arrays: if(null == [ ])

[Jonathan M Davis]

On Monday, May 14, 2012 14:08:17 Gor Gyolchanyan wrote:
> Hi! I have a small question:
> Is the test for a null array equivalent to a test for zero-length array?
> This is particularly interesting for strings.
> For instance, I could return an empty string from a toString-like function
> and the empty string would be printed, but If I returned a null string,
> that would indicate, that there is no string representation and it would
> cause some default string to be printed.
> So, the question is, if a null array is any different from an empty array?

A null array is equal to an empty array.

assert(null == []);
assert(null == "");

It's when you use is that things change. An array "is null" only if it's ptr 
property is null, which is true only for uninitialized arrays and [] (the 
compiler doesn't bother allocating memory for [], so it's basically the same 
as null). However, since "" is a string literal, it _does_ have memory 
allocated to it, so

assert([] is null);
assert("" !is null);

So, it all comes down to the ptr property. An array is considered to have a 
length of 0 if its length property is 0 (which null arrays do). It's only 
considered to be null if its ptr property is null. == uses the length 
property, ignoring the ptr property as long as the lengths are equal (and 
checking each of the elements refered to by the ptr property if the lengths 
aren't equal), whereas is specifically checks whether the ptr properties of the 
two arrays are equal.

Personally, I think that conflating null and empty like this is atrocious, but 
that's how the language works, and we're stuck with it.

- Jonathan M Davis