[code]
	int[] list;

	list = new int[0];

	std.stdio.writeln("Is Null ? ", (list is null));
[/code]

Result is "Is Null? true".

Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.

On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
> [code]
> 	int[] list;
>
> 	list = new int[0];
>
> 	std.stdio.writeln("Is Null ? ", (list is null));
> [/code]
>
> Result is "Is Null? true".
>
> Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.


Long discussion: http://forum.dlang.org/thread/rrrtkfosfnfuybblexow@forum.dlang.org

On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
> [code]
> 	int[] list;
>
> 	list = new int[0];
>
> 	std.stdio.writeln("Is Null ? ", (list is null));
> [/code]
>
> Result is "Is Null? true".
>
> Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.

Yes, it's correct behaviour. `array is null` checks whether array.ptr is null, which is the case for a 0-length array.

void main()
{
	auto a = new int[0];
	writeln(a.ptr);      //a.ptr is null
	
	auto a2 = new int[1];
	writeln(a2.ptr); //a2.ptr is not null
	a2 = a[0..$];    //Slice off the only element of a2
	writeln(a2.ptr); //Now a2.ptr is null
}

On Saturday, October 10, 2015 15:20:02 tcak via Digitalmars-d-learn wrote:
> [code]
>   int[] list;
>
>   list = new int[0];
>
>   std.stdio.writeln("Is Null ? ", (list is null));
> [/code]
>
> Result is "Is Null? true".
>
> Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.

It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0.

- Jonathan M Davis

On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote:
> It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0.
>
> - Jonathan M Davis

Look at my second example.

On Sunday, 11 October 2015 at 00:18:54 UTC, Meta wrote:
> On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote:
>> It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0.
>>
>> - Jonathan M Davis
>
> Look at my second example.

Sorry, I thought you were responding to me.

On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote:
> On Saturday, October 10, 2015 15:20:02 tcak via Digitalmars-d-learn wrote:
>> [code]
>>   int[] list;
>>
>>   list = new int[0];
>>
>>   std.stdio.writeln("Is Null ? ", (list is null));
>> [/code]
>>
>> Result is "Is Null? true".
>>
>> Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.
>
> It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0.
>
> - Jonathan M Davis

The situation is that the "length" parameter comes from user. Also the item values come from user as well. I create the array with "length" parameter. At another part of code, I check firstly whether the array is created [code] if( array is null ) [/code], then the items are checked for validation.

On Saturday, 10 October 2015 at 15:46:51 UTC, Meta wrote:
> On Saturday, 10 October 2015 at 15:20:04 UTC, tcak wrote:
>> [code]
>> 	int[] list;
>>
>> 	list = new int[0];
>>
>> 	std.stdio.writeln("Is Null ? ", (list is null));
>> [/code]
>>
>> Result is "Is Null? true".
>>
....
> }

Do I miss the point?
Shouldn't have "new int[0]" , "[]" , ".length = 0" and "null" the same meaning like "array is empty"? I never understood why [] and null have different meanings in other languages.

And following the discussion...In my mind, aren't pointer operations dangerous?...yes,sometimes necessary...but always like an operation on a open heart.

Regards, Ozan

On Sunday, October 11, 2015 05:10:34 tcak via Digitalmars-d-learn wrote:
> On Saturday, 10 October 2015 at 20:07:11 UTC, Jonathan M Davis wrote:
> > On Saturday, October 10, 2015 15:20:02 tcak via Digitalmars-d-learn wrote:
> >> [code]
> >>   int[] list;
> >>
> >>   list = new int[0];
> >>
> >>   std.stdio.writeln("Is Null ? ", (list is null));
> >> [/code]
> >>
> >> Result is "Is Null? true".
> >>
> >> Is this the correct behaviour? I would expect compiler to point to an address in the heap, but set the length as 0. So, it wouldn't return null, but the length would be 0 only.
> >
> > It basically didn't bother to allocate an array on the heap, because you asked for one with a length of zero. Efficiency-wise, it makes no sense to allocate anything. You wouldn't be doing anything with the memory anyway. The only way that you're going to get an array of length 0 which doesn't have a null ptr is to slice an array down to a length of 0.
> >
> > - Jonathan M Davis
>
> The situation is that the "length" parameter comes from user. Also the item values come from user as well. I create the array with "length" parameter. At another part of code, I check firstly whether the array is created [code] if( array is null ) [/code], then the items are checked for validation.

In general, because of how arrays tend to conflate null and empty, it's a bad idea to differentiate between null and empty with arrays. I don't know exactly what you're doing, but there's no reason to check for null before iterating over an array, because null == "" and null == []. You'll never get a segfault from operating on a null array unless you try and do something with its ptr property explicitly. Almost everything treats a null array the same as an empty array. If you really need to have a null value for arrays, consider using std.typecons.Nullable to wrap the array.

- Jonathan M Davis