Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: cast of a bound pointer: why ?
Message-ID: <763@taumet.com>
Date: 12 Jun 91 16:22:55 GMT
References: <1991Jun11.191016.9873@beaver.cs.washington.edu>
Organization: Taumetric Corporation, San Diego
Lines: 39

pauld@cs.washington.edu (Paul Barton-Davis) writes:

>My understanding of this position is that it really boils down to an
>syntactic nicety:

>	ptr to member function != ptr to function

>Can anyone explain the necessity for this distinction, and perhaps
>more importantly, can anyone comment on the possibility that it will
>no longer be possible to use this construct ?

It is not a syntactic nicety, but a fundamental semantic issue.

A non-static member function requires a "this" pointer, which is
implicitly supplied by the compiler when you call it.  That is, given
	class C { ... foo(); ... } c;
when you write
	c.foo();
it really means something like this (with artificial syntax):
	C::foo(&c);

If you take the address of C::foo, cast it to an ordinary function
pointer, and call foo via that pointer, no "this" pointer is supplied.
	int (*fp)() = (int(*)())C::foo;
	fp();		// call C::foo with no "this" pointer
Function C::foo presumably uses "this", and will fail.

Now suppose foo() is a virtual function.  The actual function to be
called depends on the actual object it is called in conjuction with:
	C *p = ...;
	p->foo();
might call C::foo(), or another foo() in class C's hierarchy.  Therefore,
the pointer-to-member-function must carry with it enough data about 
the virtual function so that the right virtual function is called.
A simple address is not sufficient.  (There are further complications
with multiple inheritance when adjustment to "this" is required.)
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com