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

[Jonathan M Davis]

> On 03.08.2011 18:18, Jonathan M Davis wrote:
> > On Thursday 04 August 2011 00:27:12 Mike Parker wrote:
> >> On 8/3/2011 11:23 PM, simendsjo wrote:
> >>> On 03.08.2011 15:49, bearophile wrote:
> >>>> simendsjo:
> >>>>> void main() {
> >>>>> assert(is(typeof("") == typeof("".idup))); // both is
> >>>>> immutable(char)[]
> >>>>> 
> >>>>> assert("" !is null);
> >>>>> assert("".idup !is null); // fails - s is null. Why?
> >>>>> }
> >>>> 
> >>>> I think someone has even suggested to statically forbid "is null" on
> >>>> strings :-)
> >>>> 
> >>>> Bye,
> >>>> bearophile
> >>> 
> >>> How should I test for null if not with "is null"? There is a difference
> >>> between null and empty, and avoiding this is not necessarily easy or
> >>> even wanted.
> >>> I couldn't find anything in the specification stating this difference.
> >>> So... Is it a bug?
> >> 
> >> This is apparently a bug. Somehow, the idup is clobbering the pointer.
> >> You can see it more clearly here:
> >> 
> >> void main()
> >> {
> >> 
> >> assert("".ptr);
> >> 
> >> auto s = "".idup;
> >> assert(s.ptr); // boom!
> >> 
> >> }
> > 
> > I don't know if it's a bug or not. The string _was_ duped. assert(s ==
> > "") passes. So, as far as equality goes, they're equal, and they don't
> > point to the same memory. Now, you'd think that the new string would be
> > just empty rather than null, but whether it's a bug or not depends
> > exactly on what dup and idup are supposed to do with regards to null.
> > It's probably just a side effect of how dup and idup are implemented
> > rather than it being planned one way or the other. I don't know if it
> > matters or not though. In general, I don't like the conflation of null
> > and empty, but is this particular case, you _do_ get a string which is
> > equal to the original and which doesn't point to the same memory. So, I
> > don't know whether this should be considered a bug or not. It depends on
> > what dup and idup are ultimately supposed to do.
> > 
> > - Jonathan M Davis
> 
> I would think it's a bug, but strings doesn't quite behave as regular
> references anyway...
> But why should dup/idup change the semantics of the array?
> 
> void main() {
> // A null string or empty string works as expected
> string s1;
> assert(s1 is null);
> assert(s1.ptr is null);
> assert(s1 == ""); // We can check for empty even if it's
> null, and it's equal to ""
> assert(s1.length == 0); // ...and length even if it's null
> s1 = "";
> assert(s1 !is null);
> assert(s1.ptr !is null);
> assert(s1.length == 0);
> assert(s1 == "");
> 
> // the same applies to null mutable arrays
> char[] s2;
> assert(s2 is null);
> assert(s2.ptr is null);
> assert(s2 == "");
> assert(s2.length == 0);
> // but with .dup/.idup things is different!
> s2 = "".dup;
> //assert(s2 !is null); // fails
> //assert(s2.ptr !is null); // fails
> assert(s2.length == 0); // but... s2 is null..?
> assert(s2 == "");
> assert(s2 == s1);
> }

If you look at the spec ( http://d-programming-language.org/arrays.html ), it 
says:

dup: Cre­ate a dy­namic array of the same size and copy the con­tents of the 
array into it.

idup: Cre­ate a dy­namic array of the same size and copy the con­tents of the 
array into it. The copy is typed as being im­mutable. D 2.0 only


This is _exactly_ what dup and idup are doing. You get a new array with the 
exact same size and contents. null doesn't factor into it at all. So, per the 
spec, there's no bug here at all. dup and idup promise _nothing_ with regards 
to null.

It may be that it would be better if dup and idup returned an array which was 
null if the original was null, and that would also be within the spec, but 
what dup and idup do at the moment _does_ follow the spec.

So, feel free to file a bug report on it. Maybe it'll get changed, but the 
current behavior follows the spec. And given how arrays don't generally treat 
empty and null as being different, I wouldn't really expect an array to stay 
null if you do _anything_ to it other than simply pass it around or check its 
value. In this case, you're creating a new array, and D just doesn't generally 
care about null vs empty when it comes to arrays. I wouldn't argue that that's 
a good thing (because I don't really think that it is), but because of that, 
you can't really expect much to treat null and empty as being different. And 
in this particular case, it's not only debatable as to whether it matters, but 
the current behavior is completely within the spec.

- Jonathan M Davis