设计类继承

题目链接：https://www.nowcoder.com/practice/699ba050e2704591ae3e62401a856b0e?tpId=107&&tqId=33421&rp=1&ru=/ta/beginner-programmers&qru=/ta/beginner-programmers/question-ranking
看了那么多答案，这个应该是最标准的，既用到了初始化列表很高端的写法，又实现了虚函数重写，实现了多态

#include<iostream>
using namespace std;
class shape{
private:
    int x;
    int y;
public:
    shape() : x(0), y(0){ }
    shape(int a, int b) : x(a), y(b){ }
    virtual void GetArea() = 0;
};
class Circle : public shape{
private:
    double radius;
    const static double PI;
public:
    Circle() : radius(0) { }
    Circle(int r) : radius(r) { }
    virtual void GetArea(){
        double S;
        S = PI * radius * radius;
        cout << S << endl;
    }
};
const double Circle::PI = 3.14;
class Retangle : public shape{
private:
    int w;
    int h;
public:
    Retangle() : w(0), h(0){ }
    Retangle(int x, int y) : w(x), h(y){ }
    virtual void GetArea(){
        double S;
        S = w * h;
        cout << S << endl;
    }
};
class Square : public Retangle{
private:
    int a;
public:
    Square() : a(0){ }
    Square(int x) : a(x){ }
    virtual void GetArea(){
        double S;
        S = a * a;
        cout << S << endl;
    }
};
int main(){
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    Retangle re(a, b);
    Circle C(c);
    Square s(d);
    shape* p;
    p = &re;
    p->GetArea();
    p = &C;
    p->GetArea();
    p = &s;
    p->GetArea();
    return 0;
}