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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Call by reference vs. call by value
Date: 1996/07/22
Message-ID: <Duy5ur.4Dt@world.std.com>#1/1
X-Deja-AN: 170539113
references: <31F10E50.726@egr.uri.edu> <dewar.837901109@schonberg>
 <DuwxvJ.G4t@world.std.com> <dewar.837995690@schonberg>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-07-22T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.837995690@schonberg>, Robert Dewar <dewar@cs.nyu.edu> wrote:
>If you want to think more about this issue, worry about packed arrays
>too. Requiring call by reference would mean that ALL packed arrays have
>to be passed using general bit pointers, which would be unacceptably
>inefficient in the normal case where slices are not passed. Same
>thing for records, all records would have to be passed by bit address,
>just in case the record you are passing is a field in a rcord with a
>record rep clause.

Pascal solved this problem in a simple (but rather ugly) way: It's
illegal to pass a component of a packed array or record as a parameter.
The *programmer* must declare a temp variable, and make a copy.  And
Pascal doesn't have slices.

I suspect part of the reason for Ada's rules (which of course predate
the DS annex by a decade) is to avoid the ugliness of the Pascal rule.
Unfortunately, the Ada rule introduces an implementation dependence that
I find uncomfortable.

- Bob