Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!apple!rutgers!dptg!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: deleting objects with virtual bases
Summary: You need a virtual destructor
Message-ID: <9903@alice.UUCP>
Date: 14 Sep 89 15:03:12 GMT
References: <1984@hcr.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 50

In article <1984@hcr.UUCP>, stan@hcr.UUCP (Stan Jarzabek) writes:
- 
- Is it legal to delete an object with a virtual base through base pointer?
- 
- class A  { int a; };
Use

	class A
	{
		int a;
	public:
			virtual ~A() {}
	};

- 
- class B : virtual public A  { int b; };
- 
- class C : public B  { int c; };
- 
- main()
- {
- 	A* ap = new C;  
- 	delete ap;
- 
- 	C* cp = new C;  //core dumped
- }
- 
- As 'ap' does not really point to the beginning of the area allocated by 'new C',
- 'delete ap' causes a problem. (In 2.0, operator delete() calls free().)
- In fact, a subsequent call to operator new() causes core dump. The same problem 
- may occur when you delete an object with multiple bases through a base pointer.
- 
- The language manual does not address this issue: is this a language definition 
- problem, the problem of current implementation, or the thing a user should be 
- responsible for?
- 
- Stan Jarzabek

All this will work fine if the base class has a virtual destructor.
It can be empty.  The virtual function mechanism takes care of fixing
up the ``this'' pointer so the called function ses the object it
expects.

As a general rule, any class that has any virtual functions should
have a virtual destructor.

-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229