Re: Ranges and (non)static constraints

[Jean-Pierre Rosen <rosen@adalog.fr>]

Dmitry A. Kazakov a �crit :
> On Thu, 16 Nov 2006 12:02:15 +0100, Maciej Sobczak wrote:
> 
>> type T is range 1 .. N;
>> type U is new Integer range 1 .. M;
>>
>> N must be static, but M does not have to.
>> Why and what is the real difference between T and U?
> 
> Informally, the second is an abbreviation for:
> 
>    type <anonymous> is new Integer;
>    subtype U is <anonymous> range 1..M;
> 
> so "range" refers to a subtype. On the contrary, in the first, "range"
> refers a type. So the difference.
> 
> Is it real? I don't think so. 

Sorry, but there is a huge difference. With U, the range cannot be 
outside the range of Integer, under penalty of Constraint_Error. T is 
valid as long as the compiler offers a big enough integer type, and if 
it doesn't, it won't compile.

Actually, the second form should never be used: you are relying on 
Integer, a non-portable type that plays absolutely (or almost) no 
special role in Ada. Why derive from Integer, rather than from 
Long_Integer, or anything else?

If you want dynamic bounds, remember that anything "dynamic" has to have 
an upper limit at some point. It is thus better to write:

type Biggest_T is range 1 .. Absolute_Max_Expectable_Value;
subtype T is Biggest_T range 1 .. N;
-- 
---------------------------------------------------------
            J-P. Rosen (rosen@adalog.fr)
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