[Qt-interest] QLabel text () returns QString::null aftersetting empty QString

[Frank Hemer]

On Monday 13 September 2010 09:10:28 Constantin Makshin wrote:
> If I understood you correctly, you want to check if widget contents (text)
> have been changed, right? Then you could use something like this:
>
> if (!(oldText.isEmpty() && newText.isEmpty()) // (1)
>     && newText != oldText)
> {
>     // ...
> }
>
> (1) prevents the code from being triggered in your 'QString("") !=
> QString::null' case.

Would work but unfortunatelly in my case I tried to use the 'QString::null' 
condition too - so this approach was not sufficiant.

> I guess widgets may prefer QString::null to QString("") because the former
> is shared between all QString objects and therefore helps to save some
> memory — the fastest way to make a string empty without memory
> reallocations is to set both the first character and length property to
> zero, but since some widgets like labels are usually used to display some
> static or rarely-changed text, that truncation would be a waste of memory.

Thats what I would have expected.
However a quick check after Tiagos answer revealed the following:

QLabel: returns QString::null if set to ""
QLineEdit: never returns QString::null
QTextEdit: never returns QString::null
QComboBox: always returns exactly to what it was set before.

To my feel, all these elements should behave as the QComboBox. Especially when 
I discovered that QTextEdit in Html mode for example returns changed and 
modified html after a write-and-following-read operation and even worse, 
changes again after a second read operation (bug was filed but not accepted) 
which makes tracking user interaction even more difficult.

Oh well - I love qt but these little things that can be so much time 
consuming:-(

Frank

> On Thursday 09 September 2010 23:18:30 Frank Hemer wrote:
> > On Thursday 09 September 2010 19:23:43 Scott Aron Bloom wrote:
> > > -----Original Message-----
> > > From: qt-interest-bounces at trolltech.com
> > > [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Frank Hemer
> > > Sent: Thursday, September 09, 2010 10:19 AM
> > > To: qt-interest at trolltech.com
> > > Subject: Re: [Qt-interest] QLabel text () returns QString::null
> > > aftersetting empty QString
> > >
> > > On Thursday 09 September 2010 19:07:25 Thiago Macieira wrote:
> > > > Em Quinta-feira 09 Setembro 2010, às 15:38:33, Frank Hemer escreveu:
> > > > > Is the behavior mentioned in the subject intended/known or is it a
> > > > > bug that needs to be filed?
> > > >
> > > > Unless otherwise specified in the documentation, you shouldn't rely
> > > > on methods keeping the null-vs-empty setting.
> > >
> > > Thanx for clearing this.
> > >
> > > However this makes it impossible to track if objects (QLabel,
> > > QLineEdit, QTextEdit and probably others) have been modified bec.
> > > comparing former state to later state is no more possible:-( ... for
> > > code that
> > > differentiates between QString ("") and QString::null ...
> > >
> > > Frank
> > > ------------------------------
> > >
> > > Not unless you simply tag it when you watch for the textChanged
> > > signals.. Though for QLabel, the user cant modify the label directlky
> > > so Im not sure how it would be so difficult to track.
> > >
> > > Scott
> >
> > Well ... thats just the point: QLabel is not a real problem here as it
> > can be worked around by adding tracking code ... but what about the other
> > QWidget derived classes that return QString? Using textChanged is of no
> > use here since it already hides the possible 'null' string.
> >
> > I'm using this in a scripting environment - the only way would be to
> > derive from the base class and overwrite _ALL_ methods that somehow
> > modify the contained string ... I'm sure it is way easier to fix the
> > class itself to return what it  was set to before ...
> >
> > Frank
>
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