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From: Jean-Pierre Rosen <rosen@adalog.fr>
Newsgroups: comp.lang.ada
Subject: Re: Correct use of variants
Date: Mon, 07 Nov 2005 13:52:32 +0100
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Date: 2005-11-07T13:52:32+01:00
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Maciej Sobczak a �crit :
> Hi,
> 
> Consider this:
> 
> with Ada.Text_IO;
> with Ada.Integer_Text_IO;
> use Ada.Text_IO;
> use Ada.Integer_Text_IO;
> 
> procedure Hello is
> 
>    type Chooser is (First, Second);
> 
>    type Discriminated(Which : Chooser) is
>       record
>          case Which is
>             when First =>
>                X : Integer;
>             when Second =>
>                Y : Integer;
>          end case;
>       end record;
> 
>    A : Discriminated := (Which => First, X => 7);
> 
> begin
> 
>    Put(A.X);   -- prints 7
>    New_Line;
> 
> --   does not compile:
> --   A := (Which => Second, Y => 5);
> -- 
> --   Put(A.Y);   -- should print 5
> --   New_Line;
> 
> end Hello;
> 
> 
> What is the correct way to achieve the declaration of A with the 
> possibility of changing the discriminant later? One of the ways is to 
> use *some* default value for the discriminant, but I might have 
> difficulty deciding which value is better than the others for this role. 
> An arbitrary choice would create a misleading message to the reader that 
> the particular value is considered to be "best" or "idle" or "starting" 
> or whatever. 
Providing a default value for the discriminant is the way to achieve 
this. Why this rule? Here is how it goes:
1) You want to declare a "mutable" (unconstrained) object:
    A : Discriminated;
2) A discriminant is too important to be left uninitialized
3) Therefore, declaring unconstrained objects is allowed only if the 
discriminants have default values. QED.

> In addition, having a default value for the discriminant 
> means that it's not obligatory to initialize the variable A while it's 
> declared and I would like to keep this enforced.
> 
Oh, I see your confusion here. You are not required to initialize your 
variable even if you don't provide default values for the discriminants, 
you just need to state the constraint:
   A : Discriminated (First);

In the form you use, the constraint comes from the initial value which 
makes it "look" unconstrained. BTW, such a declaration was not allowed 
(for variables) in Ada83.

You can force initialization of variables by giving it unknown 
discriminants, but the type has to be private. If you don't mind a 
run-time rather than compile-time check, you can do the following:

function Raise_PE return integer is
begin
    raise Program_Error;
    return 1; -- Required by the compiler
end;

type Must_Be_Initialized is
    record
       Junk : integer := Raise_PE;
       ...
    end record;
-- 
---------------------------------------------------------
            J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr