#ifndef MYCLASS_H
#define MYCLASS_H

#include <QtGui>

class myclass : public QWidget {
    Q_OBJECT
public:
    explicit myclass(QWidget *parent = 0);
    virtual ~myclass();
    void paintEvent(QPaintEvent *event);
    virtual QSize sizeHint() const;
    virtual QSize minimumSizeHint() const;
private:

};

#endif // MYCLASS_H

#include "myclass.h"

myclass::myclass(QWidget *parent) {
    setWindowTitle("test");
    resize(50, 50);
    setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
    repaint();
}

myclass::~myclass() {
}

void myclass::paintEvent(QPaintEvent *event) {
    QPainter painter(this);
    painter.drawLine(0,0,50,50);
    painter.drawLine(0,50,50,0);
}

QSize myclass::sizeHint() const {
    return QSize(50,50);
}

QSize myclass::minimumSizeHint() const {
    return QSize(50,50);
}

#include "myclass.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    myclass myobject;
    myobject.show();

    return a.exec();
}

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}