char[] == null

[Kagamin via Digitalmars-d-learn]

On Thursday, 19 November 2015 at 03:53:48 UTC, Meta wrote:
> On Wednesday, 18 November 2015 at 23:53:01 UTC, Chris Wright 
> wrote:
>> ---
>> char[] buffer;
>> if (buffer.length == 0) {}
>> ---
>
> This is not true. Consider the following code:
>
> import std.stdio;
>
> void main()
> {
> 	int[] a = [0, 1, 2];
>         //4002E000 3
> 	writeln(a.ptr, " ", a.length);
>         //Is not triggered, obviously
>         assert(a == null);
> 	
> 	a.length = 0;
>         //4002E000 0
> 	writeln(a.ptr, " ", a.length, " ", a);
>         //Is not triggered, not as obvious
>         assert(a == null);
> }
>
> There are cases when an array may have 0 length but a non-null 
> pointer. If you want to check if an array's length is 0, you 
> must explicitly check its length member. Checking if an array 
> is equal to null only compares its pointer field to null. It 
> does *not* check the length.

Comparison is ok, weird behavior manifests in boolean context:
void main()
{
	int[] a = [0, 1, 2];
         //4002E000 3
         assert(a != null);
         assert(!!a); //passes, ok
	
	a.length = 0;
         //4002E000 0
         assert(a == null);
         assert(!!a); //still passes!
}