I’m retired, I am starting part time study for a degree in mathematics and software development this year and I intend to write a collection of basic calculators in different languages and eventually start a GNU graphical calculator project in my favorite language. Take a moment to help me by listing some of the different technologies that are used to write for multiple operating systems simultaneously or that make it easy to port applications between different operating systems.
I am so far committed to writing a simple calculator in Swift which I am told will then compile for everything Apple and another in C++ with SDL bindings that I am told will be easy to port to all the major operating systems. Do you know of any other systems that I should use to write/port a third, fourth or fifth little calculator for/to a wide range of platforms?