句柄类的简单运用

C++中尽量少用指针，但是，有的时候，我们又不得不使用指针。

 

因此，当我们使用指针的时候我们可以考虑采用智能指针方式。(可以有效的防止内存泄露和访问不存在的对象)

 

智能指针常见又用二种

 

一。引用计数（定义一个计数类来进行管理）

二。句柄类（对我们需要用的指针来定义一个类进行包装他）

 

以下代码是我学习句柄类打的代码:

 

#include <iostream> #include <string> #include <cstddef> #include <vector> #include <algorithm> #include <numeric> #include <set> class Item_base { public: Item_base(const std::string &book = "", double dpr = 0.0): isbn(book), price(dpr) { } std::string Book() const; virtual double net_price(std::size_t n) const; virtual Item_base* clone() const; virtual ~Item_base() { } virtual void debug() const; friend std::ostream& operator<<(std::ostream &os, const Item_base &item); protected: double price; private: std::string isbn; }; std::string Item_base::Book() const { return isbn; } double Item_base::net_price(std::size_t n) const { return price * n; } Item_base* Item_base::clone() const { return new Item_base(*this); } void Item_base::debug() const { std::cout << "debug Item_base:isbn" <<isbn << "/tprice:" << price << std::endl; } class Disc_item: public Item_base { public: Disc_item(const std::string &book, double dpr, unsigned count, double dis): Item_base(book, dpr), unCount(count), discount(dis) { } virtual double net_price(std::size_t n) const = 0; virtual Disc_item* clone() const = 0; virtual ~Disc_item() { } protected: unsigned unCount; double discount; }; class Bulk_item: public Disc_item { public: Bulk_item(const std::string &book = "", double dpr = 0.0, unsigned count = 0, double dis = 0.0): Disc_item(book, dpr, count, dis) { } virtual double net_price(std::size_t n) const; virtual Bulk_item* clone() const; virtual ~Bulk_item() { } virtual void debug() const; }; Bulk_item* Bulk_item::clone() const { return new Bulk_item(*this); } double Bulk_item::net_price(std::size_t n) const { if (n > unCount) { return n * price * discount; } else { return Item_base::net_price(n); } } void Bulk_item::debug() const { std::cout << "debug Bult_item:isbn" << Book() << "price:/t" << price << "/tmin count:" << unCount << "/tdiscount" << discount << std::endl; } class Limit_item: public Disc_item { public: Limit_item(const std::string &book = "", double dpr = 0.0, unsigned count = 0, double dis = 0.0): Disc_item(book, dpr, count, dis) { } virtual double net_price(std::size_t n) const; virtual Limit_item* clone() const; virtual ~Limit_item() { } virtual void debug() const; }; Limit_item* Limit_item::clone() const { return new Limit_item(*this); } double Limit_item::net_price(std::size_t n) const { if (n <= unCount) { return n * price * discount; } else { return unCount * price * discount + (n - unCount) * price; } } void Limit_item::debug() const { std::cout << "debug Limit_item:isbn" << Book() << "/tprice" << price << "/tmax count" << unCount << "/tdiscount:" << discount << std::endl; } //句柄类 class Sales_item { public: Sales_item(): p(0), use(new unsigned(1)) { } Sales_item(const Item_base &item): p(item.clone()), use(new unsigned(1)) { } Sales_item(const Sales_item &item): p(item.p), use(item.use) { ++*use; } Sales_item& operator=(const Sales_item &item); ~Sales_item(); Item_base& operator*() { return *p; } const Item_base& operator*() const { return *p; } Item_base* operator->() { return p; } const Item_base* operator->() const { return p; } private: void dp_use() { if (--use == 0) { delete p; delete use; } } private: Item_base *p; unsigned *use; }; Sales_item& Sales_item::operator =(const Sales_item &item) { ++*use; dp_use(); p = item.p; use = item.use; return *this; } Sales_item::~Sales_item() { dp_use(); } std::ostream& operator<<(std::ostream &os, const Item_base &item) { std::cout << item.isbn << "/t" << item.net_price(10) << std::endl; return os; } bool compare(const Sales_item &item1, const Sales_item &item2) { return item1->Book() < item2->Book(); } //简单的购物篮 class Tasket { typedef bool (*Cmp)(const Sales_item &item1, const Sales_item &item2); public: Tasket(): books(compare) {} void add_item(const Sales_item &item); double total() const; private: std::multiset<Sales_item, Cmp> books; }; void Tasket::add_item(const Sales_item &item) { books.insert(item); } double Tasket::total() const { double sum = 0.0; for (std::multiset<Sales_item, Cmp>::const_iterator iter = books.begin(); iter != books.end(); iter = books.upper_bound(*iter)) { sum += (*iter)->net_price(books.count(*iter)); } return sum; } int main() { Tasket task; //购物篮 task.add_item(Sales_item(Bulk_item("11", 10, 2, 0.9))); task.add_item(Sales_item(Bulk_item("11", 10, 2, 0.9))); task.add_item(Sales_item(Bulk_item("11", 10, 2, 0.9))); std::cout << task.total() << std::endl; system("pause"); return 0; }