How can I make my own GUI library e.g. SFML?
This question is for the purpose of understanding how GUI libraries work, not to actually make my own library.
I asked what I needed to do so. Can I open my editor and write some C++ code, importing some libraries and that's it. Or it's more complicated?
Creating a shared and static library with the gnu compiler [gcc] : http://www.adp-gmbh.ch/cpp/gcc/create_lib.html
A library is "just" a bunch of classes and functions, so you write it like a normal program. The only thing that is different is when you compile it: you have to create a static/dynamic library file.