Generic instantiation with constrained subtype

Generic instantiation with constrained subtype

[Simon Wright]

I've been trying to write some test cases for the Booch Components and
came up with this.

The test case is written as a generic, to be instantiated with the
appropriate Container type. This is fine for the definite containers
(ie, ones like No_Problem below) but fails for the Bounded forms, which
use the Unconstrained/Constrained technique.

This is the example:

-----------------------------------------------------------------------
package Instantiation_Problem is

   type Base is tagged null record;

   type No_Problem is new Base with null record;

   type Unconstrained (Constraint : Positive)
   is new Base with null record;

   subtype Constrained is Unconstrained (Constraint => 10);

   generic
      type T is new Base with private;
   package G is end G;

   package NP is new G (No_Problem);

   package F is new G (Constrained);
   --                  ^--------------------18:24

   package U is new G (Unconstrained);
   --                  ^--------------------21:24

end Instantiation_Problem;
-----------------------------------------------------------------------

and GNAT (GCC 4.3.0) says

instantiation_problem.ads:18:24: constraint on actual is incompatible with formal
instantiation_problem.ads:18:24: instantiation abandoned
instantiation_problem.ads:21:24: actual subtype must be constrained
instantiation_problem.ads:21:24: instantiation abandoned

Am I going to have to rethink this? Have I used the wrong way of
specifying the generic parameter? Is GNAT confused? (note, at 21:24 it's
asking for a subtype, so why complain at 18:24?).

Ideally I'd like it if any solution could work with Ada 95 - these are
the Booch Ada 95 Components!

--S

Re: Generic instantiation with constrained subtype

[Jeffrey R. Carter]

Simon Wright wrote:
> 
>       type T is new Base with private;

type T (<>) is new Base with private;

eliminates the compiler errors. It may not be suitable for your needs, however.

-- 
Jeff Carter
"Go and boil your bottoms."
Monty Python & the Holy Grail
01

Re: Generic instantiation with constrained subtype

[Simon Wright]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org> writes:

> Simon Wright wrote:
>>
>>       type T is new Base with private;
>
> type T (<>) is new Base with private;
>
> eliminates the compiler errors. It may not be suitable for your needs,
> however.

Hmm, I need to be able to say

   generic
      type T (<>) is new Base with private;
   package G is
      O : T;
   end G;

Ah! I can provide an initialising value!

   generic
      type T (<>) is new Base with private;
      V : T;
   package G is
      O : T := V;
   end G;

   A : Constrained;

   package F is new G (Constrained, A);

-- and it works fine! Thanks for the help ..