Error: passing xxx as 'this' argument of xxx discards qualifiers?

The objects in the std::set are stored as const StudentT . So when you try to call getId() with the const object the compiler detects a problem, namely you're calling a non-const member function on const object which is not allowed because non-const member functions make NO PROMISE not to modify the object; so the compiler is going to make a safe assumption that getId() might attempt to modify the object but at the same time, it also notices that the object is const; so any attempt to modify the const object should be an error. Hence compiler generates error message.

Member functions that do not modify the class instance should be declared as const.

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