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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: newbie Q: storage management
Date: 1997/05/04
Message-ID: <dewar.862773676@merv>#1/1
X-Deja-AN: 239333846
References: <5k5hif$7r5@bcrkh13.bnr.ca> <JSA.97Apr30173548@alexandria>
 <dewar.862625257@merv> <5kgtf1$js1@bcrkh13.bnr.ca>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-05-04T00:00:00+00:00
List-Id: <comp.lang.ada>


Kaz says

<<to supply GC, a maximally portable Ada program cannot rely on its presence. It
would be foolhardy to write a program that expects the implementation to locate
unreachable objects and reclaim them, when the language doesn't require the
facility.>>


Actually the issue is not so much what the language requires (as I explained
in a previous post, it is hard to require GC in any formal sense). What is
important is what you wll actually find out there.

As an example, consider the following. There is nothing in the Ada RM that
requires a implementation to support more than one digit of precision for
floating-point. This means that, by the same reasoning as Kaz is using
above, a maximally portable Ada program should not have a declaration like

   type x is digits 6;

However, in the real world, no one knows any Ada implemnetation where
Max_Digits is less than 6, so the above declaration is in practice
entirely portable, even though not blessed by the language.

On the other hand, you cannot get much of a hint from the RM that the
declaration

   type x (y : integer := 0) is record
     q : string (1 .. y);
   end record;

is problematic, and indeed it will work on some implementations. But if
you know the real world of Ada implemntations, then you will know that
the above variant record declaration is not a good idea, and will cause
unexpected storage errors on some (most?) Ada 95 implementations.