MainWindow structure

This generated class is a perfect yet simple example of Qt framework usage; we will dissect it together. As mentioned previously, the MainWindow.ui file describes your UI design and the MainWindow.h/MainWindow.cpp files define the C++ object where you can manipulate the UI with code.

It's important to take a look at the MainWindow.h header file. Our MainWindow object inherits from Qt's QMainWindow class:

#include <QMainWindow> 
 
namespace Ui { 
class MainWindow; 
} 
 
class MainWindow : public QMainWindow 
{ 
    Q_OBJECT 
 
public: 
    explicit MainWindow(QWidget *parent = 0); 
    ~MainWindow(); 
private: 
    Ui::MainWindow *ui; 
}; 

As our class inherits from the QMainWindow class, we will have to add the corresponding #include at the top of the header file. The second part is the forward declaration of Ui::MainWindow, as we only declare a pointer.

Q_OBJECT can look a little strange to a non-Qt developer. This macro allows the class to define its own signals/slots through Qt's meta-object system. These features will be covered later in this chapter in the section Signals and slots.

This class defines a public constructor and destructor. The latter is pretty common but the constructor takes a parent parameter. This parameter is a QWidget pointer that is null by default.

QWidget is a UI component. It can be a label, a textbox, a button, and so on. If you define a parent-child relationship between your window, layout, and other UI widgets, the memory management of your application will be easier. Indeed, in this case, deleting the parent is enough because its destructor will take care of also deleting its child recursively.

Our MainWindow class extends QMainWindow from the Qt framework. We have a ui member variable in the private fields. Its type is a pointer of Ui::MainWindow, which is defined in the ui_MainWindow.h file generated by Qt. It's the C++ transcription of the MainWindow.ui UI design file. The ui member variable will allow you to interact with your C++ UI components (QLabel, QPushButton, and so on), as shown in the following figure:

If your class only uses pointers or references for a class type, you can avoid including the header by using forward declaration. That will drastically reduce compilation time and avoid circular dependencies.

Now that the header part is done, we can talk about the MainWindow.cpp source file.

In the following code snippet, the first include is our class header. The second one is required by the generated Ui::MainWindow class. This include is required as we only use a forward declaration in the header:

#include "MainWindow.h" 
#include "ui_MainWindow.h" 
 
MainWindow::MainWindow(QWidget *parent) : 
    QMainWindow(parent), 
    ui(new Ui::MainWindow) 
{ 
    ui->setupUi(this); 

In many cases, Qt generates good code using the initializer list. The parent argument is used to call the QMainWindow superclass constructor. Our ui private member variable is also initialized.

Now that ui is initialized, we must call the setupUi function to initialize all the widgets used by the MainWindow.ui design file:

As the pointer is initialized in the constructor, it must be cleaned in the destructor:

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