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Date: 13 Dec 91 17:24:40 GMT
From: elroy.jpl.nasa.gov!swrinde!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!ds
inc!gvlf3.gvl.unisys.com!email!parkhill@ames.arc.nasa.gov  (parkhill)
Subject: Re: 'SIZE attribute of a type
Message-ID: <5907@email.sp.unisys.com>
List-Id: <comp.lang.ada>

warack@dip.eecs.umich.edu (Christopher Warack) writes:
> In article <5898@email.sp.unisys.com> parkhill@email.sp.unisys.com (parkhill)
 writes:
> >>>Image a compiler was smart enough to understand baised number ranges. 
> >>
> >>>   type b is range 9 .. 10; 
> >>>        or 
> >>>   type b is range 2**31 - 2 .. 2**31 - 1;
> >>
> >>
> >>>If the compiler can generate code that only uses 1 bit then b'Size 
> >>>should return 1.
> >> 
> >>Oh, that compilers were so cleverly written! 
> >>
> >
> >> Deleted text.
> >
> >My point on the compiler having the capability to use one bit is that I
> >find the utility of type_name'size is now questionable.  Lets say the
> >compiler is capable of using 1 bit but won't do it in most situations.
> >How can any programmer use type_name'size to any practical purpose?  It 
> >seems less useful than Entry_Name'Count.  However, I am sure that the 
> >justification for the change to type_name'size is rock solid.
> 
> Seems to me that the only reason you'd want the minimum number of bits
> used to represent a type is if you wanted to implement that type with the
> minimum number of bits allowed, eg, in a maximally packed record using
> a "somewhat" portable rep clause.  (Not the most beautiful piece of code I
> can imagine).
> 
> If you wanted to know the number of bits in a certain type of object (e.g,
> a variable of a type) use object'size -- that's what it's for.
> 
> -- Chris
> 
> -- 
> Christopher A. Warack                   warack@eecs.umich.edu
> Graduate Dept, EECS			(313) 665-4789
> University of Michigan



You are quite correct,  one of the only reasons you would want to use the
minimum number of bits that minimum bits are required.  I am often 
required to write device drivers in Ada.  The device being communicated
with sets the protocol.  Some of the military devices I communicate with
used biased representation for input and output values.  If the compiler
could generate code for this I wouldn't have to do it.  I think
portability goes or when you are doing memory mapped I/O to external devices.

Yes, I know Object_Name'Size tells you the number of bits used by an
object.  What is Type_Name'Size for??

Robert Parkhill