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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: "access constant" discriminant
Date: 19 Feb 2003 18:20:45 -0800
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tmoran@acm.org wrote in message news:<3CS1a.55972$2H6.1357@sccrnsc04>...
> >What is it that you want to do?
>    In the case at hand, I'm using the idiom of a limited controlled type
> whose initialization/finalization grabs/releases a resource lock, and the
> access discriminant tells which resource.
>   type Lock_Type(Resource : access Resource_Type) is new
>   Ada.Finalization.Limited_Controlled with ...
> and later
>   Locker : Lock_Type(Resource'unchecked_access);
>   ...
> But this won't work when Resource is an "in" parameter, eg a function
> parameter.  I suppose Ada is telling me that grabbing a lock is
> effectively modifying the resource, and thus something that shouldn't be
> done when the resource is an "in" parameter, but the question remains, why
> can't one say
>   type Lock_Type(Resource : access constant Resource_Type) ...
> Did the designers foresee and forestall my use with a function?

If your resource is limited, then use the Rosen Trick, and add a
handle to the type's representation:

procedure Op (Resource : in Resource_Type) is

  Lock : Lock_Type (Resource.Handle.Resource);
begin

where Handle is 

type Handle_Type (Resource : access Resource_Type) is
   limited null record;

type Resource_Type is
   limited record 
      Handle : Handle_Type (Resource_Type'Access);
      ...
   end record;

If your resource type isn't limited, then you can safely use
Address_To_Access_Conversions the type is pass-by-reference.