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From: "John B. Matthews" <nospam@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Operation can be dispatching in only one type
Date: Wed, 02 Dec 2009 07:35:39 -0500
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Date: 2009-12-02T07:35:39-05:00
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In article <1jcbtmi5rztyp$.norvlhez9i9$.dlg@40tude.net>,
 "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote:

Georg Bauhaus wrote:

> >  From the Java point of view, there are no false positives
> > or negatives.
> 
> Of course there are, because it is a halting problem. So you need 
> some optimistic or pessimistic estimation of:
> 
> if False then
>    Print (X.Tire); -- Is this illegal? If yes, that is a false positive
> end if;

It would be indeterminate but for doing "a specific conservative flow 
analysis." In the specific cases of boolean constant expressions and if 
statements,

  if (true) { spare = new Tire(); } // spare assigned
  if (1 == 1) { spare = new Tire(); } // spare assigned
  if (false) { spare = new Tire(); } // spare unassigned
  if (1 == 2) { spare = new Tire(); } // spare unassigned
  if (always()) { spare = new Tire(); } // spare unassigned

  static boolean always() { return true; }

In particular, spare remains unassigned even though the function 
always() is always true.

In Ada, a certain compiler may warn 'variable "N" is assigned but never 
read' for the following:

N : Integer;
if False then N := 1; end if;

I'm not especially advocating applying the Java rules to Ada, but I like 
the warning.

<http://java.sun.com/docs/books/jls/third_edition/html/defAssign.html>

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>