I had never done anything with exceptions before so I found this one quite confusing, especially since it doesn't specify that you are supposed to create a new class. For anyone else wondering,

BadLengthException(n)

is a class constructor, not just an external function. The variable "e" is an object of this class, and "what()" is a function inside the class.

For anyone else unfamiliar with exceptions, an exception consists of a "throw" and a "catch". A "throw" is followed by an object or variable. When the "throw" is called, the program will jump to the "catch" using said object/variable as an input to the catch. In this case, the throw constructs a new object of class "BadLengthException", which is then used as the input to the catch (this is labelled as "e").

The code that I used is as follows:

/* Define the exception here */
class BadLengthException {
    private: 
        int n;
    public:
        BadLengthException(int errornumber) {
            n = errornumber;
        }
    
        int what() {
            return n;
        }
};