Path: utzoo!attcan!uunet!brunix!doorknob!jak
From: jak@cs.brown.edu (Jak Kirman)
Newsgroups: comp.lang.c++
Subject: Deriving new stream types: tstream
Message-ID: <JAK.91Jan31095444@bimini.cs.brown.edu>
Date: 31 Jan 91 14:54:44 GMT
Sender: news@brunix.UUCP
Reply-To: jak@cs.brown.edu
Organization: Department of Computer Science, Brown University
Lines: 47

I would like to create a type which behaves precisely as does an ostream
except that the constructor would take two ostream references, and every
operation would affect both ostreams equally.

This would be very useful for debugging, where I often want a complete
transcript of the debugging messages in a file, and also some of the
debugging messages on the terminal (a run-time switch can make different
outputs go to the file or to the tstream).

If I spent a long time poring over the manuals, I could probably figure
out how to do this right, but I thought I would first ask if anyone else
has done this, or anything similar enough to provide either a cookbook
approach or some hints on what to avoid or worry about.

I managed to get something similar working by deriving a type
tfilebuf from filebuf, and a type tstreambase from ios, containing a
tstreambuf, then to derive tstream from ostream and tstreambase.

                      ios
      filebuf          |
            |     Tstreambase    ostream
          Tfilebuf        \      /
                           Tstream

However, this was aesthetically unpleasing, and less general than I
would have liked.  I also only redefined the overflow function; I am a
little concerned that some of the other virtual functions might also get
called at some point.

Clearly, in order to get the functionality of an ostream, I need to
inherit from ostream, but since an ostream has data, I can only contain
one ostream; I now have an assymetry between the two ostreams which are
being printed to: one is a base class and one is a member.  Also, why
don't all the output functions defined on ostream resolve to calling a
single function, like "put"?  That surely would have made inheriting
from ostream much simpler.

I confess to not understanding the workings of streams fully, and would
appreciate a brief, higher level description of how to derive from them
than is presented by the man pages.

Thanks.
                                Jak                            jak@cs.brown.edu
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I wouldn't recommend sex, drugs or insanity for everyone, but they've
always worked for me.
                                                       -- Hunter S. Thompson