When comparing gtkmm vs FLTK, the Slant community recommends FLTK for most people. In the question“What are the best cross-platform C++ GUI toolkits ?” FLTK is ranked 4th while gtkmm is ranked 11th. The most important reason people chose FLTK is:
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Specs
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Pros
Pro Pure C++ and if you hate IDE's, it is perfect
Pro Easy memory management
Pro Fairly lightweight
gtkmm is focused on only one thing: the GUI. It does not provide auxiliary features/APIs like in Qt.
Pro Simplicity
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Pro Fast
Well-designed widgets, coded with careful attention to rendering/execution speed.
Pro Stable
FLTK code developed more than 10 years ago still compiles and runs perfectly, without changes.
Pro Lightweight
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.
Pro GUI designer
Fast Light User-Interface Designer (FLUID) included.
Cons
Con Documentation is lacking
Con Can be too simple for some projects
FLTK offers far fewer widgets than most other toolkits.
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.