SFML vs FLTK

In short, SFML is free for any use (commercial or personal, proprietary or open-source). You can use SFML in your project without any restriction. You can even omit to mention that you use SFML -- although it would be appreciated.

If you hate something about the way SFML handles graphics, you can just combine it with OpenGL. It's completely smooth and works as expected, without any additional dependencies.

You can ask questions on their own personal forum which is full of users, and their wiki is constantly being maintained. They even have an IRC.

SFML 2.2 brought forth Android & iOS functionality, and SFML games work on Linux, Mac and Windows out of the box, since SFML is written with OpenGL.

SFML is very well documented, even with short examples of use for many functions and modules. Furthermore, there are books like 'SFML essentials' and 'SFML for game development' which teach you how to use this library to its fullest.

If you are thinking about using OpenGL, look no further, you can open a window, and handle events in less than 15 lines, and it provides input, time, and even networking, plus alot more. It has become my favorite c++ library :D

SFML is a collection of modular, well designed libraries you can implement an engine or game on top of. The API provides tons of good documentation and is very straightforward to use. You can get a game up and running with SFML quite quickly and with minimal effort.

An SFML project's code-base is usually clean and easy to read. All public SFML classes are under the namespace "sf" so it is easy to tell which code is yours and which belongs to SFML (of course you can stop this by typing using namespace sf;).

There's support for many languages besides C++, current supported languages are: C, .NET, Crystal, D, Euphoria, Go, Java, Julia, Nim, OCaml, Pascal, Python, Ruby and Rust, and this list is constantly growing.

SFML is extremely beginner friendly and even provides pre-built libraries for your IDE of choice on Windows. Besides the fact that it's extremely well documented, they also have a set of tutorials that walk you through every module.

SFML is one of the few good C++ frameworks out there to actually make full use of the language. It's extremely well optimized and it plays well with anything you throw at it.

It's simple design and lack of more advanced C++ features makes it easy for beginners.

Well-designed widgets, coded with careful attention to rendering/execution speed.

FLTK code developed more than 10 years ago still compiles and runs perfectly, without changes.

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.

Fast Light User-Interface Designer (FLUID) included.