Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!husc6!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: destructors, derived classes, and pointers
Message-ID: <8372@alice.UUCP>
Date: 30 Oct 88 17:31:55 GMT
Article-I.D.: alice.8372
References: <16219@agate.BERKELEY.EDU>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 47

In article <16219@agate.BERKELEY.EDU>, pete@violet.berkeley.edu (Pete Goodeve) writes:

> The problem is that all the managing procedure has is a pointer to the
> object (of the base type naturally).  Now I would expect that a delete call
> on this pointer would detect the correct destructor to invoke before it
> actually disposed of the memory, but NOOO!  It calls ONLY the parent type
> destructor!!!  So much for nice neat cleanup...

The solution to your problem is to declare the destructor as
virtual in the base class.  For more detail, see my article
``An example of dynamic binding in C++'' in the Journal of
Object-Oriented Programming, vol. 1, #3.

Example of the syntax:

	class Node: {
	// stuff
	public:
		Node();
		virtual ~Node();
	// more stuff
	};

	class SpecialNode: public Node {
	// stuff
	public:
		SpecialNode();
		~SpecialNode();
	// more stuff
	};

You only need to make the destructor virtual in the base class.
You don't -- and indeed, can't -- make any constructors virtual.

Caveat: some versions of the C++ translator have a bug
that doesn't get things quite right unless every derived class
has an explicit destructor.  To avoid getting bitten by this bug,
write a destructor even if you don't need it:

	class NodeWithEmptyDestructor: public Node {
	// stuff
	public:
		~NodeWithEmptyDestructor() { }
	};
-- 
				--Andrew Koenig
				  ark@europa.att.com