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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: accessibility check failed
Date: Sun, 29 Nov 2015 20:15:57 +0100
Organization: cbb software GmbH
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Date: 2015-11-29T20:15:57+01:00
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On Sun, 29 Nov 2015 10:03:25 -0800 (PST), Serge Robyns wrote:

> I'm facing a "mysterious" accessibility check failed error.

Nothing mysterious anonymous access + checks is an endless source of
problems in Ada.

> I'm having the following function:
> 
>    overriding function Get_Client
>      (Self        : not null access T_Data_Store;
>       Client_Name : in              T_Client_Name)
>       return T_Client'Class
>    is
>       Cursor : Client_Map.Cursor;
>    begin
>       Cursor := Self.Clients.Find (Client_Name);
>       if Cursor = Client_Map.No_Element then
>          return T_Client_DAO'(No_Client
>                                   with Store => Self);

Try Self.all'Unchecked_Access.

(Don't feel guilty about Unchecked_Access, all means are good in the holly
war against evil access checks... (:-))

> Researching the web for the accessibility check failed all refers to
> returning a reference to a local variable which is indeed disappearing on
> the execution of the return statement.  But in my case, I'm just storing
> the reference I did receive from the parent.

Yes, but how much up you pass that reference? The rules are too crude. Most
of alarms they generate are false alarms. I don't remember a case
accessibility check indeed helped to detect a bug.

> I did find a suggestion to replace anonymous access types to named ones
> but  that does not compile for the interface class for which the above
> function is an implementation.
 
It is a poor advice when you design a reusable component. Anonymous access
type is the most general type of reference imposing least constraint on the
clients. Thus it is the type to be used in all interfaces.

In the implementations a named access type is indeed the best and safest
choice.

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