The Qt toolkit is a C++ GUI library developed over 6 years to be cross-platform. It began development in 1991 and was released under commercial and open source licenses in 1995. Qt uses signals and slots to connect GUI components to program logic, separating the user interface from code. It provides a full 2D graphics system and supports OpenGL for 3D graphics. Qt handles platform differences and includes classes for files, networking, and other OS functions.

1. Introduction to Qt
The IDE for C++

2. THE QT STORY
 Haavard Nord and Eirik Chambe-Eng (the original
developers of Qt and the CEO and President, respectively, of
Trolltech) began development of "Qt" in 1991, three years
before the company was incorporated as Quasar
Technologies, then changed the name to Troll Tech and then
to Trolltech.
 Qt is developed by an open source project, the Qt Project,
involving both individual developers as well as developers
from Nokia, Digia, and other companies interested in the
development of Qt.

3.  The Qt toolkit is a multi-platform C++ GUI toolkit (class library) that has
been developed over a 6 year period.
 The company Troll Tech AS was founded in 1994 to secure future
development of Qt.
 On May 20, 1995, Qt was made available under commercial and non-
commercial GNU licenses. The non-commercial license grants any
developer the right to use Qt to develop software for the free software
community.
 It was ten months before the first commercial license was purchased.
The European Space Agency purchased the second.
 Around 1997, Qt was chosen as the code basis for the KDE linux
desktop environment.
 Qt 3.0 was released in 2001 with Windows, Unix, Linux, Embedded
Linux, and Mac OS X libraries.

4. WHY CROSS-PLATFORM GUI TOOLKITS
 Increases target market.
 May provides the same look-and-feel across
platform. Reduces training and documentation
costs.

5. STATEGIES FOR IMPLEMENTING CROSS-PLATFORM GUIS
 API Layering – Mapping one API to many others
 Example – wxWindows – Win32 API on top of Motif or
Xt API under Unix.
 Advantages – easy to write, 100% compatible native
look and feel.
 Disadvantages –
 slower
 problems mapping to vastly different API architectures
 Lowest common denominator – i.e. no pop-up help anywhere
 Objects required a C++ wrapper to work with them in C++

6. QT ASSISTANT
 All documentation is
available through the
trolltech web site.
 Qt Assistant is a Qt help
browser that runs under
Windows.
 It has with search and
indexing features that make
it quicker and easier than the
web.

7. `QT DESIGNER
 Widgets and forms
created with Qt Designer
integrated seamlessly
with programmed
code, using Qt's signals
and slots
mechanism, that lets you
easily assign behavior to
graphical elements. All
properties set in Qt
Designer can be changed
dynamically within the
code.
Furthermore, features like
widget promotion and
custom plugins allow you
to use your own
components with Qt
Designer.

8. A SIMPLE EXAMPLE
/* HELLOWORLD.CPP */
1 #include <qapplication.h>
2 #include <qlabel.h>
3
4 int main(int argc, char **argv) {
5
6 QApplication myapp(argc, argv);
7
8 Qlabel *mylabel = new Qlabel(“Hello World”, 0);
9 mylabel->resize(100, 200);
10
11 myapp.setMainWidget(mylabel);
12 mylabel->show();
13 return myapp.exec();
14 }

9. LINE-BY-LINE
1 #include <qapplication.h>
2 #include <qlabel.h>
Always #include any Q types referenced in code.
6 QApplication myapp(argc, argv);
Creates an object to manage application-wide resources. Passes
argc and argv because Qt supports a few command line arguments of
its own.
8Qlabel *mylabel = new Qlabel(“Hello World”, 0);
Creates a QLabel widget on the heap. A widgets is any visual
element in a user interface. Widgets can contain other widgets. For
example a window may contain a QMenuBar, QToolBar, QStatusBar,
and other widgets. The 0 parameters says that that the label is a
stand-alone window, is not inside another window.

