Book Image

Mastering Qt 5 - Second Edition

By : Guillaume Lazar, Robin Penea
Book Image

Mastering Qt 5 - Second Edition

By: Guillaume Lazar, Robin Penea

Overview of this book

Qt 5.11 is an app development framework that provides a great user experience and develops full capability applications with Qt Widgets, QML, and even Qt 3D. Whether you're building GUI prototypes or fully-fledged cross-platform GUI applications with a native look and feel, Mastering Qt 5 is your fastest, easiest, and most powerful solution. This book addresses various challenges and teaches you to successfully develop cross-platform applications using the Qt framework, with the help of well-organized projects. Working through this book, you will gain a better understanding of the Qt framework, as well as the tools required to resolve serious issues, such as linking, debugging, and multithreading. You'll start off your journey by discovering the new Qt 5.11 features, soon followed by exploring different platforms and learning to tame them. In addition to this, you'll interact with a gamepad using Qt Gamepad. Each chapter is a logical step for you to complete in order to master Qt. By the end of this book, you'll have created an application that has been tested and is ready to be shipped.
Table of Contents (16 chapters)

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; 
}