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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Help - Constructors - ASAP.
Date: 1998/08/01
Message-ID: <m31zr1wvo1.fsf@mheaney.ni.net>#1/1
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NNTP-Posting-Date: Fri, 31 Jul 1998 21:30:57 PDT
Newsgroups: comp.lang.ada
Date: 1998-08-01T00:00:00+00:00
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dennison@telepath.com writes:

> But, a naieve reader would look at that and think that they could get
> back an object of *any* type in the class (as I have just proven :-)
> ), when in actuality they always get back the same class of object.

If you don't like returning the class-wide type, then declare a
constructor that returns the specific type, but is not primitive.  The
simplest way to do this is to declare the constructors in a nested
package:

package P is

   type T is tagged private;

   <primitive ops>

   package Constructors is

      function New_T (I : Integer) return T;

   end Constructors;

private
...
end P;


> If I read you correctly, what you were worried about was developers
> screwing up and forgetting to override the default
> constructors. Nothing about this solution prevents that. Now instead
> of getting the default constructor, our hapless client will compile
> with the class-wide constructor. They get an exception at runtime
> either way. But now the client, who is just as capable of screwing up
> as the subclass author, could goof and use the class-wide constructor
> on his own.

You can detect type mismatches at compile-time by using a constructor
that returns the specific type (not the class-wide type), and is
non-primitive.  See the example above.

> Hmm. I believe the language rules state that no more primitive
> operations may be declared after the type is "finalized". It may be
> possible to force this by doing something like deriving a dummy
> subclass before the constructor is declared.

I'm not sure what you mean.  If you want non-primitive operations, then
just declare them in a nested package.  Don't use the freezing rules
(see RM95 13.14) to try to do this; that would be confusing.