.init property for char[] type

[Steven Schveighoffer]

On Tue, 22 Sep 2009 09:53:52 -0400, Justin Johansson  
<procode at adam-dott-com.au> wrote:

> Daniel Keep Wrote:
>
>> > Big difference if you pass char[] variable .ptr to a C function.
>
>> In general, if you pass a string to a C function you should send it
>> through toStringz first.  If you don't, you're just begging for  
>> segfaults.
>
> Agreed .. fair enough.
>
> Actually I'm more interested in the semantics for default initialized  
> char[].
> Does it have exactly the same semantics as an empty string (in general D  
> or runtime library, Phobos et. al. context)?

A null string *is* an empty string, but an empty string may not be a null  
string.

The subtle difference is that the pointer points to null versus some data.

A non-null empty string:

  - May be pointing to heap data, therefore keeping the data from being  
collected.
  - May reallocate in place on appending (a null string always must  
allocate new data on append).

It's a difficult concept to get, but an array is really a hybrid type  
between a reference and a value type.  The array is actually a value type  
struct with a pointer reference and a length value.  If the length is  
zero, then the pointer value technically isn't needed, but in subtle  
cases, it makes a difference.  When you copy the array, the length behaves  
like a value type (changing the length of one array doesn't affect the  
other), but the array data is referenced (changing an element of the array  
*does* affect the other).

I think plans are to make the array a full reference type, and leave  
slices as these structs (in D2).  This probably will clear up a lot of  
confusion people have.

I hope this helps...

Oh, and BTW, you can pass string literals to C functions, but *not* char[]  
variables.  Always pass them through toStringz.  It generally does not  
take much time/resources to add the zero.

-Steve