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From: george.priv@gmail.com
Newsgroups: comp.lang.ada
Subject: Re: How to build Objects containing itself?
Date: Fri, 20 Jun 2008 15:42:08 -0700 (PDT)
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Date: 2008-06-20T15:42:08-07:00
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On Jun 20, 6:06 pm, "snoopysal...@googlemail.com"
<snoopysal...@googlemail.com> wrote:
> Hi!
>
> So, here's my next question about object orientation in Ada.
>
> In languages like Ruby, C++ or Java, the following construct is
> possible (here in Java):
>
> class State {
>     HashMap<char, State> transitions = new HashMap<char, State>();
>
> }
>
> Perhaps you're wondering, what this code is meant to be. Some time ago
> I programmed a stack-structure simulating a trie as part of a
> incremental search-algorithm. The idea was to have a state-object
> containing a mapping of characters pointing to state-objects again.
> So, the result is a data-structure containing instances of its own
> data type again.
>
> As object orientation is a bit different in Ada, the previous code
> example would be something like this in Ada:
>
> package State is
>
>     type State is tagged
>         record
>             Transitions : Transition_Maps.Map;
>         end record;
>
>     package Transition_Maps is new Ada.Containers.Ordered_Maps
>         (Key_Type => Character,
>          Element_Type => State,
>          "<" => "<",
>          "=" => "=");
>     use Transition_Maps;
>
> end State;
>
> As you can see, we have a "What had been there first? The hen or the
> egg?"-problem: The State-record requires an instance of
> Transition_Maps.Map which is declared after the State-record's
> definition. But because Transition_Maps.Map needs a State as
> Element_Type changing the order ends in the same problem.
>
> So, has anybody an idea, how to solve this paradox?
>
> Thanks,
> Matthias

I am not sure what do you mean by including C++ there, but in Java all
objects are dynamic references.  So in order to make your type
equivalent to Java one should use 1) references when defining the map
and 2) Advance declaration of types (that will be also true for C++).
Otherwise you will have infinitely recursive type declaration that
compiler guards against.

   type State;
   type State_Access is access all State;

     package Transition_Maps is new Ada.Containers.Ordered_Maps
        (Key_Type => Character,
         Element_Type => State_Access,
         "<" => "<",
         "=" => "=");
   use Transition_Maps;

   type State is tagged
        record
            Transitions : Transition_Maps.Map;
        end record;