Qt Opengl QMainWindow

glwidget.h

#ifndef GLWIDGET_H
#define GLWIDGET_H

#include <QGLWidget>

class GLWidget : public QGLWidget
{
public:
        GLWidget(QWidget *parent);

        ~GLWidget();
        void initializeGL();
        void paintGL();
        void resizeGL(int w, int h);
};

#endif // GLWIDGET_H

glwidget.cpp

#include "glwidget.h"

GLWidget::GLWidget(QWidget *parent)
                : QGLWidget(parent)
{}

GLWidget::~GLWidget()
{}

void GLWidget::initializeGL()
{
    qglClearColor( Qt::black );
}

void GLWidget::paintGL()
{
    glClear( GL_COLOR_BUFFER_BIT );

    glLoadIdentity();
    qglColor( Qt::white );
    glBegin(GL_TRIANGLES);
    glVertex3f( 0.0f, 1.0f, 0.0f);
    glVertex3f(-1.0f,-1.0f, 0.0f);
    glVertex3f( 1.0f,-1.0f, 0.0f);
    glEnd();
}

void GLWidget::resizeGL(int w, int h)
{
    h = h < 1 ? 1 : h;
    glViewport( 0, 0, (GLint)w, (GLint)h );
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QMainWindow>
#include <QMenuBar>
#include <QAction>
#include <QApplication>
#include "glwidget.h"
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
private:
    GLWidget *w;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    w=new GLWidget(this);
    setCentralWidget(w);

    QMenu *fileMenu = new QMenu("&File");
    fileMenu->addAction("&Close", qApp, SLOT(quit()), Qt::CTRL + Qt::Key_Q);
    menuBar()->addMenu(fileMenu);
}

MainWindow::~MainWindow()
{

}

main.cpp

#include <QtGui/QApplication>
#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

opengl.pro

QT += opengl
TARGET = opengl
TEMPLATE = app
SOURCES += main.cpp \
    mainwindow.cpp \
    glwidget.cpp
HEADERS += mainwindow.h \
    glwidget.h