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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Future Ada language revisions?
Date: 1998/10/06
Message-ID: <EACHUS.98Oct6183832@spectre.mitre.org>#1/1
X-Deja-AN: 398426442
References: <6um7on$db5$1@nnrp1.dejanews.com>
 <EACHUS.98Oct2164706@spectre.mitre.org>
 <Pine.BSF.4.02A.9810031123180.20730-100000@shell5.ba.best.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-10-06T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <Pine.BSF.4.02A.9810031123180.20730-100000@shell5.ba.best.com> Brian Rogoff <bpr@shell5.ba.best.com> writes:

  > Yes, this is nice, though I (perhaps wrongly) feel that this solution is
  > kind of heavy in terms of performance costs, especially if I want to have 
  > side-effecting functions on objects with value semantics.

  1) You can do that!  The compiler may warn you that you are being
naughty, but you knew that.  The idiom is to use an attribute
definition clause to overlay an internal object:

with Text_IO;
procedure Cheat_Test is
   Test_String: String := "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
   Junk: Character;

   function Cheat(X: in String) return Character is
     Temp: Character := X(X'First);
     Overlay: String(X'Range);
     for Overlay'Address use X'Address;
   begin
     Overlay(X'First) := 'Z';
     return Temp;
   end Cheat;

begin

   Text_IO.Put_Line(Test_String);
   Junk := Cheat(Test_String);
   Text_IO.Put_Line(Test_String);

end Cheat_Test;

   Using GNAT on a Sun this program produces no warnings and:

spectre% cheat_test
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZBCDEFGHIJKLMNOPQRSTUVWXYZ
spectre% 

   If you look at RM 13.3(13), we went to a great deal of effort to
write it to leave erroneousness to the programmer.  If you know what
you are doing, there is no problem:

   "If an Address is specified, it is the programmer's responsibility
to ensure that the address is valid; otherwise, program execution is
erroneous."

  > Eh? READERS will look at the interface to the function and see an out mode 
  > parameter, so they'll only be surprised if they don't read the spec.
  > "Against stupidity the gods themselves contend in vain", right?

  Correct, but irrelevant.  If the style in Ada is that functional
notation means that--in spite of the realities above--the effect of a
function on its parameters is benign, then a function which does
violence to its parameters will always be unexpected.

  > I see "distributed cost" has become overloaded now ;-). I think if a shop 
  > had a rule of "no access mode" or no "_Type" in type names or whatever
  > there is also a similar cost, but I don't think that means that should be 
  > language rules enforcing restrictions here.

  No, in Ada design it has always been used to mean a "feature" which
makes things more difficult even if it is not used.  If you use the
above approach, there is no distributed cost--you take the
responsibility for what happens if the argument is a constant, and any
compiler optimizer will know that an address clause is bad juju--the
hardware may even play with the value.

--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...