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Date: Sun, 02 Jul 2006 11:18:24 +0200
From: Georg Bauhaus <bauhaus@futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: Constant record components
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Date: 2006-07-02T11:17:11+02:00
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Jeffrey R. Carter wrote:


>       Ada.Text_IO.Put_Line ("Component is not constant");

> You get 3 guesses which string is output, and the 1st 2 don't count. 
> Clearly, this does not meet the OP's interest in components that cannot 
> be changed after initialization.

Not sure whether it is clear that the OP wanted records initialized
twice, as in

   A : Rec.R := Rec.Make (1);
begin
   A := Rec.Make (2); 

To enforce the contract of the  constructor function `make` ("call once
for each object") and still not making the records or a component limited
how about this workaround then:

package Rec is

   type R is tagged private;

   procedure make(m: INTEGER; result: in out R);
      -- `m` is the initial value for the immutable component
      -- pre: result has not been initialized

   function immutable(thing: R) return INTEGER;

private

   type R is tagged record
      mutable: INTEGER;
      data: INTEGER;
      identity: POSITIVE'base := 0;
   end record;

end Rec;


Imagine a protected Sequence in the body that generates unique
positive values for the .identity part of R. Make raises Program_Error
whenever called with a Result that has .identity /= 0.

A limited component Immutable looks like another option to me.

(Yes, there are no immutable non-limited components other than
discriminants, but possibly there is a solution, sort of,
that works reasonably well even though it takes some space?)