From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,c6b9934af04bce01
X-Google-Attributes: gid103376,public
From: John English <je@bton.ac.uk>
Subject: Re: A question to the "access-type lifetime rule"
Date: 2000/04/04
Message-ID: <38E9E6A5.AC631EEA@bton.ac.uk>#1/1
X-Deja-AN: 606543772
Content-Transfer-Encoding: 7bit
References: <38E89AB9.7D33E88C@hamilton.physik.uni-konstanz.de>
X-Accept-Language: en
Content-Type: text/plain; charset=us-ascii
X-Complaints-To: news@bton.ac.uk
X-Trace: saturn.bton.ac.uk 954853074 25667 193.62.183.204 (4 Apr 2000 12:57:54
 GMT)
Organization: University of Brighton
Mime-Version: 1.0
NNTP-Posting-Date: 4 Apr 2000 12:57:54 GMT
Newsgroups: comp.lang.ada
Date: 2000-04-04T12:57:54+00:00
List-Id: <comp.lang.ada>

Bernd Rinn wrote:
> 
> Hi,
> 
> I am new to Ada and I am looking for an elegant way to get around a
> restriction posed by the "access-type lifetime rule" for
> access-to-subprogram types. First of all: does anyone know, why the
> access-type lifetime matters and not the access-variable lifetime? (This
> should prevent dangling pointers as well, I guess.)
> 
> I want to write an numerical integration function, that is defined in a
> separate package as
> 
> type Integrand_Type is access function ( X : Long_Float ) return
> Long_Float;
> function Integrate(Integrand : Integrand_Type; Lower_Bound, Upper_Bound
> : Long_Float) return Long_Float;
> 
> Now I would like to call this from a main procedure and pass an access
> attribute of a nested function of the main procedure as `Integrand'. Is
> there a way to reach this? (Unchecked_Access is not an allowed attribute
> for subprograms and I do not want to generate a package of it's own for
> the integrand function.)

If you declare the package where Integrate is declared like this:
  generic
  package X is ...

then in the same declaration section as the nested function you say
this:
  package Inner_X is new X;

and voila, you have smuggled the declarations from X into the inner
scope (the access level of Inner_X being the level at which the
instantiation occurs).

-----------------------------------------------------------------
 John English              | mailto:je@brighton.ac.uk
 Senior Lecturer           | http://www.it.bton.ac.uk/staff/je
 Dept. of Computing        | ** NON-PROFIT CD FOR CS STUDENTS **
 University of Brighton    |    -- see http://burks.bton.ac.uk
-----------------------------------------------------------------