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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Anonymous access types
Newsgroups: comp.lang.ada
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Date: Fri, 8 Jun 2007 14:01:09 +0200
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Date: 2007-06-08T13:58:38+02:00
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On Fri, 08 Jun 2007 02:29:48 -0700, adam.betts155@gmail.com wrote:

> I am having some trouble with anonymous access types, behaviour which
> I do not understand. I have 2 questions:
> 
> 1) This one is bizarre and quite non-deterministic. I *sometimes* get
> a "finalize/adjust raised exception" with the following bit of code:
> 
> declare
>       t_ptr : tree_pointer := new tree'(get_tree(g));
>       anon_t_ptr : access tree'class := new tree'(get_tree(g));
>       lca : least_common_ancestor(anon_t_ptr);
> begin
> end;

This leaks, where these objects get deallocated?

> with the following type declarations:
>  tree_pointer is access tree'class;
>  type least_common_ancestor (tree_ptr: access constant tree'class);
> 
> It seems that there is sometimes a problem with the lca declaration,
> since the exception is never raised when anon_t_ptr is replaced with
> t_ptr.

I guess it is because of accessibility checks. t_ptr and anon_t_ptr have
different accessibility levels. When you call to least_common_ancestor,
which probably has its parameter of some access type declared in an
enclosing scope, then anon_t_ptr being converted fails on accessibility
check. In the result you get a snowball of misleading secondary faults.

(never use pointers if you can)

> 2) I want to return an anonymous access type from a function:
> 
> function get_tree (g: graph) return non null access tree is
> begin
>   return new tree(g.t);
> end get_tree;
> 
> The access type is not null when I use this function. However, when I
> run the code and try to do operations on the access type, it is not
> pointing in the right place. For example, the returned tree does not
> have vertices and edges even though I know it does.

   new tree(g.t);

does not initialize the object beyond standard initialization. You probably
meant

   new tree'(<some initial value>);

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de