Path: utzoo!mnetor!uunet!munnari!otc!mikem
From: mikem@otc.oz (Mike Mowbray)
Newsgroups: comp.lang.c++
Subject: Re: Virtual Constructors
Message-ID: <306@otc.oz>
Date: 3 Feb 88 22:49:16 GMT
References: <1788@ubc-cs.UUCP>
Organization: OTC Development Unit, Australia
Lines: 98

In article <1788@ubc-cs.UUCP>, coatta@ubc-csgrads.uucp (Terry Coatta) says:

 > Can someone post (re-post) some example code which uses virtual destructors

Here's a sketch of a library we use internally here. (Apologies if some of
the code isn't exactly right - it's just intended to convey the idea.)

Consider a linked list class where everything on the list is derived from a
"Dlink" class which has a virtual destructor:

	class Dlink
	{
		friend class Dlist;

		void	unlink();	// tidy up on either side

	    protected:
		Dlink  *prev, *next;
	    
		Dlink();		// set prev and next to null, say
		virtual ~Dlink();	// call unlink()
	};

	void Dlink::unlink()
	{
	    if (prev)
		prev->next = next;
	    if (next)
		next->prev = prev;
	    next=prev=NULL;
	}

	Dlink::~Dlink()
	{
	    unlink();
	}

	//------------------------------------------

	class Item1 : public Dlink
	{
		// .....
	    public:
		Item1();
		~Item1();		// do some other stuff to clean up

	class Item2 : public Dlink
	{
		// .....
	    public:
		Item2();
		~Item2();		// do some other stuff to clean up
	};
	//-------------------------------------------

	class Dlist
	{
		Dlink  key;
	    public:
		Dlist()		{ key.prev = key.next = &key; }
		~Dlist();	// Call the destructor for every Dlink on list

		void put(Dlink *d);
	};

	void Dlist::put(Dlink *d)
	{
	    d->unlink();		// for safety
	    d->prev = key->prev;
	    d->next = &key;
	    key->prev->next = d;
	    key->prev = d;
	}

	Dlist::~Dlist()
	{
	    while (key.next != &key)
		delete key.next;
	}
	//----------------------------------------------

Then you can put install Item1's and Item2's on a Dlist, and the correct
destructors will be called when the Dlist is deleted.

 > and is there such a thing as virtual constructors?

This would make no sense. A thing can't be created if one doesn't know exactly
(at compile time) what one wants to create.


			Mike Mowbray
		    Systems Development
			|||| OTC ||

		    ACSnet:  mikem@otc.oz
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