Question. Does QT still have a licensing problem for closed-source apps on mobile platforms?
Last I saw, QT's GPL/LGPL license required you to open-source the code that was statically linked to it. That wasn't much of a problem for desktop platforms, as you could just package up all of QT into its own QT.DLL (or the equivalent) and link to it dynamically. But this isn't allowed on stuff like iOS, so you were required to buy a license for those.
Only parts (non-core) of QT are GPL, most are LGPL
And it's a bit of a misconception that LGPL requires statically linked things to be open sourced: in that area it only requires that the LGPL components be replaceable (and the source for the LGPL components be available). In the context of static linking, that can be done by releasing pre-link binaries (object files) with instructions for linking them.
Of course, that's likely more work than just making the source available under a proprietary license (ie: provide source code so LGPL components can be replaced).
If you're using Qt GPL components, then yes, you'll need to open source your application (or pay for a Qt License)
author here;
IANAL but I think for iOS it's still required to either open-source your code under GPL or to buy a commercial license as you said.
But IIRC there is work being done to bring dynamic linking to iOS as well, since apple allowed it with iOS 8 or so.
edit: can't comment any further currently, I will try to answer everything once I'm unblocked.
Qt itself is LGPL now, so it no longer requires you to open-source code linked to it.
Some of the language bindings that aren't maintained by Digia are still GPL, though. An example of this is PyQt, and the PyQt team's intransigence on the GPL is why PySide exists.
Last I saw, QT's GPL/LGPL license required you to open-source the code that was statically linked to it. That wasn't much of a problem for desktop platforms, as you could just package up all of QT into its own QT.DLL (or the equivalent) and link to it dynamically. But this isn't allowed on stuff like iOS, so you were required to buy a license for those.