I kept it as a struct, since that is what was provided. For those that may not already know, a struct is essentially the same as a class - only that it is 'public' by default, opposed to classes which are 'private' by default.

solution

struct BadLengthException{
    int n;
    BadLengthException(int n){
        this->n = n;
    }
    int what(){
        return n;
    }
};