C++ 句柄类的原理以及设计

multimap<Base> basket;
Base base;
Derived derive;
basket.insert(base);	//ok,add copy of base;
basket.insert(derive);	//ok,but derive sliced down to its base part.

//Base.h
#pragma once
class Base
{
public:
	Base(void);
	virtual ~Base(void);
	virtual Base* clone() const;
	virtual void fine() const;
private:
	int mb;
};

//Base.cpp
#include "Base.h"
#include <iostream>
using namespace std;


Base::Base(void):mb(12)
{
}


Base::~Base(void)
{
}


Base* Base::clone() const
{
	return new Base(*this);
}


void Base::fine() const
{
	cout<<"Base fine function"<<endl;
}

//Derive.h
#pragma once
#include "base.h"
class Derive :
	public Base
{
public:
	Derive(void);
	virtual ~Derive(void);
	virtual Derive* clone() const;
	virtual void fine() const;
private:
	int md;
};

//Derive.cpp
#include "Derive.h"
#include <iostream>
using namespace std;


Derive::Derive(void):Base(),md(13)
{
}


Derive::~Derive(void)
{


}


Derive* Derive::clone() const
{
	return new Derive(*this);
}


void Derive::fine() const
{
	cout<<"Derive fine function"<<endl;
}

//Handles.h 
#pragma once
#include "Base.h"
#include <iostream>
class Handles
{
public:
	Handles(void);
	Handles(const Base&);
	Handles(const Handles& h);
	~Handles(void);


	const Base* operator->()const;
	const Base& operator*()const;
	Handles& operator=(const Handles&);


	
private:
	Base*	p;
	std::size_t*	use;
	void dec_use()
	{
		if(--*use == 0)
		{
			delete p;
			delete use;
			std::cout<<"delete resource"<<std::endl;
		}
	}


};

//Handle.cpp 
#include "Handles.h"


Handles::Handles(void):p(NULL),use(new size_t(1))
{
}


Handles::Handles(const Handles& h):p(h.p),use(h.use)
{
	++*use;
}


Handles::Handles(const Base& item):p(item.clone()),use(new std::size_t(1))
{
}
const Base& Handles::operator*()const
{
		if(p) 
			return *p;
		else
			throw std::logic_error("unbounded Handles");
}


const Base* Handles::operator->()const
{
		if(p) 
			return p;
		else
			throw std::logic_error("unbounded Handles");
}


Handles& Handles::operator=(const Handles& h)
{
	++*h.use;
	dec_use();
	p = h.p;
	use = h.use;
	return *this;
}


Handles::~Handles()
{
	dec_use();
}

//main.cpp
#include <iostream>
#include "Handles.h"
#include "Derive.h"
#include <vector>
using namespace std;
void main()
{
	vector<Handles> mb;
	Base b;
	Derive d;
	mb.push_back(Handles(b));
	mb.push_back(Handles(d));
	mb[0]->fine();
	mb[1]->fine();
	system("pause");
}