必须为多态基类声明 virtual 析构函数

class Animal {
public:
	Animal()
	{
		cout << "Animal constructing" << endl;
	}
	virtual ~Animal() {
		cout << "Animal destructing" << endl;
	}
	virtual void MakeSound() const = 0 {}

};

class Cat :public Animal {
public:
	Cat() {
		cout << "Cat constructing" << endl;
	}
	~Cat() {
		cout << "Cat destructing" << endl;
	}
	virtual void MakeSound() const{
		cout << "Cat make sound." << endl;
		Animal::MakeSound();
	}
};
int main() {
	Animal * pcat = new Cat;
	pcat->MakeSound();
	delete pcat;//如果基类的析构函数里没有使用 virtual ，这里会导致Cat的析构函数无法执行。
	return 0;
}