exercism.io is a croud-sourced mentorship platform for learning to program. In my opinion, they do a lot of things right. In particular, an exercise on exercism.io consists of a descriptive README file and a set of test cases implemented in the target programming language. The tests have two positive sides: You learn to do test-driven development, which is good. And you also have an automated validation suite. Of course, a test can not give you feedback on your actual implementation, but at least it can give you an idea if you have managed to implement what was required of you. But that is not the end of it. Once you have submitted a solution to a particular exercise, other users of exercism.io can comment on your implementation. And you can, as soon as you have submitted the first implementation, look at the solutions that other people have submitted to that particular problem. So knowledge transfer can happen both ways from there on: You can learn new things from how other people have solved the same problem, and you can also tell other people about things they might have done in a different way. These comments are, somewhat appropriately, called nitpicks on exercism.io.
Now, exercism has recently gained a C++ track. That track is particularily fun, because it is based on C++11, Boost, and CMake. Things that are quite standard to C++ development these days. And the use of C++11 and Boost makes some solutions really shine.