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From: dld@F.GP.CS.CMU.EDU (David Detlefs)
Newsgroups: comp.lang.c++
Subject: Re: Filesystem classes: how to?
Message-ID: <DLD.90Jun8182234@F.GP.CS.CMU.EDU>
Date: 8 Jun 90 22:22:34 GMT
References: <1567@swbatl.sbc.com> <1642@ns-mx.uiowa.edu>
Organization: CMU CS Department
Lines: 44
In-reply-to: williams@umaxc.weeg.uiowa.edu's message of 8 Jun 90 15:32:21 GMT

Dave
Brian Gilstrap sends another scenario, and asks for comments:

If you absolutely, positively, can't use virtual functions to do
whatever you need to do (and it's a different way of thinking, I've
run in to quite a number of situations were I think "shoot, there's no
*way* I'm gonna be able to use virtual functions!" but I was
eventually able to), well, If you've really gotta be able to get a
CharacterFile out of something your program is thinking of as a
FileSystemNode, then I'd do something like this:

class FileSystemNode {
  // ...
  public:
    virtual CharacterFile* toCharacterFile() { return NULL; }
}

class CharacterFile : virtual public FileSystemNode {
  public:
    CharacterFile* toCharacterFile() { return this; }
}

Given a FileSystemNode object, I can then invoke toCharacterFile and
get back either NULL or a pointer to the object (safely) considered as a
CharacterFile.  Note that if there is ever a *really* good C++ compiler
it may be able to optimize the invocations of these virtual functions
away entirely, since they are inline.

Drawbacks:

By symmetry, you'd want a toX function in FileSystemNode for every
subclass X.  Therefore, when you add a new subclass Y to FileSystemNode,
you need to add a new toY function to the definition of
FileSystemNode.  This can require a lot of recompilation (virtual
table size changes, for one thing), but no reprogramming -- previous
code stays correct.

For what it's worth...

--
Dave Detlefs			Any correlation between my employer's opinion
Carnegie-Mellon CS		and my own is statistical rather than causal,
dld@cs.cmu.edu			except in those cases where I have helped to
				form my employer's opinion.  (Null disclaimer.)