When comparing FLTK vs GTK+, the Slant community recommends GTK+ for most people. In the question“What are the best cross-platform GUI toolkits?” GTK+ is ranked 8th while FLTK is ranked 16th. The most important reason people chose GTK+ is:
Ranked in these QuestionsQuestion Ranking
Pro GUI designer
Fast Light User-Interface Designer (FLUID) included.
Uses a limited and lightweight design and restricts itself to solely GUI functionality. Because of this restriction, the FLTK hello world example is only about 100 KiB.
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Pro Language bindings
Pro Nicer user interface when using PyGi
Creating cross platform enterprise apps with PyGi and with the help of Gtk Themes makes them look great.
Con Does not look like a native application
Because it uses non-native widgets, it doesn't look like a native application on any platform.
Con Can be too simple for some projects
FLTK offers far fewer widgets than most other toolkits.
Con Terrible Integration
Looks and feels like an alien on all other platforms than GNOME.