Re: Unclear error message - please help

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

Martin Krischik <krischik@users.sourceforge.net> writes:

> Alfred Hilscher wrote:
> 
> >   subtype
> >     Sixteen_Bits is Integer range 0..65535;
> 
> The name "Sixteen_Bits" suggest a different implementation:
> 
> type Sixteen_Bits is mod 2 ** 16;
> for Sixteen_Bits'Size use 16;
> 
> A "range 0..65535" needs 17 bits and an "is Integer" is usually 32 bit.

I don't really agree with that advice.  Modular types should be used
only in certain rare circumstances.  (They are, perhaps, my least
favorite feature of Ada.)

Anyway, Sixteen_Bits is 16 bits.  I mean, Sixteen_Bits'Size = 16.
There's no implementation choice here -- that's required of all
implementations.  And you can use rep clauses or pragmas Pack
to ensure that objects of this subtype are actually allocated
16 bits.  Without such clauses or Pack, the compiler is free
to do whatever it likes -- probably allocate 32 bits for each object.
17 bits is pretty unlikely!  ;-)

Arithmetic will be done in the base range, however -- you'll get
overflows past Integer'First..Integer'Last, not 0..2**16-1.

> See
> 
> http://en.wikibooks.org/wiki/Ada_Programming/Types/range
> http://en.wikibooks.org/wiki/Ada_Programming/Types/mod
> 
> for more info.

- Bob