Re: Uninitialized Variables

[robert_dewar@my-dejanews.com]

In article <rracine.7.000B4B63@draper.com>,
  rracine@draper.com (Roger Racine) wrote:
> One of the few areas where Ada programs exhibit
non-deterministic behavior
> (i.e. different behavior for different runs) is when
variables are not set
> before their use.
>
> I am curious as to why the language designers did not
> think this was sufficiently common enough of an error to
> protect against it.  I understand  that having an initial
> value takes more memory and more CPU, but so do
> constraint checks.

You miss the philosophical and design objections to junk
initial values. These arguments are familiar so I won't
repeat them in detail here!

> And there could always have been a pragma to allow
> uninitialized variables (akin to pragma SUPPRESS).

Ada 95 of course has a pragma, but it has the opposite
sense, you use pragma Normalize_Scalars if you want
initializations. The intention of this pragma is precisely
to avoid non-determinism. Why is it not enough for you?

> Does anyone know why there is not at least a required
> link-time warning message for variables possibly used
> before being set?

First, a full analysis at link time of all possible static
flows through a program would be a very heavy burden to
place on a compiler. It is far out of reach of what could
be done in existing implementation frameworks, and I would
have significant efficiency concerns.

Second, requiring warning messages is a big bogus in
anyway. Such requirements are not testable, and not
well-defined. I think it reasonable to let the marketplace
determine the need for warnings.

It is certainly useful for a compiler to do a reasonable
job of detecting obvious cases (try using -gnatwu with
GNAT 3.11p).

Robert Dewar

-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/       Search, Read, Discuss, or Start Your Own