Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!mcsun!ub4b!kulcs!herman
From: herman@kulcs.uucp (Herman Moons)
Newsgroups: comp.lang.c++
Subject: are 'friend's really necessary ??
Message-ID: <169@pollux.kulcs.uucp>
Date: 16 Mar 90 16:23:52 GMT
Reply-To: herman@kulcs.UUCP ()
Organization: Katholieke Universiteit Leuven, Dept. Computer Science
Lines: 51


I don't know whether this question appeared earlier on the net.
Are there any *good* reasons for having the friend concept in C++ ?

The example given in Lippman's book:

           class Screen;

           Screen myScreen;

           cout << myScreen;  // friend needed to implement this


does not seem convincing.  The friend mechanism is used in this case to
circumvent a bad design choice.  After all, << is an operator defined in
the stream classes, while outputting an object logically belongs to the
object itself (you say to the object: output yourself).  
Wouldn't it be better to say:

          class Screen {
              ostream& operator>>= (ostream& out); // no friend needed here
          };

          myScreen >>= cout;
          yourScreen >>= myScreen >>= cout;


Discussions with my colleagues didn't provide an example were friends were
really necessary.  We could always find ways to implement our classes such
that the information hiding principle wasn't violated.  We also couldn't
find a good example where data abstraction resulted in a serious loss of
efficiency (we could always find sensible inline functions that provided
fast access to the needed information).

So, if you have an example that really proves the necessity of the friend
mechanism, please post it.  After all, if there is no real necessity, the
mechanism is just an unnessary loophole in the language to defeat the 
information hiding principle.


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Herman Moons                                   Katholieke Universiteit Leuven
                                               Dept. of Computer Science
Tf : +32 (16) 20.06.56                         Celestijnenlaan 200A
Fax: +32 (16) 20.53.08                         B-3030 Heverlee-Leuven
                                               Belgium
e-mail: herman@kulcs.uucp
	herman@blekul60.bitnet
	herman@cs.kuleuven.ac.be

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