Re: Type safety, C++ and code generation

Georg Bauhaus wrote:

type ranged_type<int, 0, 250> Speed;
Speed s1, s2, s3; // with some values
s1 = s2 + s3; // OK
s1 = s2 * s3; // not OK

The addition is fine, but the multiplication should not be provided,
because speed multiplied by speed is not a speed. Can you extend your
class so that the compiler will refuse to compile the second operation
above?
(Ada, anyone? :) )


Just so it is visible:


procedure useop is

s1, s2, s3: SPEED; -- with some values

begin
s3 := s1 * s2;
end useop;

8. s3 := s1 * s2;
|
>>> cannot call abstract subprogram "*"

where

package Op is

type SPEED is range 0 .. 250;

function "*"(a, b: SPEED) return SPEED is abstract;

end Op;

I like it, although there is some potential problem with this approach. It uses the "negative logic" - in other words, specifies what is forbidden, not what is allowed - so it's more prone to errors than the "positive logic", where you specify what *is* supported instead. In the "positive logic" approach you can start with the default setting (no operations) and the compiler will point you to each new operation usage in your code - then, you can either consciously extend the type definition to cover the new use case or reject it if it was a bug. It's much more cumbersome with the "negative logic", where the default setting provides no protection at all.


--
Maciej Sobczak : http://www.msobczak.com/
Programming : http://www.msobczak.com/prog/
.

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