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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: "accessibility check failed" on Node
References: <kqfrrv$lsb$1@news2.open-news-network.org>
Date: Thu, 27 Jun 2013 06:46:43 -0400
Message-ID: <85obarvnjg.fsf@stephe-leake.org>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:182111
Date: 2013-06-27T06:46:43-04:00
List-Id: <comp.lang.ada>

Thomas Schmidt <TC.Schmidt@gmx.net> writes:

> I'm new to Ada. 

Welcome!

> Now what I don't understand: Why does "findNode1" work well while
> "findNode2" raises PROGRAM_ERROR at the line indicated below?

Accessibility refers to where an object is declared, relative to where
its type is declared.

> Given the following declarations:

It's better if you post a complete compilable example, so we can compile
it, and maybe fix the bug.


>   function findNode2
>     (graph : DirectedGraph;
>      aNode : access Node'Class)
>      return  Node_Class_Access is
>
>      position : constant NodeSets.Cursor :=
>        graph.nodes.Find (aNode.all'Access);	-- <<< Raises
> PROGRAM_ERROR with "accessibility check failed"

First, a style note: since aNode is already an access, aNode.all'Access
is just aNode. Is there some reason you wrote it this way?

The accessiblity check applies to the object that aNode designates. If
the type Node is declared at library level, but the object is at some
lower level in a subprogram, then when that subprogram returns, the
object will disappear, and the access value will become invalid. Ada
forbids this. 

If you post a complete example, we can give more precise help.

-- 
-- Stephe