Hi All,

The following code will throw an exception:
  char[] s;
  assert( s.dup  is null); // OK
  assert("".dup !is null); // FAILED

"".dup is expectly also an empty string. Is this a compiler bug?

--Qian

On Mon, 04 May 2009 09:46:57 -0400, Qian Xu <quian.xu@stud.tu-ilmenau.de> wrote:

> Hi All,
>
> The following code will throw an exception:
>   char[] s;
>   assert( s.dup  is null); // OK
>   assert("".dup !is null); // FAILED
>
> "".dup is expectly also an empty string.
> Is this a compiler bug?
>

I think you might have a bug?

"".dup is the same as s.dup, not sure why you would expect it to be not-null.

-Steve

Steven Schveighoffer wrote:

> On Mon, 04 May 2009 09:46:57 -0400, Qian Xu <quian.xu@stud.tu-ilmenau.de> wrote:
> 
>> Hi All,
>>
>> The following code will throw an exception:
>>   char[] s;
>>   assert( s.dup  is null); // OK
>>   assert("".dup !is null); // FAILED
>>
>> "".dup is expectly also an empty string.
>> Is this a compiler bug?
>>
> 
> I think you might have a bug?
> 
> "".dup is the same as s.dup, not sure why you would expect it to be not-null.
> 
> -Steve

They are not the same. s is null. "" is an empty string. empty string and null are definitely two thing.

Steven Schveighoffer wrote:

> I think you might have a bug?
> 
> "".dup is the same as s.dup, not sure why you would expect it to be not-null.
> 
> -Steve

If I have not explained clearly. Here is the full code:

  char[] s;
  assert(s     is null);
  assert(s.dup is null);

  assert(""     !is null); // OK
  assert("".dup !is null); // FAILED

At least the last two lines behave not consistent. Either both are failed, or both are passed.

On Mon, 04 May 2009 10:22:49 -0400, Qian Xu <quian.xu@stud.tu-ilmenau.de> wrote:

> Steven Schveighoffer wrote:
>
>> I think you might have a bug?
>>
>> "".dup is the same as s.dup, not sure why you would expect it to be
>> not-null.
>>
>> -Steve
>
> If I have not explained clearly.
> Here is the full code:
>
>   char[] s;
>   assert(s     is null);
>   assert(s.dup is null);
>
>   assert(""     !is null); // OK
>   assert("".dup !is null); // FAILED
>
> At least the last two lines behave not consistent.
> Either both are failed, or both are passed.


OK, your original post was this:

  assert( s.dup  is null); // OK
  assert("".dup !is null); // FAILED


The compiler always returns a null array if you dup an empty array.  The reason being: why allocate memory for something that is zero length?

If anything, the oddity is this line:

assert("" !is null);

But of course, no memory is allocated for literals, so at least needless memory allocation does not occur.

To be consistent, I think assert("" is null); should pass, but it's a minor inconsistency at best.

I usually never compare arrays to null for this reason...

-Steve

Steven Schveighoffer wrote:
> On Mon, 04 May 2009 10:22:49 -0400, Qian Xu <quian.xu@stud.tu-ilmenau.de> wrote:
> 
>> Steven Schveighoffer wrote:
>>
>>> I think you might have a bug?
>>>
>>> "".dup is the same as s.dup, not sure why you would expect it to be
>>> not-null.
>>>
>>> -Steve
>>
>> If I have not explained clearly.
>> Here is the full code:
>>
>>   char[] s;
>>   assert(s     is null);
>>   assert(s.dup is null);
>>
>>   assert(""     !is null); // OK
>>   assert("".dup !is null); // FAILED
>>
>> At least the last two lines behave not consistent.
>> Either both are failed, or both are passed.
> 
> 
> OK, your original post was this:
> 
>   assert( s.dup  is null); // OK
>   assert("".dup !is null); // FAILED
> 
> 
> The compiler always returns a null array if you dup an empty array.  The reason being: why allocate memory for something that is zero length?
> 
> If anything, the oddity is this line:
> 
> assert("" !is null);

If I remember correctly, string literals are stored with a null appended, so as to make them easier to use with OS calls, etc. Could it be that "" stores a 1-byte string, consisting with just this null? Then this behavior would be understandable.

> But of course, no memory is allocated for literals, so at least needless memory allocation does not occur.
> 
> To be consistent, I think assert("" is null); should pass, but it's a minor inconsistency at best.
> 
> I usually never compare arrays to null for this reason...
> 
> -Steve

On Mon, 04 May 2009 12:09:09 -0400, Georg Wrede <georg.wrede@iki.fi> wrote:

> If I remember correctly, string literals are stored with a null appended, so as to make them easier to use with OS calls, etc. Could it be that "" stores a 1-byte string, consisting with just this null? Then this behavior would be understandable.

Yes, that makes complete sense, thanks.

-Steve