Re: generics and records

[spambox@volja.net]

Dmitry A. Kazakov wrote:
> >> 2. Inheritance from a record type with known components:
> >>
> >> type Common_Base is tagged record
> >>    A : Float; -- This will be visible in Foo
> >> end record;
> >>
> >> generic
> >>    type Record_Type is new Common_Base with private;
> >> package Foo is
> >>    ...
> >>
> >> Instantiation:
> >>
> >> type My_Record is new Common_Base with record
> >>    ... -- These new components will be inaccessible in Foo!
> >> end record;
> >> package My_Foo is new Foo (My_Record);

> You can't do that. Ada has a contract model of generics. That means
that
> Foo in the example above will never directly see the whole record,
only its
> Common_Base part. The primitive operations defined on Common_Base
will. So
> if Foo has to access all record, the only way to do it is to express
what
> Foo should do in terms of primitive operations defined on
Common_Base. They
> can dispatch to the specific operations defined on the actual type.
> Usually, it is sufficient for all purposes and also is a good OO-ish
> programming style. But there is no way to write a generic unit
working with
> whatever components of an actual parameter. Ada does not provide
abstract
> record interfaces with enumeration of components and their types at
run
> time. Though you can implement something close to that using map
container
> types instead of record types. Probably this is what you actually
need. But
> again, to work through primitive operations is better.

Yes, I understand. I was too eager to try your solution out, failed and
phrased my question poorly. I'm perfectly content with a record like
Common_Base above. But where should its definition be put? It can't go
into the package specs, where the "generic ..." part belongs. But if
it's not there, how can it be seen? -- "type Record_Type is new
Common_Base with private;" 
andrej