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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: [Q] To initialise or not.
Date: 1996/04/27
Message-ID: <DqJ9D6.345@world.std.com>#1/1
X-Deja-AN: 151741547
references: <484274071wnr@diphi.demon.co.uk> <3180CF00.17D5@lmtas.lmco.com>
 <dewar.830583555@schonberg>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-04-27T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.830583555@schonberg>, Robert Dewar <dewar@cs.nyu.edu> wrote:
>The whole idea is to guarantee predictability by initializing everything
>to a known and consistent value, WITHOUT necessarily providing a valid
>value on which the proram could (incorrectly) depend.

Right.  In fact, the compiler is *supposed* to initialize to an
*invalid* value, whenever possible, in order to cause an exception.
The reason this is not *required* is that some types don't have any
invalid values.  For example, type Integer likely uses up all the
available bits for representing valid values, and we didn't want to
require the compiler to store an extra bit in those cases.  Requiring
that would complicate the compiler, cause inefficiency, and cause
trouble when interfacing to other languages.

Another example: in a packed array of Booleans, you get one bit per
component, which doesn't leave any extra bits for invalid values.  A
stand-alone Boolean variable, on the other hand, will probably get 8
bits, or 32 bits, so pragma Normalize_Scalars will cause "X: Boolean;"
to be initialized to some bit pattern that represents neither True nor
False.

- Bob