学步园

对于下面的程序：

#include <iostream>
#include <string>
using std::string;
//using std::size_t;
using std::endl;
using std::cout;
class Item_base{
    public:
        Item_base(const string &book="",double sales_price=0.0):
            isbn(book),price(sales_price){
                cout<<"base:constructor"<<endl;
            }
        string book()const {return isbn;}
        virtual double net_price(size_t n)const
        {
            return n*price;
        }
        Item_base(const Item_base&ib):isbn(ib.isbn),price(ib.price){
            cout<<"base:copy constructor"<<endl;
        }
        Item_base& operator=(const Item_base&rhs){
            if(this != &rhs){
                isbn=rhs.isbn;
                price=rhs.price;
            }
            cout<<"base:assign"<<endl;
            return *this;
        }
        virtual ~Item_base(){
            cout<<"base:destructor"<<endl;
        }
    private:
        string isbn;
    protected:
        double price;
};
class Bulk_item:public Item_base {
    public:
        Bulk_item(const string &book="",double sales_price=0.0,size_t qty=0,double disc_rate=0.0):
            Item_base(book,sales_price),min_qty(qty),discount(disc_rate){
                cout<<"bulk:constructor"<<endl;
            }
        double net_price(size_t cnt)const{
            if(cnt>=min_qty)
                return cnt*(1-discount)*price;
            else
                return cnt*price;
        }
        Bulk_item(const Bulk_item& bi):Item_base(bi),min_qty(bi.min_qty),discount(bi.discount){
                cout<<"bulk:copy constructor"<<endl;
        }
        Bulk_item& operator=(const Bulk_item&rhs)
        {
            if(this !=&rhs){
                Item_base::operator=(rhs);
                min_qty=rhs.min_qty;
                discount=rhs.discount;
            }
            cout<<"bulk:assign"<<endl;
            
            return *this;
        }
        ~Bulk_item()
        {
            cout<<"bulk:destructor"<<endl;
        }
    private:
        size_t min_qty;
        double discount;
};
class Lds_item:public Item_base {
    public:
        Lds_item(const string &book="",double sales_price=0.0,size_t qty=0,double disc_rate=0.0):
            Item_base(book,sales_price),max_qty(qty),discount(disc_rate){
                cout<<"lds:constructor"<<endl;
            }
        double net_price(size_t cnt)const{
            if(cnt<=max_qty)
                return cnt*(1-discount)*price;
            else
                return cnt*price-max_qty*discount*price;
        }
        Lds_item(const Lds_item& li):Item_base(li),max_qty(li.max_qty),discount(li.discount){
            cout<<"lds:copy constructor"<<endl;
        }
        Lds_item& operator=(const Lds_item&rhs){
            if(this !=&rhs){
                Item_base::operator=(rhs);
                max_qty=rhs.max_qty;
                discount=rhs.discount;
            }
            cout<<"lds:assign"<<endl;
            return *this;
        }
        ~Lds_item(){
            cout<<"lds:destructor"<<endl;
        }
    private:
        size_t max_qty;
        double discount;
};
void func1(Item_base obj)
{
    cout<<"func1"<<endl;
}
void func2(Item_base& obj){
    cout<<"func2"<<endl;
}
Item_base func3(){
    cout<<"func3"<<endl;
    Item_base obj;
    return obj;
}
Bulk_item func4(){
    cout<<"func4"<<endl;
    Bulk_item obj;
    return obj;
}
int main(){
    Item_base obj;
    cout<<"01========"<<endl;
    func1(obj);
    cout<<"02========"<<endl;
    func2(obj);
    cout<<"04========"<<endl;
    obj=func3();
    cout<<"05========"<<endl;
    Item_base *p=new Item_base;
    cout<<"06========"<<endl;
    delete p;
    cout<<"07========"<<endl;
    Bulk_item bobj;
    cout<<"08========"<<endl;
    func1(bobj);
    cout<<"09========"<<endl;
    func2(bobj);
    cout<<"10========"<<endl;
    bobj=func4();
    cout<<"11========"<<endl;
    Bulk_item *q=new Bulk_item;
    cout<<"12========"<<endl;
    delete q;
    cout<<"13========"<<endl;
    return 0;
    cout<<"14========"<<endl;
}

这是C++primer 第4版的习题15.20，答案给出的解释是：