智能指针与句柄详解（二）

前言
智能指针与句柄详解（一）中讲了句柄类的实现，本章将用C++模板来实现句柄类。

#ifndef _Sales_item_h
#define _Sales_item_h

#include "item_base.h"
#include <iostream>
using namespace std;
template <class T>
class Sales_item 
{
public:
    Sales_item();
    Sales_item(const Sales_item& sales_item);
    Sales_item(const T& item_base);
    ~Sales_item();

    Sales_item& operator=(const Sales_item& sitem);
    const T* operator->() const;
    const  T& operator*() const; 

private:
    T *m_item_base;
    int *m_useCount;

    void decr_use();
};

template <class T>
Sales_item<T>::Sales_item()
    :m_item_base(NULL)
    ,m_useCount(new int(1))
{
    cout<<"Sales_item constructor"<<endl;
}

template <class T>
Sales_item<T>::Sales_item(const Sales_item& sales_item)
    :m_item_base(sales_item.m_item_base)
    ,m_useCount(sales_item.m_useCount)
{
    cout<<"Sales_item copy constructor"<<endl;
    ++*m_useCount;
}

template <class T>
Sales_item<T>::Sales_item(const T& item_base)
    :m_item_base(item_base.clone())
    ,m_useCount(new int(1))
{
    cout<<"Sales_item item_base constructor m_useCount = "<<*m_useCount<<endl;
}

template <class T>
Sales_item<T>& Sales_item<T>::operator=(const Sales_item& sitem)
{
    bool isSame = this->m_item_base == sitem.m_item_base;
    cout<<"Sales_item assignment isSame = "<<isSame<<endl;
    if (!isSame) {
        ++*sitem.m_useCount;
        decr_use();
        m_item_base = sitem.m_item_base;
        m_useCount = sitem.m_useCount;
    }
    return *this;
}

template <class T>
Sales_item<T>::~Sales_item()
{
    cout<<"Sales_item destruct"<<endl;
    decr_use();
}

template <class T>
const T* Sales_item<T>::operator->() const
{
    cout<<"Sales_item operator->"<<endl;
    if (m_item_base) {
        return m_item_base;
    }
    else throw ("unbound Sales_item");
}

template <class T>
const T& Sales_item<T>::operator*() const
{
    cout<<"Sales_item operator*"<<endl;
    if (m_item_base) {
        return *m_item_base;
    }
    else throw ("unbound Sales_item");
}

template <class T>
void Sales_item<T>::decr_use()
{
    cout<<"Sales_item decr_use m_useCount = "<<*m_useCount<<endl;
    if (-- *m_useCount == 0) {
        delete m_item_base;
        delete m_useCount;
    }
}

#endif

输出结果与智能指针与句柄详解（一）一致。

总结：
这里的类模板其实主要多了“template ”关键字，当然类成员函数也定义在了.h文件中。想了解具体情况可以参考《C++ Primer中文版（第四版）》中第十六章 模板与泛型编程。