[Qt-interest] Overwriting public slots and qobject_cast

[Álvaro Morais]

A Quinta 04 Junho 2009 17:24:30 John McClurkin você escreveu:
> Álvaro Morais wrote:
> > Hi.
> >
> > I want to make a custom progressBar that changes color with
> > percentage. For example 25% will give a highlight red, 50% Yellow,
> > etc...
> >
> > It will also automatically give the password entropy of a QString, so
> > the setValue SLOT will become private, since there is no need to
> > change it from outside.
>
> According to the docs even if a slot is private, another object can
> still connect a signal to it and it will be called when the signal is is
> emitted, so I see little point to trying to make the slot private.
>
> > I'm using the promote to custom widget on the QtDesigner.
> >
> > My question is: What is the best way of overwriting the public slots?
> >
> > is it safe to do this:
> >
> > class PasswordQuality : public QProgressBar
> > {
> > ...
> > private slots:
> >     virtual void setValue ( int value ) {
> > 	qobject_cast<QProgressBar*>(this)->setValue(value);
> >
> > 	... //code to change the palette.
> > 	}
> > };
>
> You don't keed to use qobject_cast here because the this pointer is a
> pointer to a QProgressBar type object. Qobject_cast is used when you are
> not sure when of the type of a pointer to type QWidget. In this
> situation, qobject_cast will return a NULL if the pointer is to an
> incompatible type, for example if you are expecting a pointer to a
> QMainWindow but the pointer received is to a QDialog.
>
> > Am I using qobjet_cast the right way?
> >
> > Thanks in advance.
>
> You are subclassing QProgressBar to add functionality. I would suggest
> an alternative suggestion. Create a new slot, say setEntropy(int), and
> connect that to the signal that normally would be connected to a
> QProgreasBar setValue slot. Then, in setEntropy do the color change and
> call QProgressBar setValue.
>
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One of the points that I'm creating subclass is for the learning experience of 
QT, and not for the functionality of this specific implementation. I probably 
will later change for your suggesting.
As I said in my previous post I had a lot of bad assumptions based on a 
execution error, that made me assume wrong things. I already figured this out.

Thanks for your reply, and clarification. Your  explanation about qobject_cast 
was particular useful.

Álvaro Morais
-- 
Neither Emacs or Vi are my primary editors...
..and I'm not ashamed!!!