Adding new functionality to a class - c++

Question

I'm studying the Qt4 library and I want to add some functionality to all the children of QWidget in my project. I need all widgets to have mousePressEvent overridden. Obviously I do not need to override it in every particular widget I use(I use many of them and I they to follow DRY rule) , instead I create a Foo class:
class Foo : public QWidget
{
Q_OBJECT
protected:
/* implementation is just a test */
virtual void mousePressEvent(QMouseEvent*) { this->move(0,0); }
};
And derive my button from it:
class FooButton : public QPushButton , public Foo
{
public:
FooButton (QWidget* parent) : QPushButton(parent) { };
};
But the event seems to be not overridden... What am I doing wrong?
Thanks in advance.

Answer 1

For the mousePressEvent access, try QObject::installEventFilter().
http://doc.qt.digia.com/stable/qobject.html

Answer 2

You are inheriting twice now from QWidget. This is problematic (see diamond problem).
Instead, drop your Foo class and move your custom mouse press event handler to your FooButton class:
class FooButton : public QPushButton
{
Q_OBJECT
protected:
/* implementation is just a test */
virtual void mousePressEvent(QMouseEvent*) { this->move(0,0); }
public:
FooButton (QWidget* parent) : QPushButton(parent) { };
};
If this doesn't suit the design of your application, then try using virtual inheritance instead. That is, change Foo to:
class Foo : public virtual QWidget
If that doesn't help, you should follow the advice of the other answers and install an event handler.
(You can read about virtual inheritance in its Wikipedia article.)

Answer 3

I suspect that the problem is that you're inheriting from QWidget twice, once through QPushButton and once through Foo.
From the way you phrased the question, I'm assuming that you want to do this for varying kinds of widgets, and thus don't want to have to subclass QPushButton, QLabel, QCheckBox, etc. If this is not the case then you should use Nikos's answer.
If not, your best bet is probably doing to be to use an event filter.
class MousePressFilter : public QObject {
Q_OBJECT
public:
MousePressFilter(QObject *parent) : QObject(parent) { }
protected:
bool eventFilter(QObject *watched, QEvent *event) {
QWidget *widget = dynamic_cast<QWidget*>(watched);
widget->move(0,0);
return false;
}
};
And then in your Foo class constructor:
class Foo {
Foo() {
installEventFilter( new MousePressFilter(this) );
}
};