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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Iterable container as generic parameter
Date: Wed, 24 Jan 2018 21:36:22 -0600
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:50137
Date: 2018-01-24T21:36:22-06:00
List-Id: <comp.lang.ada>

"Lionel Draghi" <lionel.draghi@gmail.com> wrote in message 
news:61ba3677-0041-4dba-af9b-a5df48f3ce8a@googlegroups.com...
>I would like to make a generic function containing this simple code :
>
> for E of L loop
>   Put_Line (Image (E));
> end loop;
>
> And I would like to have this generic compatible with all "iterable 
> container".
>
> I thought it would be possible by just providing as generic parameters the 
> Image function, and an Ada.Iterator_Interfaces instantiation, but without 
> referencing the container.
> Isn't the iterator supposed to encapsulates both the cursor and the 
> container?
>
> Something like :
>
> generic
>   type Cursor is (<>);
>   with function Image (C : Cursor) return String;
>   with package Iterator_Interfaces is
>     new Ada.Iterator_Interfaces (Cursor, others => <>);
> function List_Image return String;
>
> But I couldn't figure out a simple way to do the body :
> 1. using the "for E of L" or the "for C in L.Iterate" loop refers to L, 
> the Iterable container : why should I need that here?

L is the container object that you are iterating over. An iterator interface 
describes *how* to iterate, but you also have to describe *what* data to 
iterate.

> 2. and it's not clear for me how to directly use a Cursor with First and 
> Next functions provided by Iterator_Interfaces, that need a 
> Forward_Iterator parameter.

You just call it, prefixing with the container object in question. These are 
just functions that work like any other functions.

---

As to the question you didn't ask, you clearly need the container type 
somewhere in your interface, so you can pass container objects. (Or, I 
suppose, you could reference the type used in the instance of the 
Iterator_Interfaces.) Otherwise, you just have methods but no data. And 
there isn't anything interesting that you can do with just methods!

                                    Randy.


>
> Any hints?
>
> Thanks,
>
> -- 
> -- Lionel