Re: Type of subtraction operator

[Adam Beneschan <adam@irvine.com>]

On May 28, 2:23 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> According to 4.5.3/2, each numeric type has a subtraction operator
> with this specification:
>
> function "-"(Left, Right : T) return T
>
> This means that the type of difference is the same as the type of
> operands.
>
> Which means that if T is Natural, then I should expect
> CONSTRAINT_ERROR when the result of subtraction is less than 0.
> But it does not happen:

Natural is not a type.  It's a subtype.  The subtraction operator is
defined on the underlying type, Integer's type, rather than the
subtype.  (Integer is also a subtype, rather than a type; see 3.5.4
(11).)  The result of the function is then converted to the subtype
Integer, and no Constraint_Error occurs since -2 is within Integer's
range.


> with Ada.Text_IO;
> procedure Test is
>
>    X : Natural := 5;
>    Y : Natural := 7;
>    Z : Integer := X - Y;   -- here the result is not in Natural'Range
>
> begin
>    Ada.Text_IO.Put_Line (Integer'Image (Z));
> end Test;
>
> The above happily prints -2.
> I thought initially that it is a subtype that is responsible for this,
> but defining My_Natural type as a completely new type (with
> appropriate type cast when Z is initialized) gives the same result.

My_Natural is not a type either.  It's a subtype of some new integer
base type whose base range will include both negative and positive
numbers (symmetric about zero)---see 3.5.4(1), 3.5.4(9).


> I conclude that the effective type of difference (the return type of
> the "-" operator) is T'Base instead of T.

No, it's T.  4.5.3(1) refers to T as a type, not a subtype.  Your
mistake, which is natural enough (hee hee hee), was in assuming that
My_Natural is a type.  The "-" operator is defined on the base type of
which My_Natural is a subtype.

Yep, it's confusing.  It took me some time to figure all this out too.

                                   -- Adam