异质链表部分编译不通过,该如何处理

class salary_base
{
private:
    int work_hour; //加班时间
public:
    salary_base(){work_hour=0;}
    int get_hour(){return work_hour;}
    void set_hour(int hour){work_hour=hour;}
    virtual int get_salary()=0;
};

class SeniorEngineer:public salary_base
{
public:
    int get_salary();
};

int SeniorEngineer::get_salary()
{
    return (8000+80*get_hour());
}

class LowEngineer:public salary_base
{
public:
    int get_salary();
};
int LowEngineer::get_salary()
{
    return (5000+50*get_hour());
}

class Engineer:public salary_base
{
public:
    int get_salary();
};
int Engineer::get_salary()
{
    return (4000+40*get_hour());
}

typedef struct salary_node
{
    salary_base *p;
    salary_node *next;
}salary_node;
int main()
{
    int a,b,c;
    cin>>a>>b>>c;
    SeniorEngineer obj1;
    LowEngineer obj2;
    Engineer obj3;
    obj1.set_hour(a);
    obj2.set_hour(b);
    obj3.set_hour(c);
    cout<<obj1.get_salary()<<endl;
    cout<<obj2.get_salary()<<endl;
    cout<<obj3.get_salary()<<endl;
    salary_node *head=NULL;
    salary_node *pr,*s=NULL;
    s = new salary_node;
    s->p=new SeniorEngineer();
    s->p->set_hour(a);
    head=s;
    pr=head;
    s = new salary_node;
    s->p=new LowEngineer();
    s->p->set_hour(b);
    pr->next=s;
    pr=s;
    s = new salary_node;
    s->p=new Engineer();
    s->p->set_hour(c);
    pr->next=s;
    pr=s;
    pr->next=NULL;
    int sum=0;
    int i=0;
    while(head)
    {
        sum+=head->p->get_salary();
        head=head->next;
        i++;
    }
    cout<<sum/i<<endl;
    return 0;
}