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From: anh_vo@udlp.com (Anh_Vo)
Newsgroups: comp.lang.ada
Subject: Re: Cloning a Class Wide Data Type with "new"?
Date: 6 Jun 2003 11:38:04 -0700
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"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> wrote in message news:<3EE097D5.2010901@cogeco.ca>...
> I havn't had much time to research this one yet, but I've
> got stuck trying to clone a class wide type this morning,
> on my way to work. A sample program is provided below.
> 
> I need to be able to allocate and then copy an arbitrary
> tagged type (based upon a base type) into the newly
> allocated object. There's gotta be a way to do this..
> I just can't remember it!
> 
> The problem is the operation of the form of :
> 
>    Cloned : Base_Access := new Base_Class'(Orig);
> 
> Here is a sample program that distills the problem:
> 
> 
> with Ada.Text_IO;
> use Ada.Text_IO;
> 
> procedure TProg is
> 
>     type Base_Type is tagged
>        record
>           V :      Integer;
>        end record;
> 
>     type Base_Access is access all Base_Type'Class;
>     subtype Base_Class is Base_Type'Class;
> 
>     type Derived_Type is new Base_Type with
>        record
>           Z :      Natural;
>        end record;
> 
>     -- This is the original object to be cloned
>     Orig :   Derived_Type := ( V => 32, Z => 99 );
> 
>     -- This is a local copy (easy)
>     Copied : Base_Class  := Orig;
> 
>     -- Allocate Cloned to be same as Orig (the problem)
>     Cloned : Base_Access := new Base_Class'(Orig);
> begin
> 
>     Put_Line("Cloned.V = " & Cloned.V'Img);
> 
> end TProg;
> 
> 
> TIA, Warren.

Base_Class is not a real type. Base_Type and Derived_Type are real
types. Thefore, either one of them should be used. I assume
Derived_Type was your intention in this case. Otherwise, explicit
conversion is needed. Both cases are shown below.

Cloned : Base_Access := new Derived_Type' (Orig);  -- case 1

Cloned : Base_Access := new Base_Type' (Base_Type (Orig));  -- case 2

A. Vo