char[] == null

[Chris Wright via Digitalmars-d-learn]

On Wed, 18 Nov 2015 20:57:06 +0000, Spacen Jasset wrote:

> Should this be allowed? What is it's purpose? It could compare two
> arrays, but surely not that each element of type char is null?
> 
> char[] buffer;
> if (buffer == null) {}

'null' is a value of ambiguous type. The compiler finds a set of 
compatible types for them by applying known implicit conversions. 'null' 
can implicitly convert to 'char[]'.

Arrays, of course, are tuples consisting of a start pointer and length. A 
null array is essentially {ptr = null, length = 0}. Array equality is 
implemented as, roughly:

---
if (a.length != b.length) return false;
foreach (i, v; a) {
  if (v != b[i]) return false;
}
return true;
---

(That's not quite how it's implemented; it uses runtime functions and 
indirection. But it's the same algorithm.)

This shows us that all 0-length arrays are equal. Pretty much as we'd 
expect. So your code is equivalent to:

---
char[] buffer;
if (buffer.length == 0) {}
---