026:编程填空：统计动物数量

题目：

http://cxsjsxmooc.openjudge.cn/2021t3springall2/026/

分析：

这题考的是静态成员和虚函数以及派生类成员覆盖基类成员的特性。本题的要点是：

1. 如果要在类外访问静态成员，则必须把这个静态成员放在public中。
2. 当派生类成员中有覆盖基类成员（同名）时，在派生类中访问成员时要加上类名前缀。
3. 虚拟化析构函数，实现使用基类指针下析构派生类。
4. 静态变量一定要记得初始化！！！在全局中作初始化即可。

参考代码：

#include <iostream>
using namespace std;
// 在此处补充你的代码
class Animal{
public:
    static int number;
    Animal() {}
    virtual ~Animal(){
        number--;
    }
};

class Dog:public Animal{
public:
    static int number;
    Dog(){
        Animal::number++; //wo need to point out that which "number" is.
        Dog::number++;
    }
     virtual ~Dog(){
        number--;
    }
};

class Cat:public Animal{
public:
    static int number;
    Cat(){
        Animal::number++;
        Cat::number++;
    }
    virtual ~Cat(){
        number--;
    }
};

//never forget to initiate the variable !!!
int Animal::number = 0;
int Dog::number = 0;
int Cat::number = 0;

void print() {
    cout << Animal::number << " animals in the zoo, " << Dog::number << " of them are dogs, " << Cat::number << " of them are cats" << endl;
}

int main() {
    print();
    Dog d1, d2; //class Dog
    Cat c1; //class Cat
    print();
    Dog* d3 = new Dog();
    Animal* c2 = new Cat; //class Animal, class Animal is the basic of class Dog and Cat
    Cat* c3 = new Cat; //why is it different from new Dog() ?
    print();
    delete c3;
    delete c2;
    delete d3;
    print();
}