Re: The right way to handle this difference in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 22 Jul 2004 12:23:09 +0200, vic wrote:

> Hello,
> 
> I'm starting to develope in Ada 95, but as I come from C++ language I'm
> having some difficulties about strong typing.
> 
> Please consider this type:
> 
> type unsigned_int_12_type is range 0..(2*12)-1;
> 
> and these 2 variables of the above type:
> 
> a: unsigned_int_12_type;
> b: unsigned_int_12_type;
> 
> If the code must perform a difference between a and b, say:
> 
> c := a-b
> 
> 1) which should be the type of c? I think it should be something like:
> 
> type signed_int_12_type is range -unsigned_int_12'last ..
> unsigned_int_12'last.
> 
> Is this right?

It depends!

1. In C++ spirit I would expect something like

   type unsigned_int_12_type is mod 2**12;

and the types of a,b,c all same. So that:

   a : unsigned_int_12_type := 1;
   b : unsigned_int_12_type := 2;
   c : unsigned_int_12_type := a - b; -- yields 4095

2. Well, if c should be negative then probably:

   type int_12_type is range -2**12..2**12-1;
   subtype unsigned_int_12_subtype is
      int_12_type range 0..int_12_type'Last;

   a : unsigned_int_12_subtype := 1;
   b : unsigned_int_12_subtype := 2;
   c : int_12_type := a - b; -- yields -1

Here unsigned_int_12_subtype is a subtype. Though its values cannot be
negative, temporary values can be. It is similar to the solution of
proposed by Nick Roberts, but uses a separate type int_12_type instead of
standard Integer, mainly for strong typing sake.

> 2) And I should rename a "-" operation which gets two signed_int_12_type  as
> operands and returns signed_int_12_type as result?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de