```d
if (null)
    "1".writeln;
if ("")
    "2".writeln;
if ("" == null)
    "3".writeln;
```

Output:
2
3

How to understand this?

On Saturday, 13 May 2017 at 00:36:55 UTC, mogu wrote:
> ```d
> if (null)
>     "1".writeln;
> if ("")
>     "2".writeln;
> if ("" == null)
>     "3".writeln;
> ```
>
> Output:
> 2
> 3
>
> How to understand this?

Boolean conversion on an array works on array's pointer, not it's length. So even though `"".length == 0`, `"".ptr != null`, and so `cast(bool)"" == true`.

On Saturday, 13 May 2017 at 00:59:14 UTC, Stanislav Blinov wrote:
> On Saturday, 13 May 2017 at 00:36:55 UTC, mogu wrote:
>> ```d
>> if (null)
>>     "1".writeln;
>> if ("")
>>     "2".writeln;
>> if ("" == null)
>>     "3".writeln;
>> ```
>>
>> Output:
>> 2
>> 3
>>
>> How to understand this?
>
> Boolean conversion on an array works on array's pointer, not it's length. So even though `"".length == 0`, `"".ptr != null`, and so `cast(bool)"" == true`.

```d
string s1 = null;
string s2 = "";
assert(s1 == null);
assert(s1.ptr == null);
assert(s2 == null);
assert(s2.ptr != null);
if (s1)
    1.writeln;
if (s2)
    2.writeln;
```

Output:
2

Thanks very much. This is a little bit confusing.

On Saturday, 13 May 2017 at 00:59:14 UTC, Stanislav Blinov wrote:
> On Saturday, 13 May 2017 at 00:36:55 UTC, mogu wrote:
>> ```d
>> if (null)
>>     "1".writeln;
>> if ("")
>>     "2".writeln;
>> if ("" == null)
>>     "3".writeln;
>> ```
>>
>> Output:
>> 2
>> 3
>>
>> How to understand this?
>
> Boolean conversion on an array works on array's pointer, not it's length. So even though `"".length == 0`, `"".ptr != null`, and so `cast(bool)"" == true`.

However

if ([])
    "1".writeln;

prints nothing.

So why does "" has non-null pointer while [] has null pointer? Looks inconsistent.

On 05/13/2017 06:20 AM, Yuxuan Shui wrote:
> So why does "" has non-null pointer while [] has null pointer? Looks
> inconsistent.

String literals are null-terminated, so "" needs to point at a null byte.

On Saturday, 13 May 2017 at 03:41:29 UTC, mogu wrote:
> Thanks very much. This is a little bit confusing.

There was a lot of discussion about it: https://issues.dlang.org/show_bug.cgi?id=4733, https://forum.dlang.org/thread/rrrtkfosfnfuybblexow@forum.dlang.org