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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: type access Parent'Class
Date: Mon, 21 Nov 2011 14:33:22 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2011-11-21T14:33:22-05:00
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Yukicanis <yukicanis@googlemail.com> writes:

> Dear Group,
>
> I'm new to Ada and currently playing around with its OOP features.
> Now, I have the following source:
>
> procedure Test is
>   type Parent is tagged limited null record;
>   type Parent_Access is access Parent'Class;
>   A : access Parent'Class;
>   B : Parent_Access;
>   procedure Dyn_Disp(X : access Parent'Class) is
>   begin
>     A := X;
>     B := X;
>   end Dyn_Disp;
> begin
>   null;
> end Test;
>
> When I try to complie that with GNAT 4.6.1 I get the following error
> message:
>
> test.adb:9:10: expected type "Parent_Access" defined at line 3
> test.adb:9:10: found type access to "Parent'Class" defined at line 6
>
> which I don't really understand since type "Parent_Access" is type
> access to Parent'Class, isn't it?

No, there are three different access types -- Parent_Access,
the anonymous type of A, and the anonymous type of X.  They're all
"access to Parent'Class", but they are distinct.

There are some implicit conversion rules that make "A := X;" legal.
But there is no implicit conversion to Parent_Access, so "B := X;"
is illegal.

You normally want access-to-class-wide types (and many other access
types) to be "general", which is specified by adding "all":

    type Parent_Access is access all Parent'Class;

If you do that, then you could say "B := Parent_Access(X);"
or "B := A.all'Access;".

But I suggest you avoid anonymous access types, except in certain
special cases.  They are confusing.

I also suggest you put most of your code in packages, rather than
in the main procedure.  That will also help avoid confusion, because
things behave differently when inside procedures.  For example,
you can't have any dispatching procedures unless you put the type
in a package spec.

- Bob