\title The Qt FAQ
See also the \link commonproblems.html Technical FAQ\endlink.
\section1 General Questions
\section2 What is Qt?
Qt is a multiplatform C++ GUI toolkit. It
provides application developers with all the functionality
needed to build applications with state-of-the-art graphical
user interfaces. Qt is fully object-oriented, easily
extensible, and allows true component programming.
Qt is supported on the following platforms:
\i <b>MS/Windows</b> -- 95, 98, NT 4.0, ME, 2000, and XP
\i <b>Unix/X11</b> -- Linux, Sun Solaris, HP-UX, Compaq Tru64 UNIX,
IBM AIX, SGI IRIX and a wide range of others
\i <b>Macintosh</b> -- Mac OS X
\i <b>Embedded</b> -- Linux platforms with framebuffer support.
It has become the emerging standard development
environment for IT professionals who want to:
\i Write (and maintain) a single source-tree.
\i Port their applications to multiple platforms with a simple
\i Internationalize their applications.
\i Provide their applications with native look and feel.
\i Have their applications run at native speed.
\section2 What is Trolltech?
Trolltech is an international software company with
headquarters in Oslo, the capital of Norway, and with offices
in Brisbane, Australia, and Redwood City, California. Our
flagship product is Qt, the multi-platform C++ GUI toolkit.
Trolltech ASA was founded in 1994, although, the core team of designers
started Qt's development in 1992.
More details can be found at \link troll.html Trolltech\endlink.
\section2 How can I evaluate Qt?
We offer 30 day evaluation versions of Qt on all our supported platforms.
Further details can be found here: \l http://www.trolltech.com/evaluate.html.
\section2 What does Qt cost?
This depends on your number of developers, on how many
different platforms you target, and on the edition of Qt you
choose. Currently we offer two different editions (Enterprise
Edition and Professional Edition) on four different
platforms (Microsoft Window, Unix/X11, Macintosh and Linux/embedded).
We also offer discounts for academic and research institutions.
Details on pricing can be found here:
\section2 How can I buy Qt?
To purchase, please visit the <a
webshop</a>. In the webshop, you can generate a Purchase Order, order
a Quote, or purchase directly using a credit card.
For further information and assistance, please contact Trolltech sales:
Email: \link mailto:email@example.com firstname.lastname@example.org\endlink
Phone, US office (for North America): <strong>(+1) 650-813-1676</strong>
Phone, Norway office (for others): <strong>+47 2160 4800</strong>
\section2 What kind of technical support is available for Qt?
Trolltech offers email support to commercial licensees. The first year
of Trolltech's Support and Maintenance Service is included with the
purchase of Qt Professional and Qt Enterprise Edition licenses.
Licenses can be kept up-to-date and fully supported by buying
the Support and Maintenance Service for the licenses held in each
For further information see here:
\section2 Who provides training for Qt?
A number of companies provide Qt training, in Asia, Europe and the US.
Trolltech has agreements with a number of official training
partners, who offer Trolltech approved courses. In addition
there are a number of independently developed courses run by third
parties. You can find more information about the partners and the
courses here: \l http://www.trolltech.com/training/.
\section2 Who provides consultancy for Qt?
A number of companies provide consultancy services for Qt and
Qtopia. You can find a partial list here:
\section2 Are there any books about Qt programming?
Yes, there are quite a few books on Qt, most of them
available from amazon.com. You can find a partial list at
The official Qt book is
<a href="http://www.amazon.com/exec/obidos/ASIN/0131240722/trolltech/">C++ GUI Programming with Qt 3</a>.
\section2 What is the Open Source Edition?
The Qt Open Source Edition is our non-commercial versions of
Qt. You can use this edition to create and run free
software, i.e. software that is licensed under the GNU GPL or
a similarly recognized open source license.
Qt Open Source edition is available for Unix/X11, Macintosh and
\section2 What is the idea behind having a free version of Qt?
