Declaring functions called when a mouse event ocurr in Qt - c++

Question

I'm a beginner in Qt when I run the following code I got these errors:
no void MainWindow::mousePressEvent(QMouseEvent *f) member function declared in class 'mainwindow'.
no void void MainWindow::paintEvent(QPaintEvent *e) member function declared in class 'mainwindow'.
The code is written in main.cpp file and I didn't write anything in mainwindow.cpp or mainwindow.h
The Qt code:
#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QEvent>
#include <QMouseEvent>
#include <QPainter>
void MainWindow::mousePressEvent(QMouseEvent *f)
{
QPoint point=f->pos();
int y=1;
update();
}
void MainWindow::paintEvent(QPaintEvent *e)
{
int y;
QPoint point;
QPainter painter(this);
QPen linepen(Qt::red);
linepen.setCapStyle(Qt::RoundCap);
linepen.setWidth(30);
painter.setRenderHint(QPainter::Antialiasing,true);
painter.setPen(linepen);
if(y==1)
painter.drawPoint(point);
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}

Answer 1

In header should be:
protected:
void mousePressEvent(QMouseEvent *f);
void paintEvent(QPaintEvent *e);
And includes:
#include <QMouseEvent>
#include <QPaintEvent>
Also you should write your code in mainwindow.cpp(paintEvent, and another member functions). If you will have many classes, then your main.cpp can be very hard-readable.