[Qt-interest] Overwriting public slots and qobject_cast

[John McClurkin]

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.
>