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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Subprogram Pointer in a Generic
Date: 17 Apr 2003 16:39:59 -0400
Organization: The World Public Access UNIX, Brookline, MA
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claveman@grzorgenplatz.net (Charles H. Sampson) writes:

>      I'm using a procedure that takes a procedure pointer as an
> argument.  It's pretty standard stuff:
> 
>           type Parameterless_Proc_Ptr is access procedure;
>           procedure Register (The_Proc : in Parameterless_Proc_Ptr);
> 
>      My problem occurs when I want to call this procedure from within a
> generic package.  I can't, because 3.10.2(32) says "If the subprogram
> denoted by P [the prefix of 'Access] is declared within a generic body,
> S [the access-to-subprogram type of the formal parameter] shall be
> declared within the generic body."
> 
>      Now that's no good for me at all, so I've kludged my way around it
> by taking the address of the actual argument procedure and changing it
> to a pointer using Unchecked_Conversion, a technique guaranteed to work
> for the current version of my current compiler on my current target
> machine.
> 
>      There are two questions.  (1) What are they trying to prevent by
> this restriction?

First, you can't take 'Access of a nested procedure when the access type
is less nested (nested within procedures, I mean).  That's because you
could then call the procedure after the containing procedure is done, so
the nested one no longer exists (and the variables visible to it no
longer exist).

If you do the 'Access in a generic, the compiler doesn't know (when
compiling the generic) whether the instance will be nested.  The AARM says:

    32.a  Discussion: The part about generic bodies is worded in terms of the
          denoted subprogram, not the denoted view; this implies that renaming
          is invisible to this part of the rule. This rule is partly to
          prevent contract model problems with respect to the accessibility
          rules, and partly to ease shared-generic-body implementations, in
          which a subprogram declared in an instance needs to have a different
          calling convention from other subprograms with the same profile.

>... (2) Is there a more robust way of working around it?

The solution is easy: declare the procedure you're taking 'Access of in
the private part of the generic, rather than in the body.  And don't
instantiate the generic in a nested place.

- Bob