名字在编译时识别

class Disc_item : public Item_base {
     public:
         std::pair<size_t, double> discount_policy() const
             { return std::make_pair(quantity, discount); }
         // other members as before
     };

We can access discount_policy only through an object, pointer, or reference of type Disc_item or a class derived from Disc_item:(只有Disc_item和它的派生类的对象，指针和引用可以访问discount_policy()函数)

     Bulk_item bulk;
     Bulk_item *bulkP = &bulk;  // ok: static and dynamic types are the same
     Item_base *itemP = &bulk;  // ok: static and dynamic types differ
     bulkP->discount_policy();  // ok: bulkP has type Bulk_item*
     itemP->discount_policy();  // error: itemP has type Item_base*

The call through itemP is an error because a pointer (reference or object) to a base type can access only the base parts of an object and there is no discount_policy member defined in the base class.(itemP是一个Item_base类型指针，它只能访问Item_base部分，不能访问Disc_item特有的部分)