c++ primer第15章这几个例子中的构造函数形式不太理解

//向基类构造函数传递实参p491  
class Bulk_item : public Item_base{
public:
    Bulk_item(const std::string& book,double sales_price,std::size_t qty = 0,double disc_rate = 0.0):
      Item_base(book,sales_price),min_qty(qty),discount(disc_rate){}
};


//在派生类构造函数中使用默认实参p491
class Bulk_item : public Item_base{
public:
    Bulk_item(const std::string& book,double sales_price = 0.0,std::size_t qty = 0,double disc_rate = 0.0):
      Item_base(book,sales_price),min_qty(qty),discount(disc_rate){}
};


//定义Disc_item类p492
class Disc_item : public Item_base{
public:
    Disc_item(const std::string& book = "", double sales_price = 0.0, std::size_t qty = 0, double disc_rate = 0.0):
      Item_base(book, sales_price),quantity(qty), discount(disc_rate){}
protected:
    std::size_t quantity;
    double discount;
};


//重构
//重新实现Bulk_item以继承Disc_itemp493
class Bulk_item : public Disc_item{
public:
    Bulk_item(const std::string& book = "", double sales_price = 0.0,std::size_t qty = 0,double disc_rate = 0.0):
      Disc_item(book,sales_price,qty,disc_rate){}//不太懂

      double net_price(std::size_t) const;
};