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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: prohibit certain generic instantiations in Ada 2005
Newsgroups: comp.lang.ada
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Date: Fri, 10 Feb 2006 19:25:24 +0100
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Date: 2006-02-10T19:25:15+01:00
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On 10 Feb 2006 06:18:30 -0800, matteo.bordin@gmail.com wrote:

> I understand and agree from a semantic point of view. However I would
> like to let the compiler perform some additional type checking on the
> generic instance. If the generic instantiation is not allowed, then I
> what the compiler to produce a compilation error. This makes sense if
> you are using generics as a generative tool for metaprogramming. This
> kind of things can be done in C++, which has a compilation model
> similar to Ada (for what concerns generics/templates).

I don't see how you came to this conclusion. C++ model is quite different,
especially with respect of your question - it isn't contract based. So,
basically, C++ is unable to check anything specific about the type of
template a parameter. Ada's model is contract based, so types are fully
checked. That in particular means, that there cannot be any "additional
type" checking. Probably you meant something else.

Note that the type contract can be strengthen in Ada by requiring some
additional subroutines. For example:

generic
   type S is new T with private;
   procedure Ad_Hoc (X : in out S) is <>;

Here we require a type from T'Class plus some procedure Ad_Hoc defined on
it.

However, it is a *bad* practice from software design perspective inherited
from Ada 83, which wasn't OO.

A proper way is to make T'Class + Ad_Hoc explicit. If, for whatever reason,
T is frozen, you should, as Matthew suggested, derive a new [maybe
abstract] base:

type T1 is abstract T with null record;
procedure Ad_Hoc (X : in out T1) is abstract;
   -- This is a proper contract now

generic
   type S is new T1 with private;
      -- Here we use the contract

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de