basket.h

C++Primer 经典之作

/*
 * This file contains code from "C++ Primer, Fourth Edition", by Stanley B.
 * Lippman, Jose Lajoie, and Barbara E. Moo, and is covered under the
 * copyright and warranty notices given in that book:
 * 
 * "Copyright (c) 2005 by Objectwrite, Inc., Jose Lajoie, and Barbara E. Moo."
 * 
 * 
 * "The authors and publisher have taken care in the preparation of this book,
 * but make no expressed or implied warranty of any kind and assume no
 * responsibility for errors or omissions. No liability is assumed for
 * incidental or consequential damages in connection with or arising out of the
 * use of the information or programs contained herein."
 * 
 * Permission is granted for this code to be used for educational purposes in
 * association with the book, given proper citation if and when posted or
 * reproduced.Any commercial use of this code requires the explicit written
 * permission of the publisher, Addison-Wesley Professional, a division of
 * Pearson Education, Inc. Send your request for permission, stating clearly
 * what code you would like to use, and in what specific way, to the following
 * address: 
 * 
 * 	Pearson Education, Inc.
 * 	Rights and Contracts Department
 * 	75 Arlington Street, Suite 300
 * 	Boston, MA 02216
 * 	Fax: [phone]
*/ 

#ifndef BASKET_H
#define BASKET_H

#include <iostream>
#include <string>
#include <set>
#include <map>
#include <utility>
#include <cstddef>
#include <stdexcept>

// Item sold at an undiscounted price
// derived classes will define various discount strategies
class Item_base {
friend std::istream& operator>>(std::istream&, Item_base&);
friend std::ostream& operator<<(std::ostream&, const Item_base&);
public:
    virtual Item_base* clone() const 
                       { return new Item_base(*this); }
public:
    Item_base(const std::string &book = "", 
              double sales_price = 0.0):
                     isbn(book), price(sales_price) { }

    std::string book() const { return isbn; }

    // returns total sales price for a specified number of items
    // derived classes will override and apply different discount algorithms
    virtual double net_price(std::size_t n) const 
               { return n * price; }

    // no work, but virtual destructor needed 
    // if base pointer that points to a derived object is ever deleted
    virtual ~Item_base() { } 
private:
    std::string isbn;   // identifier for the item
protected:
    double price;       // normal, undiscounted price

};

class Sales_item;

bool compare(const Sales_item &lhs, const Sales_item &rhs);

// discount kicks in when a specified number of copies of same book are sold
// the discount is expressed as a fraction used to reduce the normal price
class Bulk_item : public Item_base {
public:
    std::pair<size_t, double> discount_policy() const
        { return std::make_pair(min_qty, discount); }
    // other members as before
    Bulk_item* clone() const 
        { return new Bulk_item(*this); }
    Bulk_item(): min_qty(0), discount(0.0) { }
    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) { }

    // redefines base version so as to implement bulk purchase discount policy
    double net_price(std::size_t) const;
private:
    std::size_t min_qty;   // minimum purchase for discount to apply
    double discount;       // fractional discount to apply
};

// discount (a fraction off list) for only a specified number of copies, 
// additional copies sold at standard price
class Lim_item : public Item_base {
public:
    Lim_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), 
                 max_qty(qty), discount(disc_rate) { }

    // redefines base version so as to implement limited discount policy
    double net_price(std::size_t) const;
private:
    std::size_t max_qty;   // maximum number sold at discount
    double discount;       // fractional discount to apply
public:
    Lim_item* clone() const { return new Lim_item(*this); }
    std::pair<size_t, double> discount_policy() const
        { return std::make_pair(max_qty, discount); }
};

// use counted handle class for the Item_base hierarchy 
class Sales_item {
friend class Basket;
public:
    // default constructor: unbound handle
    Sales_item(): p(0), use(new std::size_t(1)) { }

    // attaches a handle to a copy of the Item_base object
    Sales_item(const Item_base&); 

    // copy control members to manage the use count and pointers
    Sales_item(const Sales_item &i): 
                      p(i.p), use(i.use) { ++*use; }
    ~Sales_item() { decr_use(); }
    Sales_item& operator=(const Sales_item&);

    // member access operators
    const Item_base *operator->() const { if (p) return p; 
        else throw std::logic_error("unbound Sales_item"); }
    const Item_base &operator*() const { if (p) return *p; 
        else throw std::logic_error("unbound Sales_item"); }
private:
    Item_base *p;         // pointer to shared item
    std::size_t *use;     // pointer to shared use count

    // called by both destructor and assignment operator to free pointers
    void decr_use() 
         { if (--*use == 0) { delete p; delete use; } }
};


// holds items being purchased
class Basket {
    // type of the comparison function used to order the multiset
    typedef bool (*Comp)(const Sales_item&, const Sales_item&);
public:
    // make it easier to type the type of our set
    typedef std::multiset<Sales_item, Comp> set_type;

    // typedefs modeled after corresponding container types
    typedef set_type::size_type size_type;
    typedef set_type::const_iterator const_iter;

    void display(std::ostream&) const;

    // workaround MS compiler bug: must explicitly pass function address
    Basket(): items(&compare) { }  // initialze the comparator 
    void add_item(const Sales_item &item) 
                        { items.insert(item); }
    size_type size(const Sales_item &i) const
                         { return items.count(i); }
    double total() const;  // sum of net prices for all items in the basket
private:
    std::multiset<Sales_item, Comp> items;
};

inline
Sales_item::Sales_item(const Item_base &item):
            p(item.clone()), use(new std::size_t(1)) { }

// compare defines item ordering for the multiset in Basket
inline bool 
compare(const Sales_item &lhs, const Sales_item &rhs) 
{
    return lhs->book() < rhs->book(); 
} 
#endif