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From: Stephen Leake <stephen_leake@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada Popularity: Comparison of Ada/Charles with C++ STL (and Perl)
Date: 03 Oct 2004 08:47:38 -0400
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Date: 2004-10-03T08:47:38-04:00

"Randy Brukardt" <randy@rrsoftware.com> writes:

> But I admit that I still can't figure out what it is that this "pointer to
> member" does that is different than taking an access of a component. The
> only thing that Ada doesn't have here is an equivalent to "void", which is
> of course about safety - all access types have to have a designated type.

>>From the example posted, it appears a "pointer to member" is an offset
from the start of the record, rather than an absolute pointer. So, in
psuedo-Ada : 

type A_Type is record
    A : Integer;
    B : Integer;
end record;

type A_Component_Access_Type is access component A_Type;

Access_B : A_Component_Access_Type := A_Type.B'Component_Access;
--  Note that 'Component_Access takes a _type_, not an _Object_.

D : A_Type := (1, 2);
E : A_Type := (3, 4);

F : Integer := D.Access_B.all;
G : Integer := E.Access_B.all;

Now F = 2 and G = 4. Of course, this is the same as:

F : Integer := D.B;
G : Integer := E.B;

But if the names are long and complex, the pointer to member might be
a good optimization (like a renames). I'm not clear on exactly how
that would work.

I don't see how it relates to database fields.

-- 
-- Stephe