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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Assigning a "subclass" instance to a "superclass" variable
Date: Fri, 06 Apr 2012 22:50:18 +0100
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Date: 2012-04-06T22:50:18+01:00
List-Id: <comp.lang.ada>

deuteros <deuteros@xrs.net> writes:

> I'm still trying to understand how Ada does object orientation. In
> Java I could do this:
[...]
> So in Ada I'm trying to achieve something similar with my Statement
> package: 
>
> package Statements is
>
>    type Statement is abstract tagged
>       record
>          tokens : Vector;
>          executedtokens : Vector;
>       end record;
>    
>    type Statement_Access is access all Statement'Class;
>    
>    function execute(skip: in Boolean; S: in Statement) return Integer is abstract;
>
> end Statements;
>
> So when I create a child of Statement, how to I set up the .ads file?

   with Statements;
   package Expressions is
      type Expression is new Statements.Statement with record
         ...
      end record;
      function Execute (Skip : Boolean; S : Expression) return Integer;
   end Expressions;

or

   package Statements.Expressions is
      type Expression is new Statement with record
         ...
      end record;
      function Execute (Skip : Boolean; S : Expression) return Integer;
   end Statements.Expressions;

The second form is good if the full declaration of Statement is in the
private part of Statements, since child packages can see the private
parts of their parents.