This "online-course" recreates the C++ standard library step by step and along the way not only teaches C++ language features but also software architecture, exception safety, ACIDic programming, performance optimization, etc. It won't make you a C++ guru by itself but will equip you with all the knowledge and tools to eventually reach that level on your own.
Assignments are published upon demand and everybody works at their own pace. Questions are answered and hints given during the assignment. Upon comletion one receives an in-depth review of one's solution with suggestions for improvement.
All you need to join is a basic understanding of C.