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QPushButton *button = new QPushButton("Quit");
...
button->show();

//mainwindow.h
#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

//mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "widget.h"

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

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

//widget.h
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT
public:
    explicit Widget(QWidget *parent = 0);

signals:

public slots:

};

#endif // WIDGET_H

//widget.cpp
#include "widget.h"

Widget::Widget(QWidget *parent) :
    QWidget(parent)
{
    this->setFocus();
}

//main.cpp
#include <QtGui/QApplication>
#include "mainwindow.h"
#include "widget.h"
#include <QTextCodec>

int main(int argc, char *argv[])
{
    QTextCodec::setCodecForTr(QTextCodec::codecForName("Windows1251"));
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}