Non-null objects, the Null Object pattern, and T.init

[Walter Bright]

On 1/18/2014 2:16 PM, Michel Fortin wrote:
> It works, up to a point.
>
>      void foo(RangedInt!(0, 5) a);
>
>      void bar(RangedInt!(0, 10) a)
>      {
>          if (a < 5)
>              foo(a); // what happens here?
>      }
>
> In that "foo(a)" line, depending on the implementation of RangedInt you either
> get a compile-time error that RangedInt!(0, 10) can't be implicitly converted to
> RangedInt!(0, 5) and have to explicitly convert it, or you get implicit
> conversion with a runtime check that throws.

Yes, and I'm not seeing the problem. (The runtime check may also be removed by 
the optimizer.)


> Just like pointers, not knowing about the actual control flow pushes range
> constrains enforcement at runtime in situations like this one.

With pointers, the enforcement only happens when converting a pointer to a 
nonnull pointer.

> In fact, even the most obvious case can't be caught at compile-time with the
> template approach:
>
>      void baz()
>      {
>          foo(999); // runtime error here
>      }

Sure it can. Inlining, etc., and appropriate use of compile time constraints.