Think of a class as being a template for a structure (i.e., a struct typedef).
When you call a member function of a class, you are in fact passing to that function an "invisible" argument. That is to say that you won't see this argument as a formal argument in the function's declaration, nor will you include a value for the argument when you call the member function. This invisible argument is the "this" pointer, which points to a specific instance of the class'es structure.