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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Pb with use of redefined "=" operator
Date: 1998/11/10
Message-ID: <wccww538aho.fsf@world.std.com>#1/1
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Sender: bobduff@world.std.com (Robert A Duff)
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Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1998-11-10T00:00:00+00:00
List-Id: <comp.lang.ada>

dennison@telepath.com writes:

> My understanding was that without 'Class, the tag may not even be *present*
> in the object. It isn't really needed, since every object's type is
> determinable at compile time.

But when the compiler lays out objects of the type it needs to decide
whether to include a tag field.  It doesn't know if some other part of
the program will mention 'Class (unless it has global knowledge, eg by
deferring all code generation to link time).

Consider:

    package P is
        type T1 is tagged null record;
        X: T1;
        procedure Do_Something(X: T1);
        procedure Do_Something_Else(X: T1);
    end P;

    with P; use P;
    package Q is
        type T2 is new T1 with null record;
        Y: T2;
        procedure Do_Something_Else(X: T2);
    end Q;

    with P; use P;
    with Q; use Q;
    procedure Main is
    begin
        Do_Something(P.X);
        Do_Something(Q.Y);
    end Main;

    package body P is
        procedure Do_Something(X: T1) is
	begin
	    Do_Something_Else(T1'Class(X)); <<--------------------
	end Do_Something;

        procedure Do_Something_Else(X: T1) is
        begin
            ...
        end Do_Something_Else;
    end P;

In the usual implementation model, both variables X and Y need a tag
field, but we haven't yet seen any 'Class when they are compiled.  An
alternative is to store tags with parameters and pointers, instead of
with the objects themselves.  I don't know of any compilers that do
that, and I'm not sure it's a good idea -- it seems kind of annoying to
double the size of all access-to-classwide types.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.