"" gives an empty string, while "".idup gives null

Steven Schveighoffer schveiguy at yahoo.com
Wed Aug 3 10:15:51 PDT 2011

On Wed, 03 Aug 2011 06:35:08 -0400, simendsjo <simendsjo at gmail.com> wrote:

> void main() {
>      assert(is(typeof("") == typeof("".idup))); // both is  
> immutable(char)[]
>      assert(""      !is null);
>      assert("".idup !is null); // fails - s is null. Why?
> }

An empty string manifest constant (i.e. string literal) still must have a  
valid pointer, because it's mandated that the string have a zero byte  
appended to it.  This is so you can pass it to C functions which expect  
null-terminated strings.

So essentially, there is a '\0' in memory, and "" points to that character  
with a length of 0

However, idup calls a runtime function which *purposely* asks to make a  
copy.  However, it's *NOT* required to copy the 'zero after the string'  

The implementation, knowing that a null array is equivalent to an empty  
array, is going to return null to avoid the performance penalty of  
allocating a block that won't be used.  If you append, it will simply  
allocate a block as needed.

I see no reason the runtime should waste cycles or a perfectly good  
16-byte buffer to give you an empty array.

Definitely functions as designed, not a bug.  If you would like different  
behavior, you are going to have to have a really really good use case to  
get this changed.


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