Instantiating forms, message boxes, and dialogs in your application

Qt Designer generates an XML-based layout file (which ends in .ui) for each form you create in Designer. At compile time, Qt Creator compiles the layout into a header file that constructs the components for your user interface layout. The pattern typically used by Qt applications is to construct a private layout class that the main class instantiates. Here's how it works for the main window:

#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

In mainwindow.cpp, we have:

#include "mainwindow.h"
#include "resultdialog.h"
#include "ui_mainwindow.h"
#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
{
 ...