Qt Tutorial 4 - Let There Be Widgets

Line by Line Walkthrough

    class MyWidget : public QWidget
    {
    public:
        MyWidget(QWidget *parent = 0);
    };

Here we create a new class. Because this class inherits from QWidget, the new class is a widget and may be a top-level window or a child widget (like the QPushButton in the previous chapter).

This class has only one member, a constructor (in addition to the members it inherits from QWidget). The constructor is a standard Qt widget constructor; you should always include a similar constructor when you create widgets.

The argument is its parent widget. To create a top-level window you specify a null pointer as the parent. As you can see, this widget defaults to be a top-level window.

    MyWidget::MyWidget(QWidget *parent)
        : QWidget(parent)

The implementation of the constructor starts here. Like most widgets, it just passes on the parent to the QWidget constructor.

    {
        setFixedSize(200, 120);

Because this widget doesn't know how to handle resizing, we fix its size. In the next chapter, we will show how a widget can respond to resize event from the user.

        QPushButton *quit = new QPushButton("Quit", this);
        quit->setGeometry(62, 40, 75, 30);
        quit->setFont(QFont("Times", 18, QFont::Bold));

Here we create and set up a child widget of this widget (the new widget's parent is this, i.e. the MyWidget instance).

Note that quit is a local variable in the constructor. MyWidget does not keep track of it; Qt does, and will automatically delete it when the MyWidget object is deleted. This is why MyWidget doesn't need a destructor. (On the other hand, there is no harm in deleting a child when you choose to. The child will automatically tell Qt about its imminent death.)

The QWidget::setGeometry() call sets both the widget's screen position and the size. It is equivalent to calling QWidget::move() followed by QWidget::resize().

        connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
    }

The qApp pointer is a global variable declared in the <QApplication> header file. It points to the application's unique QApplication instance.

    int main(int argc, char *argv[])
    {
        QApplication app(argc, argv);
        MyWidget widget;
        widget.show();
        return app.exec();
    }

Here we instantiate our new child, set it to be the main widget, and execute the application.