Using an Output Stream for Debugging

Background

There are many different schools of how to debug a program (yeah, I know - write it correctly from the start), but whatever tools you prefer, it is often very convenient just to use the printf approach. Well, since you are a modern C++ kind of person, you don't really want printf, you want to use an output stream, like cerr.

There's just one problem though, you are writing Windows applications, so there is no standard error output to write to. Not true, actually there is, and you've probably used it many times already. When you use one of the TRACE macros, the result ends up in the Debug pane of Visual Studio. MFC goes through great pains to turn off all this debug output in Release builds, but it can often be very convenient to keep it.

Of course, you won't see any output in Visual Studio from a release build, but there are other programs that can capture this output. A good one is DebugView from www.sysinternals.com. Incidentally, if you start this program, you'll probably see like a million messages from Internet Explorer. There you go, even Microsoft practise Release build debugging!

The Problem

So, how to direct a standard output stream to the debug terminal? In general, the correct answer is to derive from streambuf and connect the new class to a normal ostream. In this way, all stream inserters and manipulators work as expected. In this case, we can save some work by deriving from stringbuf instead. So, without further ado, here's how to do it.

The Code

First, we include the needed headers.

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

Now for the real work: Since we are using a stringbuf, we only need to override the sync function.

The sync method is what actually transfers the text in the put area to whatever output destination the streambuf uses, in this case by calling the API function OutputDebugString.

template <class CharT, class TraitsT = std::char_traits<CharT> >
class basic_debugbuf : 
    public std::basic_stringbuf<CharT, TraitsT>
{
public:

    virtual ~basic_debugbuf()
    {
        sync();
    }

protected:

    int sync()
    {
        output_debug_string(str().c_str());
        str(std::basic_string<CharT>());    // Clear the string buffer

        return 0;
    }

    void output_debug_string(const CharT *text) {}
};

Next, I specialize the output routine so it calls the Ansi or Unicode API as appropriate.

template<>
void basic_debugbuf<char>::output_debug_string(const char *text)
{
    ::OutputDebugStringA(text);
}

template<>
void basic_debugbuf<wchar_t>::output_debug_string(const wchar_t *text)
{
    ::OutputDebugStringW(text);
}

That's really all you need, but as a convenience, I also provide a class derived from basic_ostream that connects the output stream to the basic_debugbuf just created.

In order to work just like cout, you should then create a global object of type dostream or wdostream and use that to output to.

template<class CharT, class TraitsT = std::char_traits<CharT> >
class basic_dostream : 
    public std::basic_ostream<CharT, TraitsT>
{
public:

    basic_dostream() : std::basic_ostream<CharT, TraitsT>
                (new basic_debugbuf<CharT, TraitsT>()) {}
    ~basic_dostream() 
    {
        delete rdbuf(); 
    }
};

typedef basic_dostream<char>    dostream;
typedef basic_dostream<wchar_t> wdostream;

History

• 18-Apr-2001 - Original version
• 23-Nov-2001 - Updated to use a stringbuf as suggested by Jim Barry.
  Also fixed the remaining HTML markup errors that made the code un-compilable