf or fprintf(stderr....). However, I never really did this. I have always ended modifying the libraries instead, and therefore I do not have a working implementation, but I think it should be feasible in principle.