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From: "Norman H. Cohen" <ncohen@watson.ibm.com>
Subject: Re: Unbounded strings (Was: Java vs Ada 95 (Was Re: Once again,
 Ada absent from DoD SBIR solicitation))
Date: 1996/11/25
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Date: 1996-11-25T00:00:00+00:00
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Robert Dewar wrote:

> Friendly readings are fine, but I do not know what is friendly and what
> is not.
> 
> Let's look at the reference count case.  I see two friendly readings here:
> 
>   1.  An efficient implementation with reference counts is permitted, no
>       locks are needed, since if two tasks mess with the same sets of
>       variables or values, the execution is erroneous.
> 
>   2.  If reference counts are used, task locks are required, requiring
>       a kernel call on some systems, and therefore, becuase of the
>       quite unacceptable efficiency effect, reference counts are out
>       of the question.
> 
> Which of these is friendlier? Well if you don't use tasking, or if you
> use only completely separated sets of values and variables in the
> tasking case, you find 1 much friendlier, since a reference count
> implementation makes a lot of sense in these cases.
> 
> If you insist on using unbounded strings across tasks, thinking that
> the assignment semantics are like ordinary scalar types, then of course
> you might find 1 friendlier.

What I would find friendliest is a run-time system with two versions of
Ada.Strings.Unbounded.  The version that uses reference counts but no
locks would only be linked in when I said

   pragma Restrictions (Max_Tasks => 1);

Otherwise, I would get the task-safe version (presumably without
reference counts, since the cost of locking would most likely outweigh
any savings from lazy copying).

-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen