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Qt Tutorial 3 - Family Values

Files:

• tutorial/t3/main.cpp

This example shows how to create parent and child widgets.

We'll keep it simple and use just a single parent and a lone child.

 #include <QApplication>
 #include <QFont>
 #include <QPushButton>
 #include <QWidget>

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

     QWidget window;
     window.resize(200, 120);

     QPushButton quit("Quit", &window);
     quit.setFont(QFont("Times", 18, QFont::Bold));
     quit.setGeometry(10, 40, 180, 40);
     QObject::connect(&quit, SIGNAL(clicked()), &app, SLOT(quit()));

     window.show();
     return app.exec();
 }

Line by Line Walkthrough

 #include <QWidget>

We add an include of <QWidget> to get the base widget class we'll use.

     QWidget window;

Here we simply create a plain widget object. The QWidget class is the base class of all user interface objects. The widget is the atom of the user interface: It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen. A widget is clipped by its parent and by the widgets in front of it.

A widget that isn't embedded in a parent widget, like this particular widget, is called a window. Usually, windows have their own window frame and taskbar entry, provided by the window system. A widget without a parent widget is always an independent window. Its initial position on the screen is controlled by the window system.

     window.resize(200, 120);

We set the window's width to 200 pixels and its height to 120 pixels.

     QPushButton quit("Quit", &window);

A child is born. This QPushButton is created with a parent widget (window). A child widget is always displayed within its parent's area. When displayed, it is clipped by its parent's bounds. By default, it is rooted at the top-left corner of its parent, at position (0, 0).

     quit.setGeometry(10, 40, 180, 40);

The QWidget::setGeometry() function takes four arguments: The first two arguments are the x and y coordinates of the button's top-left corner. The coordinates are relative to the parent widget. The last two arguments are the button's width and height. The result is a button that extends from position (10, 40) to position (190, 80).

     window.show();

When a parent widget is shown, it will call show for all its children (except those that were explicitly hidden using QWidget::hide()).

Running the Application

The button no longer fills the entire window. Instead, it stays at position (10, 40) within the window and with a size of (180, 40), because of the QWidget::setGeometry() call.

Exercises

Try resizing the window. How does the button change? What happens to the button's height if you run the program with a bigger font? What happens if you try to make the window really small?

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