With the Open Source Edition, Trolltech has found a way to support
the free software concept. Part of our commitment is to enable
those who contribute to the free software pool to do so
without paying license fees.
\section2 Is there a low-cost version of Qt for writing shareware?
No, but Qt's commercial licenses permit you to write and sell
shareware written with Qt.
\section1 Questions about Qt's Users
\section2 How many developers use Qt?
We don't know! We can count our many thousands of commercial
licensees, but the number of free software developers
using Qt can only be guessed based on download figures, book
sales, the number of open source Qt applications available on
the web, Universities teaching with Qt, and other factors. A
conservative estimate would put the figure at around 100,000 to
150,000 developers world-wide.
\section2 Which major corporations use Qt?
See our list of reference customers at
\l http://www.trolltech.com/company/customers.html. Note that many
commercial customers do not disclose their use of Qt because they see
Qt as a competitive advantage.
\section2 Are there any well-known applications that use Qt?
There are many, e.g. Adobe Photoshop Album from Abobe
Corporation and the KDE desktop. A partial list can be found at
\section1 Questions about Developing Applications with Qt
\section2 Is there a mailing list for Qt?
Trolltech operates several mailing lists for Qt users including
\c qt-announce, \c qt-interest and \c snapshot-users.
See \link http://lists.trolltech.com Mailing lists\endlink.
\section2 Is Qt's documentation available online?
Yes, at \l http://doc.trolltech.com.
\section2 Is there a technical newsletter for Qt?
Yes, \link http://doc.trolltech.com/qq/index.html Qt
Quarterly\endlink. It is a paper-based newsletter exclusively
available to Qt licensees.
As a courtesy and convenience to all our users, a selection of
articles is also published online about one month after publication.
\section2 I think I have found a bug in Qt. Where can I report it?
Bugs should be reported to \link mailto:email@example.com
firstname.lastname@example.org\endlink. But \e please read (the very short)
\link bughowto.html How to Report a Bug\endlink page \e first.
\section1 Questions about Professional Licensing
\section2 We have purchased a license, but the developer is leaving the company
project. Can we transfer the license to another developer?
Yes. Qt licences are for the individual use of named developers. If
the developer associated with a licence leaves the organization, or
moves on to another project that does not require a Qt license,
another developer can take over the licence. You can change the name
of the developer using a licence at any time, but after a change you
must wait at least six (6) months before changing again.
\section2 Can I purchase floating licenses for Qt?
No, we do not offer floating licenses.
The reason is this: Floating licenses are based on the concept
of defining a maximum number of concurrent users. For example,
for a word processor, this means the maximum number of
instances of that application that can be running at any
time. However, for a library product like Qt, there is no main
application that is always running when the product is being
used. Hence, it is not possible to count the number of
concurrent users, and therefore floating licenses do not
#if 0 // ##### Just an idea, no clue if this is policy today
While we want to develop our software for multiple platforms, some
of our developers will only need to work on a single platform. Can
we purchase single packs for those?
No, you can only purchase single packs as long as you target
one platform only.
\section2 Can we use the Open Source Edition while developing our non-free
application and then purchase commercial licenses when we start to
No: our commercial license agreements only apply to software
that was developed with Qt under the agreement. They do not
apply to code that was developed with the Qt Open Source Edition
prior to the agreement. Any software developed with Qt
without a commercial license agreement must be released as
free/open source software.
\section1 Questions about Open Source Software Licensing
\section2 I just want to run KDE and other Qt-based software on my
machine. Can I do that with the Open Source Edition?
Yes. You may use the Qt Open Source Edition for running applications,
both at home and at work, without restrictions.
\section2 I want to put Qt, and applications that use it, on a CD-ROM or on
a web site. Can I do that with the Open Source Edition?
Yes. You may copy and redistribute the Qt Open Source Edition, both
at home and at work, without restrictions.
\section2 Can I make software with the Qt Open Source Edition and release it under
the GNU GPL, BSD, or Artistic license?
