Re: Concatenation and Characters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jim Rogers <jimmaureenrogers@worldnet.att.net> writes:

> One of the reasons default initialization is difficult for numerics
> in Ada is because Ada allows you to define your own numeric types.
> This means that the language cannot assume any particular value is
> always a correct value for any numeric type.

True.  In fact, what would you like as the default initial value of
this:

    X: Integer range 1..0;

?

> Note that the 'Valid attribute can be checked before accessing any
> variable. This attribute returns TRUE when the variable contains a
> valid value and FALSE when the variable contains an invalid value.

Not "any variable" -- it doesn't work for composite types.

Anyway, 'Valid is irrelevant to the problem of uninitialized variables.
It's silly to check 'Valid of all your variables -- much easier to 
make sure they are initialized properly.  'Valid is for validating input
data, not for detecting program bugs.

By the way, it's interesting that a pre-1983 version of Ada (or maybe it
was called Green then) required run-time detection of uses of
uninitialized variables -- an exception was raised.  I presume they got
rid of that because of concerns about run-time speed (or lack of it).
I think that was a mistake: you could always use pragma Suppress if you
need the speed.  This decision (the trade-off between the debugging aid
and speed) should be in the hands of the programmer, not the language
designer.

Another issue with run-time detection is that it sometimes requires
extra bits to be allocated, which might mess up representations
(e.g., if you're interfacing to hardware).

- Bob