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class Animal {
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public:
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    static num; //static int num;
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    Animal(int _a);
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    int age;
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    virtual void noise()=0;
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};
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//Can not give defination to a virtual function
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Animal::Animal(int _a) :age(_a){num++;}
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void Animal::noise(){
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    cout<<"animal voice"<<endl;
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}
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class Dog : public Animal {
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public:
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    Dog();
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    void noise() override;
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    voideat(int bone);
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};
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Dog::Dog(): Animal(1){num++;}
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void Dog::noise() {
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    cout<<"wang"<<endl;
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}
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voidDog::eat(int bone){
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    cout<<bone<<endl;
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}
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class Cat: public Animal{
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public:
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    Cat();
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    void noise() override;
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    voiddrink();
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};
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//Need to define constructor if defined in public:Cat();
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void Cat::noise() {
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    cout<<"miao"<<endl;
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}
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voidCat::drink(){
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    cout<<"drink water"<<endl;
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}
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void bark(Animal *a){
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    a->noise();
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}
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int main()
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{
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    Animal a(0); //pure virtual class
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    Animal * pa= new Animal(1); //same
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    Animal * pd= new Dog(10);
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    pd->noise(); pd->eat(10);//base class pointer cannot point to function only belongs to subclass
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    Animal * pc= new Cat();
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    pc->noise(); pc->drink();
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    Dog d(10); Cat c();
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    d.noise(); d.eat();
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    c.noise(); c.drink();
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    bark(d); bark(c);//bark(&d); bark(&c); or use "->"
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    cout<<d.num<<endl;
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    cout<<Animal::num<<endl;
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    return 0;
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}