虚析构函数，纯虚析构函数~~~

1.基类的虚构函数必须声明为虚的。

class Rodent {
    public:
        virtual void eat() {
            cout << "Rodent eat...." << endl;
        }

        virtual void run() {
            cout << "Rodent run..." << endl;
        }

        virtual ~Rodent() = 0;

    };
    Rodent::~Rodent() {
        cout << "~Rodent..." << endl;
    }
    class Mouse :public Rodent {
    public:
        void eat() {
            cout << "Mouse eat..." << endl;
        }
        void run() {
            cout << "Mouse run..." << endl;
        }
        ~Mouse() {
            cout << "~Mouse..." << endl;
        }
    };

    class Gerbil :public Rodent {
    public:
        void eat() {
            cout << "Gerbil eat..." << endl;
        }
        void run() {
            cout << "Gerbil run..." << endl;
        }
        ~Gerbil() {
            cout << "~Gerbil..." << endl;
        }
    };

    class Hamster :public Rodent {
    public:
        void eat() {
            cout << "Hamster eat..." << endl;
        }
        void run() {
            cout << "Hamster run..." << endl;
        }
        ~Hamster() {
            cout << "~Hamster..." << endl;
        }
    };

    class BlueHamster :public Hamster {

    public:
        ~BlueHamster(){
        }
    };

2.纯虚析构函数必须有定义。没有定义的话就有：error LNK2019: 无法解析的外部符号 “public: virtual __thiscall A::~A(void)” (??1A@@UAE@XZ)，