Correctly over-loading a stringbuf to replace cout in a MATLAB mex file

Question

MathWorks currently doesn't allow you to use cout from a mex file when the MATLAB desktop is open because they have redirected stdout. Their current workaround is providing a function, mexPrintf, that they request you use instead. After googling around a bit, I think that it's possible to extend the std::stringbuf class to do what I need. Here's what I have so far. Is this robust enough, or are there other methods I need to overload or a better way to do this? (Looking for portability in a general UNIX environment and the ability to use std::cout as normal if this code is not linked against a mex executable)

class mstream : public stringbuf {
public:
  virtual streamsize xsputn(const char *s, std::streamsize n) 
  {
mexPrintf("*s",s,n);
return basic_streambuf<char, std::char_traits<char>>::xsputn(s,n);
  }
}; 

mstream mout;
outbuf = cout.rdbuf(mout.rdbuf());

Answer 1

cout is a particular character output stream. If you want a cout that writes to a file, use an fstream, particularly an ofstream. They have the same interface that cout provides. Additionally, if you want to grab their buffer (with rdbuf) you can.

Answer 2

You don't really want to overload std::stringbuf, you want to overload std::streambuf or std::basic_streambuf (if you want to support multiple character types), also you need to override the overflow method as well.

But I also think you need to rethink your solution to your problem.

cout is just a ostream, so if all classes / functions takes a ostream then you can pass in anything you like. e.g. cout, ofstream, etc

If that's too hard then I would create my own version of cout, maybe called mycout that can be defined at either compiler time or runtime time (depending on what you want to do).

A simple solution may be:

#include <streambuf>
#include <ostream>

class mystream : public std::streambuf
{
public:
    mystream() {}

protected:
    virtual int_type overflow(int_type c)
    {
        if(c != EOF)
        {
            char z = c;
            mexPrintf("%c",c);
            return EOF;
        }
        return c;
    }

    virtual std::streamsize xsputn(const char* s, std::streamsize num)
    {
        mexPrintf("*s",s,n);
        return num;
    }
};

class myostream : public std::ostream
{
protected:
    mystream buf;

public:
    myostream() : std::ostream(&buf) {}
};

myostream mycout;

And the cout version could just be:

typedef std::cout mycout;

A runtime version is a bit more work but easily doable.

Answer 3

Shane, thanks very much for your help. Here's my final working implementation.

class mstream : public std::streambuf {
public:
protected:
  virtual std::streamsize xsputn(const char *s, std::streamsize n); 
  virtual int overflow(int c = EOF);
};

...

std::streamsize 
mstream::xsputn(const char *s, std::streamsize n) 
{
  mexPrintf("%.*s",n,s);
  return n;
}

int 
mstream::overflow(int c) 
{
    if (c != EOF) {
      mexPrintf("%.1s",&c);
    }
    return 1;
}

...

mstream mout;
std::streambuf outbuf = std::cout.rdbuf(&mout);

...

std::cout.rdbuf(outbuf);