#include <QWidget>
#include <QApplication>
#include <QMainWindow>
#include <QStackedWidget>
#include <QPushButton>
#include <QString>


struct Panel : public QWidget {
    QPushButton *btn;
    Panel(QString text, QWidget *parent = nullptr) : QWidget(parent) {
      btn = new QPushButton(text, this);
      btn->show();
    }
};

struct Panel1 : public Panel {
    Panel1(QWidget *parent = nullptr) : Panel("Hello", parent) { }
};

struct Panel2 : public Panel {
  public:
    Panel2(QWidget *parent = nullptr) : Panel("World", parent) { }
};

class MainWindow : public QMainWindow {
  Panel1 *panel1;
  Panel2 *panel2;
  QStackedWidget *stackedWidget;

  public:
    MainWindow(QWidget *parent = nullptr) : QMainWindow(parent) {
      resize(640, 480);
      setFixedSize(size());
      panel1 = new Panel1(this);
      panel2 = new Panel2(this);
      stackedWidget = new QStackedWidget(this);
      stackedWidget->show();
      stackedWidget->addWidget(panel1);
      stackedWidget->addWidget(panel2);
      stackedWidget->setCurrentWidget(panel1);

      connect(panel1->btn, &QPushButton::clicked, [=]() {
        stackedWidget->setCurrentWidget(panel2);
      });
      connect(panel2->btn, &QPushButton::clicked, [=]() {
        stackedWidget->setCurrentWidget(panel1);
      });
    }
};

class App : public QApplication {
  MainWindow *mainWindow;

  public:
    App(int argc, char **argv) : QApplication(argc, argv) {
      mainWindow = new MainWindow();
      mainWindow->show();
    }
};

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

    return app.exec();
}