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From: kramer@tpwest.enet.dec.com
Newsgroups: comp.lang.c++
Subject: Restricting Type of Member in Subclass possible?
Message-ID: <1342@shodha.dec.com>
Date: 6 Jun 90 19:56:08 GMT
Sender: news@shodha.dec.com
Reply-To: kramer@tpwest.enet.dec.com ()
Organization: Digital Equipment Corporation
Lines: 28

Hi there!
Here is a beginners question:

Assume I have a class Node and a class Tree.  Tree has a protected
member: Node * p.
Now I like to add some functionality without changing Nodes and Trees.  
I like to add some marking stuff to Node, so I create a DerivedNode
class with a function mark().  And I want to be able to mark Trees which
consist entirely of DerivedNode instances, so I create a DerivedTree
with function mark().

Inside DerivedTree:mark() I call p->mark()

Sure enough, the compiler complains because *p is of type Node, and I
did not want to define
Node::mark().

What I would like to do is to state that for DerivedTrees *p is of type
DerivedNode.  Which is alright
since DerivedNode is a subclass of Node (so all stuff defined in Tree
would still work).

Is there any way of doing that?
Should I restructure my abstractions? (if so, how? (What if Node and
Tree are library classes?))

Thanks a lot!!!!
	axel