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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Ada 95 Numerics questions for the experts
Date: 1997/08/28
Message-ID: <dewar.872797072@merv>#1/1
X-Deja-AN: 268810817
References: <3401C14B.480@gsfc.nasa.gov> <dewar.872560585@merv>
 <jcreem-2608970711210001@ljd155.sanders.lockheed.com> <dewar.872630607@merv>
 <jeffrey.m.creem-2708970802350001@ljd155.sanders.lockheed.com>
 <dewar.872715585@merv>
 <jeffrey.m.creem-2808970708260001@ljd155.sanders.lockheed.com>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-08-28T00:00:00+00:00
List-Id: <comp.lang.ada>


Jeff said

<<do. Since most of my prior experience has been with
VADS I never took note of that LRM reference however I still am a fan of
the size clause being applied after the pragma pack since if the pack does
not pack the elements as tightly as you'd like the size clause gives the
compiler an oportunity to complain that it can not fit the structure into
the number of bits your have requested.>>

Indeed, the confirming size clause is definitely appropriate. In fact consider
the example that inspired the thread:

  type x is range 0 .. 7;
  for x'size use 3;

  type r is array (1 .. 8) of x;
  pragma Pack (r);
  for r'size use 24;

The size clause says here, I *really* want tight packing, the RM allowed
expansion to 4 bits is not acceptable.

Now your code will be accpted by GNAT that is doing what you want, but
rejected by a compiler taking advantage of the RM's permissiveness here.

Of course this rejection may mean you have a problem on your hands, but
at least you know right away at compile time.