entrant.cpp:28:41: error: request for member ‘get_id’ in ‘((Entrant*)this)->Entrant::course’, which is of pointer type ‘Course*’ (maybe you meant to use ‘->’ ?)
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 Translation: On line 28 of entrant.cpp, position 41, you're using the dot operator to access an attribute (get_id) of a pointer (course in namespace Entrant). This is invalid. You have to use the arrow operator to de-reference the pointer and access the members of the thing it points to.

ie you're doing
course.get_id; // valid Java.
course->get_id; // valid C++

Does this help?

At the risk of starting a cargo cult, Hungarian Notation is useful for avoiding these kinds of errors.