Checking if a string is null

[Frits van Bommel]

Regan Heath wrote:
> Max Samukha wrote:
>> Using '== null' and 'is null' with strings gives odd results (DMD
>> 1.019):
>>
>> void main()
>> {
>>     char[] s;
>>
>>     if (s is null) writefln("s is null");
>>     if (s == null) writefln("s == null");       
>> }
>>
>> Output:
>> s is null
>> s == null
>>
>> ----
>>
>> void main()
>> {
>>     char[] s = "";
>>
>>     if (s is null) writefln("s is null");
>>     if (s == null) writefln("s == null");       
>> }
>>
>> Output:
>> s == null
>>
>> ----
>>
>> Can anybody explain why s == null is true in the second example?
> 
> Not I, it's inconsistent IMO and it gets worse:
> 
> import std.stdio;
> 
> void main()
> {
>     foo(null);
>     foo("");   
> }
> 
> void foo(string s)
> {
>     writefln(s.ptr, ", ", s.length);
>     if (s is null) writefln("s is null");
>     if (s == null) writefln("s == null");
>     if (s < null)  writefln("s <  null");
>     if (s > null)  writefln("s <  null");
>     if (s <= null) writefln("s <= null");
>     if (s >= null) writefln("s <  null");
>     writefln("");
> }
> 
> Output:
> 0000, 0
> s is null
> s == null
> s <= null
> s <  null
> 
> 415080, 0
> s == null
> s <= null
> s <  null
> 
> So, "" is < and == null!?
> and <=,== but not >=!?

As Max said, you forgot to update some writeflns. The output of the 
corrected version is:
===
0000, 0
s is null
s == null
s <= null
s >= null

805BEF0, 0
s == null
s <= null
s >= null
===

Seems perfectly consistent to me. Anything with an equality comparison 
(==, <=, >=) is true in both cases, and 'is' is only true when the 
pointer as well as the length is equal.

> This all boils down to the empty vs null string debate where some people 
> want to be able to distinguish between them and some see no point.
> 
> I'm in the 'distinguishable' camp.  I can see the merit.  At the very 
> least it should be consistent!

They *are* distinguishable. That's why above code returns different 
results for the 'is' comparison...

I for one am perfectly fine with "cast(char[]) null" meaning ".length == 
0 && .ptr == null" and with comparisons of arrays using == and friends 
only inspecting the contents (not location) of the data.

Now, about comparisons: array comparisons basically operate like this:
---
int opEquals(T)(T[] u, T[] v) {              // bah to int return type
     if (u.length != v.length) return false;
     for (size_t i = 0; i < u.length; i++) {
         if (u[i] != v[i]) return false;
     }
     return true;
}

int opCmp(T)(T[] u, T[] v) {
     size_t len = min(u.length, v.length)
     for (size_t i = 0; i < len; i++) {
         if (auto diff = u[i].opCmp(v[i])) {
             return diff;
         }
     }
     return cast(int)u.length - cast(int)v.length;
}
---
(Taken from object.TypeInfo_Array and converted to templates instead of 
void*s + casting + element TypeInfo.{equals/compare} for readability)

Since both the null string and "" have .length == 0, that means they 
compare equal using those methods (having no contents to compare and 
equal length)

This is all perfectly consistent (and even useful) to me...