有大佬帮忙看看这个怎么写吗

声明一个Shape抽象类，在此基础上派生出Rectangle（矩形）和Circle（圆）类；
二者都有GetArea( )函数计算对象的面积，GetPerim( )函数计算对象的周长。
要求：请使用基类引用调用虚函数方法，实现动态多态

#include <iostream>
#define _USE_MATH_DEFINES
#include <math.h>
using namespace std;

class Shape{
public:
    virtual double GetArea()=0;
    virtual double GetPerim()=0;
};
class Rectangle:public Shape{
public:
    Rectangle(double length,double width):mLength(length),mWidth(width){}
    virtual double GetArea(){
        return mLength*mWidth;
    }
     virtual double GetPerim(){
        return (mLength+mWidth)*2;
    }
private:
    double mLength;
    double mWidth;
};

class Circle:public Shape{
public:
    Circle(double r):mR(r){}
    virtual double GetArea(){
        return M_PI*mR*mR;
    }
    virtual double GetPerim(){
       return mR*2*M_PI;
   }
private:
    double mR;
};

int main()
{
    Rectangle rect(12.8,6);
    Circle cle(4.5);
    Shape *sh = &rect;
    std::cout<<sh->GetArea()<<std::endl;
    std::cout<<sh->GetPerim()<<std::endl;
    std::cout<<"***************************"<<std::endl;
    sh = &cle;
    std::cout<<sh->GetArea()<<std::endl;
    std::cout<<sh->GetPerim()<<std::endl;
    return 0;
}