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Newsgroups: comp.lang.ada
Subject: Re: Q: Hiding the structure of a tagged type using containers
References: <1147139575.425825.290860@u72g2000cwu.googlegroups.com>
From: Stephen Leake <stephen_leake@acm.org>
Date: Tue, 09 May 2006 02:43:41 -0400
Message-ID: <uzmhrr942.fsf@acm.org>
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Date: 2006-05-09T02:43:41-04:00
List-Id: <comp.lang.ada>

brian.b.mcguinness@lmco.com writes:

> I am trying to create a tree of object classes to represent arrays.
> I want to hide the internal representation of the arrays in the
> private sections of the packages for the tagged types. However, I am
> having trouble figuring out how to do this. For example, I have an
> abstract base class called APL_Array, and I am trying to create a
> child class called APL_Array_Dimensioned that contains a list of
> dimensions (which may change over time, say if someone concatenates
> a row or column to a matrix).

I used to use APL! fun language :).

> <snip code> 
>
> but when I try to compile it, I get:
>
> # gnatmake -gnat05 apl-arrays-dimensioned.adb
> gcc -c -gnat05 apl-arrays-dimensioned.adb
> apl-arrays-dimensioned.ads:21:13: operation can be dispatching in only
> one type

The general solution to this error is to make one of the arguments
class-wide; in this case:

procedure Shape
   (A : in APL_Array_Dimensioned; Dimensions : out APL_Shape'class);

or

procedure Shape
   (A : in APL_Array_Dimensioned'class; Dimensions : out APL_Shape);

If children of APL_Array_Dimensioned don't need to override Shape, the
second declaration is more appropriate.

Another solution is to use only one tagged type. Apparently
APL_Dimension_List.Vector is visibly tagged; perhaps it does not need
to be?

-- 
-- Stephe