Cross-platform GUI framework for C++

So far I've only done command line programs in C++. I want to make GUI programs in it now though. Should I use Qt or GTK+? Or something else?

>>1
Probably SDL+some simple GUI lib, like one of these:
https://github.com/dalerank/nanogui-sdl
https://github.com/vurtun/nuklear
Note that I haven't actually tried one of these, so yeah, try at your own risk I guess.