Welcome to today's lesson! We'll be exploring a practical application of C++ sets by managing student enrollments for various courses. Imagine you're running an online course platform and need to handle enrollments, checks, and listings of students in different courses. std::unordered_set would be perfect for this kind of problem since it does not allow duplicates, ensuring that a student can't enroll in the same course more than once!

By the end of this session, you'll be well-versed in using sets for such tasks. Let’s dive in!

Here are the methods we need to implement in our enrollment system:

• void enroll(const std::string& student, const std::string& course);: This method adds a student to a course. If the student is already enrolled, it does nothing.
• bool unenroll(const std::string& student, const std::string& course);: This method removes a student from a course. It returns true if the student was enrolled and has now been removed. Otherwise, it returns false. If after unenrolling the student, the course becomes empty (no one is enrolled there), remove the course as well.
• bool is_enrolled(const std::string& student, const std::string& course);: This method checks if a student is enrolled in a course. It returns true if the student is enrolled and false otherwise.
• std::vector<std::string> list_students(const std::string& course);: This method returns a list of all students enrolled in a given course. If no students are enrolled, it returns an empty vector.

Let's look at how to implement each of these methods step-by-step.

We'll start by defining our class and then add each method one by one.

First, we define our EnrollmentSystem class:

This code initializes an EnrollmentSystem class with an unordered map named enrollments that maps courses to unordered sets of students.

Next, we implement the enroll method:

Here, the enroll method uses the insert method to add the student to the set of students for the specified course.

Let's move on to the unenroll method:

This method first checks whether the course and student exist in the enrollments. If they do, it removes the student from the course. If the course set becomes empty after removal, it deletes the course from the map. The method returns true if the student was successfully unenrolled and false otherwise.

Next, let's implement the is_enrolled method:

This method checks whether both the course and the student exist in the enrollments map, returning true if they do and false otherwise.

Finally, let's implement the list_students method:

This method returns a vector of students enrolled in the given course. If the course is not in the enrollments map, it returns an empty vector.

In today's lesson, we learned how to manage student enrollments using C++ unordered sets and maps. We implemented methods to enroll and unenroll students, check enrollments, and list students in a course. This task provided a practical way to reinforce your understanding of sets and maps in C++.

I encourage you to move on to the practice to undertake similar challenges to deepen your understanding. Keep experimenting and honing your skills. Happy coding!