Saturday, July 20, 2013

What is LGPL - why you should consider it for your next commercial application.

So I had someone ask on G+ today what frameworks to use for C++ for commercial development. They had mentioned that Qt has a commercial, paid license - but they clearly hadn't heard of the LGPL.

So I wanted to clear that up.

So what does LGPL mean?

You put a bunch of dlls in your application. This pretty much only practically affects Windows - as there is little choice on Mac to do anything else if you want to use the .app format, and in Linux you can simply specify your dependency in your package.

Note, however, that there is no real change in size in your application. In all cases its a packaging issue.

You can't modify the Qt source code without contributing those changes back. And you have to provide a Licenses.txt file with a notice in it that you are using Qt and link to Digia's website.

So don't believe anyone who says that Qt is not free for commercial development - they don't have a clue about what they're talking about.

Qt is perfect for commercial application development. Its awesome, it runs on each platform, it has a modern, simple nice API, its well maintained and easy to use, and there's no reason you can't use LGPL and switch to Commercial if you ever decide you need that extra utility.

In case you don't believe me, here is the official wording for the license:

"5. Combined Libraries.

You may place library facilities that are a work based on the Library side by side in a single library together with other library facilities that are not Applications and are not covered by this License, and convey such a combined library under terms of your choice, if you do both of the following:
  • a) Accompany the combined library with a copy of the same work based on the Library, uncombined with any other library facilities, conveyed under the terms of this License.
  • b) Give prominent notice with the combined library that part of it is a work based on the Library, and explaining where to find the accompanying uncombined form of the same work."

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