Null references (oh no, not again!)

[Andrei Alexandrescu]

Don wrote:
> Andrei Alexandrescu wrote:
>> And there is no reference type with two subtypes. It's one type in the 
>> language and one in the library. Maybe-null (the library) is a 
>> supertype of non-null (the default).
> 
> One problem I can see is with extern(C),(Windows) functions, since 
> pointers are maybe-null in C. The name-mangling has to work out.
> I can't see how this can be done without the compiler knowing SOMETHING 
> about both nullable and non-nullable types.
> At the bare minimum, you need to deal with maybe-null returns and 
> reference parameters from C functions.

Walter is thinking of making only references non-null and leaving 
pointers as they are. (I know, cry of horror.) But say pointers are also 
non-null. Then:

extern(C) MaybeNull!(void*) malloc(size_t s);

will work, provided that MaybeNull has no size overhead and that 
word-sized structs are returned in the same register as word returns (I 
seem to remember Walter told me that's the case already).

> Incidentally, uses of maybe-null in immutable types must be _extremely_ 
> rare. The usage statistics for D are likely to be skewed towards 
> non-null types, compared to Java. I'd say the 66% is a very conservative 
> lower bound.

Very good point. With a mutable null pointer, there's at least hope it 
will become something interesting later :o).


Andrei