Incorrect. The open source license is LGPL. If you dynamically link you can even keep your app code proprietary. If you want to statically link Qt then your code must be LGPL, but that also doesn't prevent you from selling the product.
If you statically link against an LGPLed library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application.
That's interesting. I expect for most common purposes LGPL and dynamic linking are probably fine, but it's good that there is a workaround for static linking.
Apple's mergeable libraries are basically dynamic libraries, but I expect on Apple platforms you'd have to provide the library source anyway since it won't be included in the OS. Of course I've never figured out how to download the LGPL source code from an iOS app - presumably there is a way other than just searching for it via google or github?
Apparently distributing object files for relinking on incompatible Unices used to be a fairly common practice in the early 90s[1], although I don’t know if it was ever done for LGPL compliance.
Yeah, I am mistaken but you do have to give away source to your product because of the viral nature of the LPGL. So it's not completely free to do what you want. And I've always been suspicious of the dynamic linking loophole.
The so-called dynamic linking loophole applies to the GPL, not the LGPL. An LGPL library can be linked and distributed with a closed source program if the end user is able to replace the library with a build of their own.
The loophole you're referring to involves creating a generic plugin interface that allows the program to use any library as long as it meets the requirements of the plugin system. If the program can function even when no plugins are present, the program cannot be said to be derivative of any plugin; it's the plugins that depend on the program, not vice versa. Therefore, you could for example develop an open source plugin that extends the functionality of your program and depends on GPL'd components and distribute that plugin binary and its source code with your closed source application.
Free as in keeping the source to your product closed-source. Otherwise you have to buy a license if you want to keep it close-source while statically linking it.
