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From: dewar@cs.nyu.edu (Robert Dewar)
Newsgroups: comp.lang.ada
Subject: Re: Please explain why this is wrong...
Date: 8 Feb 1995 22:55:15 -0500
Organization: Courant Institute of Mathematical Sciences
Message-ID: <3hc3n3$onc@gnat.cs.nyu.edu>
References: <3h8u4s$ea3@nonews.col.hp.com>
NNTP-Posting-Host: gnat.cs.nyu.edu
Date: 1995-02-08T22:55:15-05:00
List-Id: <comp.lang.ada>

Your program is clearly wrong, and the message from GNAT is quite precise.

You have a primitive operation New_Square, which is inherited when you
make a rectangle to give a function New_Square that returns a Rectangle.

Now it is not clear why you made this a primitive operation. I rather suspect
that you expect to use New_Square only for a Square, in which case it should
NOT be primitive.

If indeed you intend it to be inherited, it MUST be overridden. The reason
for this rule is simple. In the general case the derivation involves extending
the type, and how could the original function code possibly know enough to
initialize the extension -- it couldn't.

If as I suspect, you really did not intend New_Square to be inherited, but
rather intended it to serve as a constructor only for the Square case, then
you must be sure that you do not declare in as being primitive. The easiest
way to declare non-primitive operations is to nest them in a subpackages;

   package Constructors is
      function New_Square .. return Square;
   end Constructors;