Re: Eiffel and Java + Ada dispatching

[bobduff@world.std.com (Robert A Duff)]

In article <E0sIBu.MJz@syd.csa.com.au>,
Don Harrison <donh@syd.csa.com.au> wrote:
>:>generic                                         class X [T -> Some_Type] ...
>:>  type Some_Type is tagged limited private;
>:>package X ...
>:
>:I meant "type Some_Type is new Other_Type with private".
>
>Okay. What's the difference?

In the first example above, Some_Type doesn't have any primitive
operations.  You can't do much of anything with it, other than declare
objects and a few other minor things.  (Unless you pass in explicit
operations, which is what you (or somebody else?) was objecting to,
several messages ago.)

In the second example, Some_Type inherits all the primitive operations
of Other_Type.  Inside the generic, you can do dispatching calls to them
and so forth.  The actual type is required to be in the right class
(Other_Type'Class), and thus implements all of those things you're
calling.

I believe Eiffel has something similar to both, but I forget the syntax.
Is this the "class X [Some_Type]" vs. "class X [Some_Type ->
Other_Type]" thing?  You can have a generic class where the formal can
be any class, or you can require the formal to be a subclass of some
particular class.

- Bob