10. LINE-BY-LINE
9 mylabel->resize(100, 200);
Invokes the resize() member function.
11 myapp.setMainWidget(mylabel);
Make the label the main application widget. This means that closing the label windows
closes the application.
12 mylabel->show();
Invoke the show() member function to make the label visible. All widgets are created
invisible so that their properties can be manipulated without flickering. For example,
you would show a widget and then change its size and color. You would change the size
and color first, and then show the widget.
13 return myapp.exec();
Passes control of the application to Qt. At this point the application goes into “event-
driven” mode. It will just sit there until the user does something to create an even. This
is the same concept as Word. Word starts and waits for the user to do something.

11. EVENT HANDLING
 QT's new approach: signals and slots
 A widget sends out various signals
 Object methods can be declared as slots
 Compatible signals and slots can be connected or
plugged together like a telephone switchboard
(parameter types must match)
 Strict separation
 This strict separation between UI components and
program elements lends itself to component-based
programming
 Goal: separate UI from program logic

12. SIGNALS AND SLOTS
clicked_method()

13. QMAKE
 The qmake utility is typically invoked with the following three
commands]
qmake –project
qmake
make (or nmake under Windows)
 Rules:
 Be sure to place code in its own directory.
 qmake scans all subdirectories for dependencies. Do not place archive
version under a “save” subdirectory.
 If you reorganize your files, like adding a new .h, delete all the .pro and other
working files, then start over.

14. DEFINING SIGNALS AND SLOTS
 New C++ syntax for defining signals and
slots, added to public, private, etc.
class myClass : public Qobject {
Q_OBJECT //required macro, no semicolon
…
signals:
void somethingHappened();
…
public slots:
void slotDoSomething();
…
private slots:
void slotDoSomethingInternal();
…
};

15. EVENTS
 Signals: emit events
 declare as signals, otherwise normal member functions
 You don't implement them. Rather, you send them with
the (new) keyword emit
 E.g. emit(sliderChanged(5))
 Slots: receive and handle events
 Normal member functions declared as slots
 Connect: must connect signals to slots
 QObject::connect( mymenu, SIGNAL(activated(int)),
myobject, SLOT(slotDoMenuFunction(int)) );
 moc: meta object compiler (preprocessor) converts
these new keywords to real C++

16. WIDGETS
 Base class for all UI widgets
 Properties
 width, height, backgroundColor, font, mouseTracking,
backgroundPixmap, etc.
 Slots
 repaint, show, hide, move, setGeometry,
setMainWidget, etc.
 Signals:
 mouseMoveEvent, keyPressEvent, resizeEvent,
paintEvent, enterEvent, leaveEvent, etc.

17. QT, A GUI TOOLKIT
 Events processed with signals and slots
signal generates an event, e.g., button push
 slot processes the event, e.g., pop up a file dialog box
QPushButton * quitB = new QPushButton(“Quit”,...,...);
 connect (quitB, SIGNAL(clicked()), qApp, SLOT(quit());
 qApp is a global variable, of type QApplication
 one QApplication per program defined first in main()
 main returns qApp.exec()
 SIGNAL and SLOT are macros, expanded by a meta-object
 compiler (moc)
 moc generates .cpp files from user-defined Qt subclasses

18. OTHER FEATURES OF QT
 The Qt Paint Engine
 QPainter is highly optimized and contains several caching mechanisms
to speed up drawing. Under X11, it caches GCs (graphics
contexts), which often make it faster than native X11 programs.
 QPainter contains all the functionality one would expect from a professional
2D graphics library. The coordinate system of a QPainter can be
transformed using the standard 2D transformations (translate, scale, rotate
and shear).
 Qt supports Open GL for 3D graphics.
 Qt also contains a set of general purpose classes and a number of
collection-classes to ease the development of multi-platform applications.
 Qt has platform independent support for the operating system dependent
functions, such as time/date, files/directories and TCP/IP sockets.

19. Anupam Kumar Srivastava
Software Engineer
Burl Software Pvt. Ltd
E-mail: anupam.933@gmail.com