NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/11500

Text File | 1996-08-05 | 1.7 KB | 36 lines

Newsgroups: comp.lang.c++
Path: newshub.nosc.mil!news!news
From: [e-mail] (Charles H. Sampson)
Subject: Specifying Data Layout
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Computer Sciences Corporation
Date: Thu, 14 Mar 1996 17:36:43 GMT

     I've just finished reading the reference manual part of Stroustrup's
"The C++ Programming Language" (2nd ed) and have a number of questions.
This one is particularly important to a project I'm working on.  A quick
search of the FAQ didn't turn up anything, nor does it seem to appear in
the 1400+ articles currently in the comp.lang.c++ newsgroup on my news
server.

     Is there some way of precisely specifying the memory layout of a
struct?  In other words, can you say, "This struct is to occupy 3 words of
memory, element foo is to occupy bits 5-12 of word 1, element fern is to
occupy ..."?  If it can be done with a struct, can it also be done with a
class (specifying locations for the data members only)?  In this case, can
you also control the gizmo that is used to distinguish the various values
of a class hierarchy, both where it is to be allocated and the values to be
used to identify specific members of the hierarchy?

     For those who don't understand why anyone would want to do such stuff,
the issue is trying to make a class hierarchy out of data that were defined
and mapped to bits long before OO concepts arose.

     For reference, in Ada the technique for specifying the exact bits of a
struct (record) is called a representation specification and the gizmo is
called the class's tag.  (I'm not trying to wave any red flags here; just
attempting to use a precise term where it exists.)

                                Charlie