iOS Chess. GUI in Objective-C. Chess in C++.
Functional (if the slightest bit ugly) chess game conforming to all standard chess rules1, with the exception of certain draw conditions2. You know it works because while playing it, you remember how bad you are at chess.
1Including castle, en passant, and pawn promotion
2Wikipedia has a fantastic list of all possible draw conditions, available here: https://en.wikipedia.org/wiki/Rules_of_chess#Draws . This game enforces none of them except stalemate.