一个具有对象计数功能的基类

　　参考<<More Effective C++>>一书中的条款26："限制某个class所能产生的对象数量"，参照"一个用来计算对象个数的基类"中的代码，简单的将里面的代码实现。一个具有对象计数功能的基类，简单来说，就是完成一个base class，作为对象计数之用，并让诸如Printer之类的class继承它。更多内容请参考<<More Effective C++>>一书中的条款26，那里有很详细的描述，这里不再讲解。下面来看看代码是如何实现的。

　　代码如下：

#include "stdafx.h"
#include <iostream>

using namespace std;

//一个具有对象计数功能的基类
template<class BeingCounted>
class Counted
{
public:
    class TooManyObjects{};   //定义一个异常类
    static int objectCount()
    {
        return numObjects;
    }
protected:
    Counted();
    Counted(const Counted& rhs);
    ~Counted()
    {
        cout<<"销毁第 "<<numObjects<<" 个实例"<<endl;
        --numObjects;
    }
private:
    static int numObjects;           //当前产生实例的个数
    static const size_t maxObjects;  //可以产生实例的最多个数

    void init();
};

template<class BeingCounted>
Counted<BeingCounted>::Counted()
{
    init();
}

template<class BeingCounted>
Counted<BeingCounted>::Counted(const Counted& rhs)
{
    init();
}

template<class BeingCounted>
void Counted<BeingCounted>::init()
{
    if (numObjects >= maxObjects)  //产生的实例大于最大值就抛出异常
    {
        throw TooManyObjects();
    }
    cout<<"构造第 "<<numObjects + 1<<" 个实例"<<endl;
    ++numObjects;
}

//继承自具有对象计数功能的基类
class Printer : private Counted<Printer>
{
public:
    static  Printer* makePrinter();
    static  Printer* makePrinter(const Printer& rhs);

    ~Printer();
    using Counted<Printer>::objectCount;      //使基类的objectCount()在子类可见
    using Counted<Printer>::TooManyObjects;   //使基类的TooManyObjects在子类可见
private:
    Printer();
    Printer(const Printer& rhs);
};

template<class BeingCounted>
int Counted<BeingCounted>::numObjects;

const size_t Counted<Printer>::maxObjects = 5;   //最多5个

Printer::Printer()
{
}

Printer::~Printer()
{
}

Printer::Printer(const Printer& rhs)
{
}

Printer* Printer::makePrinter()
{
    return new Printer;
}

Printer* Printer::makePrinter(const Printer& rhs)
{
    return new Printer(rhs);
}

int _tmain(int argc, _TCHAR* argv[])
{
    Printer *p1 = Printer::makePrinter();
    Printer *p2 = Printer::makePrinter();
    Printer *p3 = Printer::makePrinter(*p1);
    Printer *p4 = Printer::makePrinter(*p2);
    Printer *p5 = Printer::makePrinter();

    delete p4;
    delete p5;
    p4 = Printer::makePrinter();
    p5 = Printer::makePrinter();
    try
    {
        p1 = Printer::makePrinter(); //Printer产生的实例大于5，则抛出异常
    }
    catch (Printer::TooManyObjects)
    {
        cout<<"TooManyObjects"<<endl;
    }

    delete p1;
    delete p2;
    delete p3;
    delete p4;
    delete p5;
    cout<<"剩余实例为："<<Printer::objectCount()<<endl;

    return 0;
}

　　运行界面如下：