This won't teach you C++ for a
robot, but it does have an open problem set in C++. It also takes a novel online approach to teaching - immediate community feedback. I used it & CodeAcademy to take a crash-course in javascript so I could figure out how to fix a complicated bug in someone else's code the night before a demo at work.
It's alot like ChiefDelphi - there are nuggets of wisdom, and best practices - but knowing which information to trust takes some experience.
http://exercism.io/