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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.
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.