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From: stt@inmet.inmet.com
Newsgroups: comp.lang.ada
Subject: Re: ADA Compiling Query
Message-ID: <20600085@inmet>
Date: 18 Feb 91 14:33:00 GMT
References: <5572@baird.cs.strath.ac.uk>
Nf-ID: #R:baird.cs.strath.ac.uk:5572:inmet:20600085:000:1265
Nf-From: inmet.inmet.com!stt    Feb 18 09:33:00 1991
List-Id: <comp.lang.ada>


Re: passing structured parameters by reference

Michael Feldman writes:

> PS: It seems to me that Ada9x could clarify the issue by simply requiring
> that structured parameters be passed by reference (instead of the Ada83
> rule that it's implementation-dependent). Since a program whose behavior
> depends upon the method of passing is - by definition of the LRM -
> erroneous, the only programs that would break would be erroneous ones,
> which Ada9x says it doesn't care about. So the clarification would be
> upward compatible. Ada9x-ers: what would be the objections?

Here are some important reasons for allowing by-copy parameter passing:

1) It supports parameter passing between parts of a distributed
program which don't share memory.

2) It allows a slice of a packed array to be copied into an aligned
temporary, rather than forcing every subprogram to handle a descriptor
for an unaligned parameter.

3) It allows very short arrays/records to be passed in registers
(e.g. a packed array of 16 booleans).

4) It is necessary when the actual parameter is in the form of a type
conversion, and the target and source type don't have the same
representation (e.g. one is packed and the other isn't).

S. Tucker Taft
Intermetrics, Inc.
Cambridge, MA  02138