Empty VS null array?

[H. S. Teoh]

On Mon, Oct 21, 2013 at 04:41:23PM +0100, Regan Heath wrote:
> On Mon, 21 Oct 2013 15:01:04 +0100, H. S. Teoh
> <hsteoh at quickfur.ath.cx> wrote:
> 
> >On Mon, Oct 21, 2013 at 11:53:44AM +0100, Regan Heath wrote:
[...]
> >>I agree you cannot logically have an existing array that is somehow
> >>a "null array" and distinct/different from an empty array, but
> >>that's not what I want/am asking for.  I want to use an array
> >>'reference' to represent that the array is non existent, has not
> >>been set, has not been defined, etc.  This is what null is for.
> >
> >The thing is, D slices are value types even though the elements they
> >point to are pointed to by reference. If you treat slices (slices
> >themselves, that is, not the elements they refer to) as value types,
> >then the problem goes away. If you want to have a *reference* to a
> >slice, then you simply write T[]* and then it becomes nullable as
> >expected.
> 
> True, and that's a pointer, and I am comfortable using pointers..
> however I worry this will limit the compilers ability to optimise
> somehow.. and doesn't it make the code immediately un"safe"?

No, pointers are allowed in @safe. What is not allowed is pointer
*arithmetic* and casting pointers into pointers of different types.


> >I do agree that the current situation is confusing, though, mainly
> >because you can write `if (arr is null)`, which then makes you think
> >of it as a reference type. I think that should be prohibited, and
> >slices should be treated as pure value types, and all comparisons
> >should be checked with .length (or .empty if you import std.range).
> 
> IMO, this would be preferable to the current situation even thought I
> would rather go the other way and have a reference type.  I can see
> the argument that it would be safer and easier for most users, even
> though I do not believe I am in that category.
[...]

Well, either way would work, though I do prefer treating slices as value
types. It's just cleaner conceptually, IMO. But I suppose this is one of
those things in which reasonable people may disagree.


T

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