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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Ada 83 - avoiding unchecked conversions.
Date: 1996/12/18
Message-ID: <E2MvBL.EHG@world.std.com>#1/1
X-Deja-AN: 204823984
references: <md5:8B831999BCF200C6E70994BDF6CC529F> <58np72$9qp@news.nyu.edu>
 <32B6905F.FC4@tiac.net> <dewar.850929268@merv>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-12-18T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.850929268@merv>, Robert Dewar <dewar@merv.cs.nyu.edu> wrote:
>iEric said
>"using overlays.  Apparently on that compiler (VAX Ada 1.something)
>unchecked conversion was doing some checks.
>"
>Note that the unchecked in unchecked conversion is talking about omitting
>normal compile time checks for type correctness. It is NOT talking about

Another efficiency issue about unch_conv is whether or not it makes a
copy.  An instance of U_C is a function, so one would normally assume
that calling it makes a copy of the argument.  This is what the Ada 83
RM said, and this is no doubt why U_C was inefficient in some early
version of VAX Ada.  However, the Ada 83 ARG ruled that the function
result may be returned by reference in some cases, and Ada 95 agrees
with that ruling (see 13.9(12)).  So if you do:

    Some_Procedure(Unch_Conv(X));

then Some_Procedure might actually be receiving the address of X, rather
than the address of a copy of X.  This is different (and often more
efficient) than the semantics normal function calls.  And the difference
is detectable.

- Bob