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From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.std.c++
Subject: Re: Pure virtual destructors: good or bad idea?
Summary: Pure virtual destructors can be declared but then they must be defined too
Message-ID: <11333@alice.UUCP>
Date: 14 Sep 90 14:43:54 GMT
References: <77210003@hpclscu.HP.COM>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 37
Posted: Fri Sep 14 15:43:54 1990

In article <77210003@hpclscu.HP.COM>, shankar@hpclscu.HP.COM (Shankar Unni) writes:
> PURE VIRTUAL DESTRUCTORS:
> 
> One of our users came up with an interesting construct:
> 
>    class Base {
>      virtual ~Base() = 0;	// pure virtual destructor
	virtual	void	f() = 0;  // example pure virtual function
>    };
>    
>    class Derived { /*...*/ };

If you want to do this you must define the Base destructor, e.g.

	Base::~Base() {}

It is a little-known (and perhaps unfortunate) fact that pure virtual
functions can be defined.  They can only be called through the
explicitly qualified name, e.g.,

	Base*	bp = new Derived;
	bp->Base::f();

would call Base::f() if it has been defined, and cause a link error
otherwise.  The destructor is a slightly special case (:-)), since the
compiler automatically generates the equivalent of an explicitly
qualified call to it in the destructor of the derived class.

An alternative way to define the language would be to disallow
defining pure virtual functions and (therefore) disallow pure virtual
destructors.  This would give the programmer somewhat less flexibility
but would not leave the compiler wondering whether a pure virtual
function was going to be defined or not.
-- 
		Jonathan E. Shopiro
		AT&T Bell Laboratories, Warren, NJ  07059-0908
		shopiro@research.att.com   (201) 580-4229