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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: condition true or false? ->  (-1 < sizeof("test"))
Date: Mon, 21 May 2012 08:45:41 -0700 (PDT)
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Date: 2012-05-21T08:45:41-07:00
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On Monday, May 21, 2012 8:28:44 AM UTC-7, Nomen Nescio wrote:
> Forwarding this to guys who write code in real languages to see what they
> think of this. AFAIK you cannot get something like that past the compiler=
 in
> Ada...and you would have to define a type or a subtype to even have an
> unsigned int unless you use a modular type IIRC.

The last part is pretty much correct.  If you do something like this in Ada=
:
=20
   B :=3D -1 < Some_Type'Size;

where B is a Boolean, it works like this: Some_Type'Size is a universal int=
eger, and it can be converted to any integer type.  However, in a case like=
 this where there's no variable or anything else that tells the language wh=
ich type to use, the language has a preference for "root_integer", which is=
 a signed integer type.  So the above would do what you expect (i.e. set B =
to True since 'Size can never be negative).  (The rule about preferring roo=
t_integer was added in Ada 95.  I don't know for sure whether the above is =
legal in Ada 83; I think this might have been an error.)

You could create a situation where -1 is treated as a modular type expressi=
on, so that the above would set B to False.  However, you really have to tr=
y.  For instance:

    type Word is mod 2**32;

    B :=3D -1 < Word' (Some_Type'Size);

would cause Some_Type'Size to be interpreted as a Word, and then the - and =
< operators are treated as the ones that operate on Word.  Or:

    One : constant Word :=3D 1;

    B :=3D -One < Some_Type'Size;

But you do have to have some modular type involved to get False for B, and =
you have to either use the type name in the expression, use the name of an =
object of that type, or do something like

    B :=3D Thing_Defining_Modular_Type."<" (-1, Some_Type'Size);

It won't happen implicitly.

                           -- Adam