Yes. The GNU GPL, GPL-compatible licenses, or any other
approved open source license will do. The FSF.org and
OpenSource.org web sites list approved software licenses.
\section2 I don't want to give away my source code. What do I do?
You need to buy a commercial Qt license.
\section2 Is software based on the Qt Open Source Edition really free? Does it
carry any Trolltech-specific license restrictions?
Yes, it really is free. This is because the Qt Open Source Edition
uses the GNU GPL, which forbids the imposition of any license
restrictions on software based on the Open Source Edition that would
make it non-free. And no, there are no Trolltech-specific
license restrictions on software produced using the Open Source
\section2 What if Trolltech stops releasing Open Source Editions of Qt?
We have absolutely no intention of doing that. Together with
the K Desktop Environment project, we have set up the KDE Free
Qt Foundation (see
\l http://www.kde.org/whatiskde/kdefreeqtfoundation.php) to
legally guarantee the availability of Qt for free software
development now and forever - even if circumstances beyound
our control prevent us from producing new free editions.
\section2 Using the Open Source Edition, can I make non-free software for internal
use in my company/organization?
No. Software developed with the Open Source Edition is always free
software, i.e. it can only be distributed under a free
software license. In particular, all the source code for all
the modules your software is based on, regardless of whether
they have been written by you or by others, must be free
software. This is part of our commitment to the free software
community, and enables those who contribute to the free
software pool to do so without paying license fees.
Although it is possible to write free software for internal
use, it is difficult to ensure that such software is used and
distributed legally. For example, if your free software
requires any modules that impose conditions on you that
contradict the conditions of the GNU GPL, including, but not
limited to, software patents, commercial license agreements,
copyrighted interface definitions or any sort of
non-disclosure agreement, then you cannot distribute it at
all; hence it cannot be given to consultants, employees for
their personal computers, subsidiaries, other divisions, or
even to new owners.
Consequently we recommend using commercial licenses for all
internal software development.
\section2 Is Qt Open Source Edition really free software in the GNU meaning of the
Yes - it is free software both as in "no cost" and as in
"free speech". Even more, it is actually available under the
terms of the GNU GPL. This means you can link GPL'ed software
to it, and you can take code from Qt and put it into other
\section2 If the Open Source Edition is GNU GPL, aren't there license conflicts
with incompatible open source licenses? How come people can even
run commercial software like the Opera web browser with it?
If the Open Source Edition was licensed purely under the GNU GPL,
there would be problems. However, as long as Qt-based
software is either open source or was developed under a
commercial license agreement with Trolltech, we grant
permission to compile, link and run those programs with the
Open Source Edition. This is written down in our second free software
license, the QPL.
\section2 Doesn't such an en extra license in addition to the GNU GPL
violate the terms of the GPL or at least make the whole package
No, because those extra license terms give you rights in
addition to those of the GNU GPL, including the right to
remove those extra terms.
\section2 Why is Qt Open Source Edition not distributed under the GNU Lesser
General Public License (LGPL)?
The LGPL is designed to "permit developers of non-free
programs to use free libraries" (quote from the LGPL). In
other words, if Qt Open Source Edition were LGPL'd, companies would
not have to purchase our commercial editions in order to make
commercial/proprietary software, they could just use the Open Source
Edition, free of charge. That would mean Trolltech would not
get the revenue necessary for improving and extending Qt.
Note also that the Free Software Foundation discourages the
use of the LGPL.
\section1 Questions about other programming languages
\section2 Are there bindings to languages other than C++?
Yes there are, e.g. bindings to Perl (PerlQt) and Python
(PyQt). These bindings were written by the open source
community and are not supported by Trolltech.
\section2 Can I develop commercial applications with PerlQt or PyQt or other
Yes, if you have purchased a commercial license. The licensing
conditions are the same whether you use the Qt API directly in
C++, or you use it through some API wrapper in another
programming language. All developers that write code
containing calls to the Qt API (directly or through a wrapper)
need Qt licenses.