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Newsgroups: comp.lang.ada
Date: Mon, 11 Sep 2017 15:06:44 -0700 (PDT)
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Subject: Re: Type invariants and private extensions?
From: Egil H H <ehh.public@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:48024
Date: 2017-09-11T15:06:44-07:00
List-Id: <comp.lang.ada>

On Tuesday, September 12, 2017 at 12:00:41 AM UTC+2, Victor Porton wrote:
>=20
> It seems that you don't understand me: I need different boolean functions=
=20
> for different derived tagged types. If we specify Type_Invariant or=20
> Type_Invariant'Class this is the same function because it can be specifie=
d=20
> only for the base class not my derived classes.
>=20
> Or maybe I do not understand you? If so, please explain.
>=20
> Hm, well, Type_Invariant'Class may point to dispatching function which ca=
n=20
> be overridden in the derived classes. Isn't it what you mean?
>=20
>=20

Any Invariants declared for a parent type must also hold for all derived ty=
pes, but derived types can add more. Think about what someone holding a var=
iable of Parent'Class expects. The definition of Parent promises an invaria=
nt. But what if the actual type is a child? Mayhem will ensue if the child =
does not promise the same invariant, hence it is inherited.