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# The Qt FAQ

See also the Technical FAQ.

- General Questions
  - What is Qt?
  - What is Trolltech?
  - How can I evaluate Qt?
  - What does Qt cost?
  - How can I buy Qt?
  - What kind of technical support is available for Qt?
  - Who provides training for Qt?
  - Who provides consultancy for Qt?
  - Are there any books about Qt programming?
  - What is the Open Source Edition?
  - What is the idea behind having a free version of Qt?
  - Is there a low-cost version of Qt for writing shareware?
- Questions about Qt's Users
  - How many developers use Qt?
  - Which major corporations use Qt?
  - Are there any well-known applications that use Qt?
- Questions about Developing Applications with Qt
  - Is there a mailing list for Qt?
  - Is Qt's documentation available online?
  - Is there a technical newsletter for Qt?
  - I think I have found a bug in Qt. Where can I report it?
- 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?
  - Can I purchase floating licenses for Qt?
  - Can we use the Open Source Edition while developing our non-free application and then purchase commercial licenses when we start to sell it?
- Questions about Open Source Software Licensing
  - I just want to run KDE and other Qt-based software on my machine. Can I do that with the Open Source Edition?
  - 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?
  - Can I make software with the Qt Open Source Edition and release it under the GNU GPL, BSD, or Artistic license?
  - I don't want to give away my source code. What do I do?
  - Is software based on the Qt Open Source Edition really free? Does it carry any Trolltech-specific license restrictions?
  - What if Trolltech stops releasing Open Source Editions of Qt?
  - Using the Open Source Edition, can I make non-free software for internal use in my company/organization?
  - Is Qt Open Source Edition really free software in the GNU meaning of the word?
  - 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?
  - 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?
  - Why is Qt Open Source Edition not distributed under the GNU Lesser General Public License (LGPL)?
- Questions about other programming languages
  - Are there bindings to languages other than C++?
  - Can I develop commercial applications with PerlQt or PyQt or other Qt wrappers?

## 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 allows true component programming.

Qt is supported on the following platforms:

- MS/Windows -- 95, 98, NT 4.0, ME, 2000, and XP
- Unix/X11 -- Linux, Sun Solaris, HP-UX, Compaq Tru64 UNIX, IBM AIX, SGI IRIX and a wide range of others
- Macintosh -- Mac OS X
- Embedded -- Linux platforms with framebuffer support.

It has become the emerging standard development environment for IT professionals who want to:

- Write (and maintain) a single source-tree.
- Port their applications to multiple platforms with a simple recompilation.
- Internationalize their applications.
- Provide their applications with native look and feel.
- Have their applications run at native speed.

### 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 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 Window, 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 purchase 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 here: 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 here: 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.

### 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 Linux/embedded.

### 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.

### 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 at around 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 operates 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 Open Source 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 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.

## Questions about Open Source Software Licensing

### 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.

### 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.

### 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.

### 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 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 Edition.

### 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 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.

### 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.

### Is Qt Open Source Edition 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 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.

### 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 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.

## 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.