Empty Array is Null?

[Frits van Bommel]

torhu wrote:
> Frits van Bommel wrote:
>> BCS wrote:
>>> Steven Schveighoffer wrote:
>>>>
>>>> Now, here is the weird part.  The compiler does some magic with 
>>>> arrays.  If you are comparing an array with null, it changes the 
>>>> code to actually just compare the array pointer to null.  So, the 
>>>> the following code:
>>>>
>>>
>>> no this is the weird part:
>>>
>>> IIRC this passes.
>>>
>>> char* cp = cast(char*)null;
>>> char[] ca = ap[0..15];
>>> assert(ca.ptr == null && ca.length == 15);
>>
>> It does with DMD, if you s/ap/cp/, but I'm pretty sure what you're 
>> doing is invoking undefined behavior. Or at least it should be, but I 
>> can't seem to find any mention of it in the spec...
> 
> Can't see why that would be undefined.  It's pretty clear what it means.

Yes, it means constructing a completely invalid array. :)
Though perhaps it should only be undefined if you ever try to read or 
write the elements?

> Perhaps it should be an error when the compiler detects that you're 
> setting .ptr to null but .length to nonzero.  But the compiler can't be 
> expected to detect that in the general case, so it would be of limited 
> usefulness.  A bit like disallowing comparing objects to 'null' with ==.

I didn't say it should be detected, only that the compiler should be 
well within its rights to make your code crash if you do that :P. An 
error message would also be nice of course, but by no means required.
Though as mentioned above, maybe the undefined behavior could be 
postponed until you actually try to read or write to the array.

It's quite similar to dereferencing null pointers: the compiler can 
refuse to compile code that tries to do it, but most compilers will just 
generate crashing code...