#include <iostream>
#include <string>

using std::string, std::cout, std::cin, std::endl;

class Student
{
protected:
    string name;
    int age;

public:
    Student(string _name, int _age) : name{_name}, age{_age} {}
    int schooling()
    {
        return 0; // 空泛的学生类，缺乏收费依据，直接返回0
    }
};

class MiddleStudent : public Student
{
    int books;

public:
    MiddleStudent(string _name, int _age, int _books)
        : Student(_name, _age), books{_books} {}
    int schooling()
    {
        return books * 20; // 中学生按20元单价收书本费
    }
};

class CollegeStudent : public Student
{
    int credit;

public:
    CollegeStudent(string _name, int _age, int _credit)
        : Student(_name, _age), credit{_credit} {}
    int schooling()
    {
        return credit * 120; // 大学生按120单价收学分费
    }
};

class GraduateStudent : public Student
{
    int credit;

public:
    GraduateStudent(string _name, int _age, int _credit)
        : Student(_name, _age), credit{_credit} {}
    int schooling()
    {
        return credit * 200; // 研究生按200单价收学分费
    }
};

int get_total(Student s[], int len)
{
    int total{};
    for (int i{0}; i < len; ++i)
        total += s[i].schooling();
    return total;
}

int main()
{
    MiddleStudent ms{"张三", 15, 20};
    CollegeStudent cs{"张二", 18, 150};
    GraduateStudent gs{"张大", 22, 40};
    Student s[]{ms, cs, gs};
    std::cout << get_total(s, 3) << '\n';
    // 本程序失败了，输出为0，为什么？
}