c ++中基类中受保护字段的问题

class Derived : public Base {
public:
    using Base::protected_field;
};

class Derived : public Base {
public:
    int get_protected_field() const {
        return protected_field;
    }
};

class Derived : public Base {
public:
    void set_protected_field(int value) {
        protected_field = value;
    }
};