Re: Sugestion to Multiple Inheritance

[Brian Rogoff <bpr@bpr.best.vwh.net>]

On Mon, 21 Jan 2002, Lutz Donnerhacke wrote:
> * Matthew Heaney wrote:
> Generics does not provide an abstract interface fullfilled by different
> implemenations.

Yes, generics can do exactly that. Do you understand the signature idiom
in Ada 95?

What you don't get, that tagged types give you, is the same kind of
polymorphism at run time. You pay for that run time capability with run
time dispatch and the overhead of tagging.

> An algorithm requires forward_iterators aof any kind.

So you parameterize over a forward iterator signature, which looks
something like this

generic
    type Value_Type (<>) is limited private;
    type Pointer_Type is access Value_Type;
    type Iterator_Type is private;

    with procedure Incr (Iterator : in out Iterator_Type) is <>;
    with function Succ (Iterator : in Iterator_Type)
                       return Iterator_Type is <>;
    with function Get_Value (Iterator : Iterator_Type)
                            return Value_Type is <>;
    with procedure Set_Value (Iterator : in out Iterator_Type;
                              Value : in Value_Type) is <>;
    with function Get_Pointer (Iterator : Iterator_Type)
                              return Pointer_Type is <>;
    with function "=" (X, Y : Iterator_Type)
                        return Boolean is <>;
package Forward_Iterator_Signature is
-- Look, no body! This is a signature package.
end Forward_Iterator_Signature;

and your algorithms package will look something like this

with Forward_Iterator_Signature;
generic
    with package Iterators is new Forward_Iterator_Signature(<>);
    use Iterators;
package Forward_Algorithms is
    -- Nonmutating sequence algorithms

    generic
        with function "=" ( X, Y: in Value_Type ) return Boolean;
    function Find ( Start, Finish: in Iterator_Type;
                   Value: in Value_Type ) return Iterator_Type;

    generic
        with function "=" ( X, Y: in Value_Type ) return Boolean;
    function Adjacent_Find ( Start, Finish: in Iterator_Type )
                           return Iterator_Type;

    -- ... etc., etc.
end Forward_Algorithms;

-- Brian