Thread: Auto Calling Derived Fun

Here is a real-world example of how this is done, and I need to understand how it's done. If you have used Qt before then this code should make sense.

Code:

#include <QApplication>
#include <QPushButton>
#include <QPainter>
#include <QPaintEvent>

class mBut : public QPushButton{

 public:
  void paintEvent(QPaintEvent *event){
    QPainter painter(this);
    painter.drawEllipse(2,2,100,25);
  };
};

int main(int argc, char *argv[]){

	QApplication app(argc, argv);
	mBut *p = new mBut();
	
	p->show();
	return app.exec();
}

If you are on linux with Qt you can compile it with this:

Code:

g++ -o main main.cpp `pkg-config --cflags --libs QtGui`

mBut is my derived class. It inherits QPushButton and implements it's own version of paintEvent(). When my widget is shown, mBut:: paintEvent() is automatically called. mBut is derived from QPushButton, which is derived from QAbstractButton, which is derived from QWidget. paintEvent() is a protected virtual function in QWidget.