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From: tom@runxtsa.runx.oz.au (Thomas Antalffy)
Newsgroups: comp.lang.c++
Subject: vtbl for abstract class : WHY ?
Keywords: abstract class, vtbl
Message-ID: <2206@runxtsa.runx.oz.au>
Date: 30 Aug 90 01:11:53 GMT
Organization: RUNX Unix Timeshare.  Sydney, Australia.
Lines: 23


When I make a class abstract by assigning 0 to one of its virtual methods, 
my C++ compiler generates a virtual function table and puts the address of 

		char __pure_virtual_called() 

in the appropriate slot. This later creates a link error, because there is 
no default implementation of this function.

I assume that this mechanism is there to give you the chance of putting 
error handling in __pure_virtual_called().

What puzzles me is that my compiler (Glockenspiel 2.0c) gives an error 
message if a derived class of an abstract class does not redefine the method. 
I.e. the compiler guarantees that methods of abstract classes are implemented 
in all derived classes, so it will never have to call the abstract class's 
method. In fact, I do not see why it needs a virtual function table for an 
abstract class at all.

Does anyone know the answer to any of these questions ?

Regards,
Thomas.