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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Why constant components are not permitted ?
Date: Thu, 24 Nov 2011 20:46:13 +0100
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Date: 2011-11-24T20:46:13+01:00
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David Sauvage writes on comp.lang.ada:
> Hello,
>
> given the following :
>
> type Object is record
>    Tag : constant String := "key"; -- GNAT compilation failed : constant components are not permitted
> end record;
>
> In my case I find it useful to declare my Object Tag component as a
> constant, but it is not permitted, I would be interested to learn why?

Maybe because you don't need constant components.

If the purpose of a constant component is to identify the non-tagged
type of an object, then both the programmer and the compiler know the
type at compile time.

If the purpose is to identify the tagged type of an object, then the
compiler emits a tag as part of the representation of each object for
this purpose.  This tag is normally a pointer to the table of primitive
operations for the type (see ARM 3.9(3)).  The programmer can query and
use the tag, see ARM 3.9(18.1/2 and following), also indirectly through
streams, see ARM 13.13.2(31/2, 34/2).

Constants with other purposes can be declared outside the type
definition and thus shared among all objects of the declared type.

-- 
Ludovic Brenta.