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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Problem Eliminating constructors
Date: Tue, 21 Jan 2003 10:44:37 +0100
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Date: 2003-01-21T10:44:37+01:00
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On Tue, 21 Jan 2003 02:07:29 +0500, porton@ex-code.com (Victor Porton)
wrote:

>In article <3E2BDFD8.4070805@epfl.ch>,
>	Rodrigo Garc�a <rodrigo.garcia@epfl.ch> writes:
>> Victor Porton wrote:
>>> package A is
>>> 
>>>   type A_Type is tagged
>>>     record
>>>       X: Integer;
>>>     end;
>>>     
>>>   function Create(X: Integer) return A_Type; -- guess what is the body
>>>   
>>> end A;
>>> 
>>> Then I have
>>> 
>>> type B_Type is new A.A_Type with
>>>   record
>>>     Y: Integer;
>>>   end;
>>> 
>>> Compiler requires me to override Create as the return type changes.
>>> But it is meaningless as now I need Create with two parameters
>>> instead of one.
>>> 
>>> What to do?
>> 
>>    You do not need a constructor for this simple case. This will create 
>> an A_Type object and initialize X to value 5.
>> 
>>    declare
>>       My_A : A.A_Type := (X => 5);
>[skip]
>
>Oh, Rodrigo, I oversimplified the real situation. Initialization with
>just aggregates is possible only in the example, really I need namely
>constructor functions.

Ada has constructor function. IMO what it does need is user-defined
contructors. However, most of Ada people do not share my opinion. In
any case (your statement below) constructors always *covariant*.

>Well, currently I switched to initialization by calling a procedure.
>
>It would be nice if Ada0X would allow to make a primitive operation
>non-virtual.

It is pretty dangerous, so there has to be a strong rationale for this
(i.e. to allow contravariant parameters). In your case, you could make
Create class-wide, if you want to overload it:

 function Create (X : Integer) return A_Type'Class;
   -- creates A_Type

function Create (X, Y : Integer) return B_Type'Class;
   -- creates B_Type

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de