Gearboy is a Nintendo Game Boy / GameBoy Color emulator written in C++ that runs on iOS, Raspberry Pi, Mac, Windows and Linux.
The emulator is focused on readability of source code and very high compatibility.
A lot of effort has gone into this in order to improve compatibility and keep it as simple as possible.
You can grab the emulator and the source code at GitHub: https://github.com/drhelius/Gearboy
More screenshots here.