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From: jgro@lia (Jeremy Grodberg)
Newsgroups: comp.lang.c++
Subject: Re: cast of a bound pointer: why ?
Message-ID: <1991Jun14.201641.688@lia>
Date: 14 Jun 91 20:16:41 GMT
References: <1991Jun11.191016.9873@beaver.cs.washington.edu>
Reply-To: jgro@lia.com (Jeremy Grodberg)
Lines: 31

In article <1991Jun11.191016.9873@beaver.cs.washington.edu> 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 ?

The difference is that member functions have an implied (or hidden) 
first argument which is the "this" pointer.  Thus a pointer to a member
function of class T taking an int is really a pointer to a function
taking a pointer to an object of class T, and an int.  Thus, member 
functions are not interchangable with non-member functions (the exception
being static member functions, which do *not* take a "this" pointer).

You could argue that you should be able to cast a member function to a 
non-member function with an object pointer, but a) this would not 
buy you much, if anything, and b) it would destroy the abstraction of 
member functions being associated with object, and c) (I believe) the
position of the "this" pointer in the function call is implementation 
dependant.

So conversions from pointer to member function to pointer to non-member
function (and vice versa) will be forbidden as soon as is practical.

-- 
Jeremy Grodberg      "Show me a new widget that's bug-free, and I'll show
jgro@lia.com         you something that's been through several releases."