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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Why constant components are not permitted ?
Date: Fri, 25 Nov 2011 02:23:22 -0800 (PST)
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Date: 2011-11-25T02:23:22-08:00
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Dmitry A. Kazakov wrote on comp.lang.ada:
> On Thu, 24 Nov 2011 20:46:13 +0100, Ludovic Brenta wrote:
> > Constants with other purposes can be declared outside the type
> > definition and thus shared among all objects of the declared type.
>
> The constant value may depend on the enclosing object, undesired or
> impossible to re-evaluate each time when needed.

IOW, the constant is the cached result of a function taking the rest,
or part thereof, of the object as its sole parameter.  If this result
is a constant, this can only mean that the rest of the object is
constant too, right? In this case one can say:

type T is record
   Hash_Value : Natural;
   Component_1 : Foo;
   Component_2 : Bar;
end record;

Object : constant T := (Hash_Value => Hash (Component_1, Component_2),
                        Component_1 => Component_1,
                        Component_2 => Component_2);

If not all other components are inputs into the function returning
the constant result, then it is probably desirable to split the object
into two types, such that all the "constant" components are in a type
of their own:

type Immutable_Part is record
   Hash_Value : Natural;
   Component_1 : Foo;
end record;

type T (Immutable : not null access constant Immutable_Part) is record
   Component_2 : Bar;
end record;

--
Ludovic Brenta.