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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada Annoyances
Date: Thu, 22 Jun 2017 22:22:10 +0300
Organization: Tidorum Ltd
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Xref: news.eternal-september.org comp.lang.ada:47059
Date: 2017-06-22T22:22:10+03:00
List-Id: <comp.lang.ada>

On 17-06-22 16:30 , Dmitry A. Kazakov wrote:
> On 22/06/2017 12:29, pythoner6@gmail.com wrote:
>
>> What do you mean by "re-dispatch" here? How is it different from the
>> dynamic dispatch that happens in Ada with class-wide types?
>
> Re-dispatch is dynamic dispatch when you go through the same virtual
> table twice.

I think that is inaccurate, unless you have a different definition of 
"re-dispatch" from what I remember of discussions re this topic.

Consider:

    type Parent is tagged record .... end record;

    procedure Foo (P : Parent);
    procedure Bar (P : Parent);

    type Child is new Parent with record ... end record;

    -- Child inherits Foo from Parent.

    overriding procedure Bar (C : Child);

followed by

    C : Child;

    Foo (C);

That call is not dispatching, so it does not go through the "virtual 
table". It calls the inherited Foo for Child.

Now suppose Foo (P : Parent) does a (re-) dispatching call to Bar, which 
in this example calls the overriding Bar (C : Child):

    procedure Foo (P : Parent)
    is
    begin
       Bar (Parent'Class (P));
    end Foo;

I believe this call of Bar is considered "re-dispatching", because there 
is a conversion from Parent to Parent'Class (effectively, Child). But 
there is only one use of the "virtual table".

> Since you are already there = you have determined the
> actual specific type, it does not make sense (and dangerous) to do this
> again.
>
> (I know that there are software patterns based on re-dispatch, they all
> are wrong.)

As you have often said. I still disagree. Just to let the OP know that 
opinions differ on this topic.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .