On Sun, Dec 16, 2012 at 12:48 AM, Dave Abrahams <dave@boostpro.com> wrote:
on Wed Dec 12 2012, Gottlob Frege <gottlobfrege-AT-gmail.com> wrote: [...]
Qt's QString has both empty() and isNull() and they are not always the same. Basically empty() is a zero-byte string, but null is a never-been-set-or-allocated string.
That's dumb, though.
It is the matter of value semantics vs. pointer semantics. QString is "more like" a pointer to a referenced counted string.
ie
QString name = database.get("Name");
name.empty() == true means the Name field was empty name.isNull() == true means the Name field doesn't exist in the
database.
Somewhat like optional<string>.
I am NOT saying whether this is a good thing. Just a widely known
example
of interpretation.
I AM saying it's a bad thing! :-)
I agree with you that the null state of QString is a bad thing. But this by itself does not justify interpreting an empty() string to be false. Let me explain. The conversion to bool usually tests the validity of the object so one can do some other operations on it. For instance this is what it means in iostreams (stream is in non-fail state so one can continue reading & writing to it) and pointers (not null so one can dereference it). These examples are of well understood validness of states. On the other hand, for strings, it is dubious that empty() strings are in any way invalid. All operations still can be done on them, like concatenation, find, operator [] with index <= size(), etc... Having said that, personally I do not object to empty() being interpreted as false. I am only trying to judge this suggestion objectively. Cheers, -- Yakov