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The Qt FAQ

See also the Technical FAQ.

General Questions

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 facilitates component programming.

Qt is supported on the following platforms:

Qt is considered to be the standard development environment by many IT professionals who want to:

What is Trolltech?

Trolltech is an international software company with headquarters in Oslo, the capital of Norway, and with offices in Brisbane, Australia, and Palo Alto, California. Our flagship product is Qt, the multi-platform C++ GUI toolkit.

Trolltech AS was founded in 1994, although, the core team of designers started Qt's development in 1992.

More details can be found at Trolltech.

How can I evaluate Qt?

We offer 30 day evaluation versions of Qt on all our supported platforms. Further details can be found here: http://www.trolltech.com/evaluate.html.

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 Windows, Unix/X11, Macintosh, and Linux/Embedded).

We also offer discounts for academic and research institutions.

Details on pricing can be found here: http://www.trolltech.com/pricing.html.

How can I buy Qt?

To purchase, please visit the Trolltech webshop. In the webshop, you can generate a Purchase Order, order a Quote, or buy directly using a credit card.

For further information and assistance, please contact Trolltech sales:

Email: sales@trolltech.com. Phone, US office (for North America): 1-650-813-1676 Phone, Norway office (for others): +47 2160 4800

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 subsequent year.

For further information see: http://www.trolltech.com/support/.

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 at: http://www.trolltech.com/training/.

Who provides consultancy for Qt?

A number of companies provide consultancy services for Qt and Qtopia. You can find a partial list here: http://www.trolltech.com/company/servicepartners.html.

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 http://www.trolltech.com/developer/books.html.

The official Qt book is C++ GUI Programming with Qt 3 (ISBN 0-13-124072-2).

What are the Free Editions?

The Qt Free Editions are our non-commercial versions of Qt. You can use these editions to create and run free software, i.e. software that is licensed under the GNU GPL or a similarly recognized open source license.

Free editions of Qt are available for Unix/X11, Macintosh and Linux/Embedded.

What is the idea behind having free editions of Qt?

The Free Editions are a way for Trolltech to support the free software concept, and free software developers. These editions are part of our commitment to those who contribute to the free software pool, allowing them to use Qt without paying license fees.

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.

Questions about Qt's Users

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 in the region of 100,000 to 150,000 developers world-wide.

Which major corporations use Qt?

See our list of reference customers at 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.

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 http://www.trolltech.com/products/qt/apps-using-qt.html

Questions about Developing Applications with Qt

Is there a mailing list for Qt?

Trolltech provides several mailing lists for Qt users including qt-announce, qt-interest and snapshot-users.

See Mailing lists.

Is Qt's documentation available online?

Yes, at http://doc.trolltech.com.

Is there a technical newsletter for Qt?

Yes, Qt Quarterly. 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.

I think I have found a bug in Qt. Where can I report it?

Bugs should be reported to qt-bugs@trolltech.com. But please read (the very short) How to Report a Bug page first.

Questions about Professional Licensing

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.

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 make sense.

Can we use the Free Edition while developing our non-free application and then purchase commercial licenses when we start to sell it?

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 a Qt Free Edition prior to the agreement. Any software developed with Qt without a commercial license agreement must be released as free/open source software.

Questions about Free Software Licensing

I just want to run KDE and other Qt-based software on my machine. Can I do that with the Free Edition?

Yes. You can use a Qt Free Edition for running applications, both at home and at work, without restrictions.

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 Free Editions?

Yes. You can copy and redistribute the Qt Free Editions, both at home and at work, without restrictions.

Can I make software with the Qt Free 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 are acceptable. The FSF.org and OpenSource.org web sites list approved software licenses.

I don't want to give away my source code. What do I do?

You need to buy a commercial Qt license.

Is software based on the Qt Free Editions really free? Does it carry any Trolltech-specific license restrictions?

Yes, it really is free. This is because the Qt Free Edition uses the GNU GPL, which forbids the imposition of any license restrictions on software based on a Free Edition that would make it non-free. And no, there are no Trolltech-specific license restrictions on software produced using a Free Edition.

What if Trolltech stops releasing Free 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 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 prevented us from producing new free editions.

Using a Free Edition, can I make non-free software for internal use in my company/organization?

No. Software developed with a Free 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.

In view of the above, we recommend using commercial licenses for all internal software development.

Are the Qt Free Editions really free software in the GNU meaning of the word?

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 GPL'ed software.

If the Free Editions are 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 Free 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 Free Edition. This is written down in our second free software license, the QPL.

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 GPL-incompatible?

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.

Why aren't the Qt Free Editions 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 Free Edition were LGPL'd, companies would not have to purchase our commercial editions in order to make commercial/proprietary software, they could just use a Free 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.

Questions about other programming languages

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.

Can I develop commercial applications with PerlQt or PyQt or other Qt wrappers?

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.


Copyright © 2004 Trolltech. Trademarks
Qt 4.0.0-tp1