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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Instantiating private types with discriminants?
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Date: Thu, 11 May 2006 09:44:20 +0200
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On Wed, 10 May 2006 16:52:32 -0500, Rick H wrote:

> But all this has a side-effect that I haven't considered: the fact
> that a variable declared as a class-wide type *requires* initialization
> means that the compiler can determine the variable's type statically.

No. It only means that the type should be determinable at run-time. There
are may ways to do it. You can call a function with a class-wide result.

It might be especially interesting to take a look at S'Class'Input
attribute. It "dispatches" on a "bare" tag. [ Ada 2005 will offer a
mechanism for fully user-defined things like this. See
Generic_Dispatching_Constructor. ]

> This means that the following will raise a constraint error:
> declare
>    Var_A : General_T'Class := Get (10);  -- Type_A
> begin
>    Put (Var_A);
>    Ada.Text_IO.New_Line;
>    Var_A := Get (100.0);      -- Type_B - constraint error!
>    Put (Var_A);
> end;

Yes, but this is a different thing. You cannot change tag, either
dynamically or statically determined. Type mutators are no-no in Ada's type
system.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de