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I consider myself an amateur and the following is based on my limited understanding. I also live in Germany, but copyright questions would probably cross borders anyway.

Situation

Let's assume I developed and released my (yet to be created) application for commercial purposes and used the LGPL version of QT and linked to it dynamically. Complying to the LGPL licence should be possible without releasing the code of my application core. Futhermore I decide that the LGPL version of QT is too restrictive (e.g. no static linking) and subscribe to use the commercial version QT. Any releases of my application would then be without QT source code and statically linked.

Problem

The only problem I see here is that statement in the faq entry 2.13:

If I have started development of a project using the open source version (LGPL), can I later purchase a commercial version of Qt and move my code under that license?

This is not permitted without written consent from The Qt Company. If you have already started the development with an open-source version of Qt, please contact The Qt Company to resolve the issue. If you are unsure of which license or version to use when you start development, we recommend you contact The Qt Company to advise you on the best choice based on your development needs.

Question

Can the QT Company really deny using the commercial version of QT in combination with my propriatary application core code? I read (part of) the licence text, but I cannot answer this question specifically. What really baffles me is that even when using the LGPL version of QT (as a library) I retain the exclusive rights to the application core and never have to release the source under any licence (when properly done, of course). To rephrase: In what way can the commercial licence force me to obtain my own source code not using the LGPL version of QT?

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Can the QT Company really deny using the commercial version of QT in combination with my propriatary application core code?

This is a commercial company and this is about a commercial contract. They can do whatever they want in their contracts.

In what way can the commercial licence force me to obtain my own source code not using the LGPL version of QT?

Again, in a commercial contract -- and if you agree to it-- they can do whatever they want which may or may not be entirely legit under some jurisdictions, but a contract becomes the law between its parties.

My take here is that QT wants to ensure that they always have full control of how its software is combined with copyleft-licensed code as their business is based on the full ownership of the code and dual licensing which that can only achieve if they control the code fully

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  • Thanks for answering. I understand what you say, but when viewing code as the written form of an Idea or art piece, the way I got this idea (using the LGPL version) should not prevent me from writing it down again using another software-licence. Assuming of course I never lost any rights to my idea.
    – Araeos
    Commented Aug 20, 2016 at 10:18
  • Copyright has nothing to do with ideas and commercial contracts can be has weird as they can get (and often are). But yes in general I would assume that the QT commercial contracts are not that far reaching. To make a parallel with employment contracts, some of them assign all your ideas to your employer (even though this may not be legally enforceable in many jurisdictions AFAIK). And of course they also demand your first new born and so on . ;)
    – Philippe Ombredanne
    Commented Aug 20, 2016 at 10:25

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