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

[Steven Schveighoffer]

On Wed, 03 Aug 2011 14:26:54 -0400, simendsjo <simendsjo at gmail.com> wrote:

> Schveighoffer also states it is as designed.
> But it really doesn't behave as one (at least I) would expect.
> So in essence (as bearophile says), "is null" should not be used on  
> arrays.
>
> I was bitten by a bug because of this, and used "" intead of "".idup to  
> avoid this, but given D doesn't distinguish between empty and null  
> arrays, this doesn't feel very safe now..

I would recommend against depending on the difference between null and  
not-null-but-empty arrays.  But in any case, "".idup is mainly pointless,  
there is never a need to idup a string, since it's already immutable (and  
therefore can be passed wherever you need it).

> In the code in question I have a lazy initialized string. The problem is  
> that I would see if it has been initialized, but an empty string is also  
> a valid value. Because I shouldn't check for null, I now have to add  
> another field to the struct to see if the array has been initialized.  
> This feels like a really suboptimal solution.

Where is it that you need to use idup?  I think you may be using that  
without need (or if you are using code that violates immutability, that  
code is incorrect), but I don't know what your code looks like so I might  
be wrong.

In any case, there may be a better way to do what you want, without the  
extra field.

At the very least, here is a function that can help you:

myIdup(string s)
{
    return s.length == 0 ? "" : s.idup;
}

Note that this kind of thing *ONLY* works for strings, because string  
literals are not null.  For normal arrays, I wouldn't expect this to work.

-Steve