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(iostream-295.info)Overflow

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Simple output re-direction by redefining `overflow'
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   Suppose you have a function `write_to_window' that writes characters
to a `window' object.  If you want to use the ostream function to write
to it, here is one (portable) way to do it.  This depends on the
default buffering (if any).

     #include <iostream.h>
     /* Returns number of characters successfully written to WIN. */
     extern int write_to_window (window* win, char* text, int length);
     
     class windowbuf : public streambuf {
         window* win;
       public:
         windowbuf (window* w) { win = w; }
         int sync ();
         int overflow (int ch);
         // Defining xsputn is an optional optimization.
         // (streamsize was recently added to ANSI C++, not portable yet.)
         streamsize xsputn (char* text, streamsize n);
     };
     
     int windowbuf::sync ()
     { streamsize n = pptr () - pbase ();
       return (n && write_to_window (win, pbase (), n) != n) ? EOF : 0;
     }
     
     int windowbuf::overflow (int ch)
     { streamsize n = pptr () - pbase ();
       if (n && sync ())
         return EOF;
       if (ch != EOF)
         {
           char cbuf[1];
           cbuf[0] = ch;
           if (write_to_window (win, cbuf, 1) != 1)
             return EOF;
         }
       pbump (-n);  // Reset pptr().
       return 0;
     }
     
     streamsize windowbuf::xsputn (char* text, streamsize n)
     { return sync () == EOF ? 0 : write_to_window (win, text, n); }
     
     int
     main (int argc, char**argv)
     {
       window *win = ...;
       windowbuf wbuf(win);
       ostream wstr(&wbuf);
       wstr << "Hello world!\n";
     }