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From: kevin.cline@gmail.com (Kevin Cline)
Newsgroups: comp.lang.ada
Subject: Re: Ada Popularity: Comparison of Ada/Charles with C++ STL (and Perl)
Date: 30 Sep 2004 17:04:31 -0700
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Date: 2004-09-30T17:04:31-07:00
List-Id: <comp.lang.ada>

"Matthew Heaney" <mheaney@on2.com> wrote in message news:<415c36c0$0$91010$39cecf19@news.twtelecom.net>...
> "Kevin Cline" <kevin.cline@gmail.com> wrote in message
> news:e749549b.0409291942.5790386d@posting.google.com...
> >
> > There seems to be some confusion between creating a pointer to a
> > particular member of a particular record, and the C++ pointer to
> > member type.
> >
> > In C++:
> [snip]
> 
> There seems to be some confusion between creating an offset to a record
> component, and simply using an Ada function pointer:
> 
> In Ada:
> 
> declare
>    type T is record
>       A, B : Integer;
>    end record;
> 
>    function A (O : T) return Integer is
>    begin
>      return O.A;
>    end;
> 
>    function B (O : T) return Integer is
>    begin
>       return O.B;
>    end;
> 
>    type Query_Type is access function (O : T) return Integer;
> 
>    Query : Query_Type := A'Access;
> 
>    O1 : T := (3, 5);
>    O2 : T := (4, 6);
> 
> begin
> 
>    pragma Assert (Query (O1) = 3);
>    pragma Assert (Query (O2) = 4);
> 
>    Query := B'Access;
> 
>    pragma Assert (Query (O1) = 5);
>    pragma Assert (Query (O2) = 6);
> 
> end;
> 
> 
> 
> > The availability of pointers to members makes it possible to concisely
> > express the mapping between the a data member and the "A" column of a
> > database table.
> 
> The availability of pointers to local functions makes it possible to
> concisely express the mapping between the A record component and the "A"
> column of a database table.

I don't personally consider this to be particularly concise.  There
were two lines C++ code, one to declare the data member:
  int a;
and one to create the table entry:
  db.insert("PERSON.SSN", &Person::ssn).

There are at least six lines of Ada code: the member declaration, a
function to access the data member, the instantiation of the insert
function for members of that type, and finally the call of the
instantiated function.  Not much of a savings over simply writing
separate create, read, and update functions for the record type.