Qt moving from LGPL to commercial halfway through [closed]

Question

I've never understood this bit about licensing on the Qt website.

Qt Commercial Developer License The Qt Commercial Developer License is the correct license to use for the development of proprietary and/or commercial software with Qt where you do not want to share any source code.

You must purchase a Qt Commercial Developer License from us or from one of our authorized resellers before you start developing commercial software. The Qt Commercial Developer License does not allow the incorporation of code developed with the Qt GNU LGPL v. 2.1 or GNU GPL v. 3.0 license versions into a commercial product.

If you are starting to develop an app while you're not sure if you'll ever want to sell it (using LGPL), how would they prevent you from moving to the commercial license at some point? As long as the API is the same you simply recompile / link, no?

What am I missing?

Answer 1

I believe that the text only refers to code that has already been distributed under LGPL, and therefore cannot be closed-sourced by switching Qt license.

I think you have nothing to worry about: nobody know/cares where the undistributed code you wrote came from (Commercial Qt or LGPL Qt). As long as it hasn't been released under a LGPL license, nothing can happpen.

Answer 2

It is the Commercial licence that adds the restriction. When they sell their software under their own commercial licence, they are free to add restrictions within the limits of the law. They are allowed to stipulate that you cannot licence software under their commercial licence that was developed using the LGPL version of the software. It is difficult for them to police, certainly, but they may just be relying on most people to do the right thing (and the fact that their Commercial licence isn't too expensive anyway).

There is nothing stopping you from selling software developed with the LGPL version, and you don't have to publish your source code. The only real consequence of using the LGPL is that you cannot technically link to the QT libraries statically. That isn't common with Qt so it's not much to worry about. And of course you don't get the same support.

To release using the commercial version, you need to purchase the commercial version from Qt (which gets you a licence code). The source is the same, but you are not using the commercial 'form' of Qt unless you buy it or steal it.