TITLE: output to multiple streams

(Newsgroups: comp.lang.c++.moderated, 25 Sep 96)

PROBLEM: steve@brecher.reno.nv.us (Steve Brecher)

[wants to 'tee' streamed output]

instead of

cout << this << that << other;
lout << this << that << other;

I'd like to write something like

my_dual_ostream << this << that << other;


RESPONSE: herbs@cntc.com (Herb Sutter)

[ I believe Herb is the author of "Learn C++ in 21 Days"
  or some title like that. -adc ]

You might try the following (this is just a quick hack for
demonstration purposes):

  /* op<< is global because my compiler doesn't support member
     templates, and the _targets list is public because my
     compiler also doesn't let me make the global op<< a friend */

  class multiway_ostream
  {
  public:
    void add( ostream& os ) { _targets.push_back(&os); };
    list<ostream*> _targets;
  };

  template<class T>
  multiway_ostream& operator<<(multiway_ostream& mos, const T& t)
  {
    for( list<ostream*>::iterator i = mos._targets.begin(); 
         i != mos._targets.end(); 
         ++i )
    {
      *(*i) << t;
    }
    return mos;
  }

Now you can multicast to as many ostreams as you want:

  int main( int argc, char* argv[])
  {
    ofstream of1( "test1.out" );
    ofstream of2( "test2.out" );

    multiway_ostream mos;
    mos.add( cout );
    mos.add( of1 );
    mos.add( of2 );

    mos << "Hello" << " world" << endl; // goes to all three ostreams

    return 0;
  }

Note that I deliberately didn't inherit from ostream because I didn't
want a multiway_ostream passed around and used as an ostream.  Without
spending some time looking at ostream's definition, I'm not sure that
all of the inherited state and functions would be appropriate.  It's
fine for this use, though.