Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!ulysses!hector!jss
From: jss@hector.UUCP (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: What is C++ doing?
Keywords: behind my back in C, that is...
Message-ID: <11565@ulysses.homer.nj.att.com>
Date: 20 May 89 17:08:00 GMT
References: <2671@ssc-vax.UUCP>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
Organization: AT&T Bell Laboratories
Lines: 33

In article <2671@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes:
>
>  Anyway, Jerry's posting is a perfect example of my ignorance ;-).
>What *is* a _vptr?  I have also heard reference to the VTABLE
>in previous postings.  What are these animals?
>

When a class has virtual member functions its representation contains
a pointer (called a virtual pointer, or "vptr" for short)  to a table
(called a virtual table or "vtbl" for short)  that has entries
indicating the actual functions to be called when a virtual function
is called.

Thus given something like

	struct Base {
		virtual void f() ;
		} ;

	struct Derived : public Base {
		void f() ;
		} ;


	void fct(Base* b) { b->f() ; }
		// actual call involves using vptr to find the vtbl
		// entry that says exactly how to invoke f.
		// This is neccessary because Base::f or Derived::f
		// must be called depending on whether b points to an
		// object that was allocated as a Base or as a Derived.

Jerry Schwarz
AT&T Bell Labs, Murray Hill