C++ output stream proxy that adds a prefix to every line

debug.cpp

#include "debug.h"

debug_stream debug;

debug.h

#ifndef DEBUG_H
#define DEBUG_H

#include <iostream>

class debug_stream {
public:
  debug_stream() : new_line(true), on(false) {}

  template<class T>
  debug_stream &operator<<(T val) {
    if(new_line) {
      std::cerr << "-- ";
      new_line = false;
    }
    std::cerr << val;
    return *this;
  }

  debug_stream &operator<<(std::ostream& (*fn)(std::ostream&)) {
    std::cerr << fn;
    if(fn == (std::ostream& (*)(std::ostream&)) std::endl) {
      new_line = true;
    }
    return *this;
  }

  bool on;

private:
  bool new_line;
};

extern debug_stream debug;

#endif

This code comes from my work for CS 130A (PA2: Binary Search Trees, AVL Trees, and Splay Trees). Once I realized I didn't need it, I didn't want to throw it away, so I packaged it into a Gist. :)