C++初学遇到编译通过，却执行失败的问题

Question

我去掉main部分后可以执行，请大佬为我指点迷津。

首先十分感谢您能打开我的问答。

问题是出现在main里的指针P上吗
错误提示
纯虚函数.obj : error LNK2001: unresolved external symbol "public: virtual double __thiscall Body::gets(void)" (?gets@Body@@UAENXZ)
纯虚函数.obj : error LNK2001: unresolved external symbol "public: virtual double __thiscall Body::getv(void)" (?getv@Body@@UAENXZ)
Debug/纯虚函数.exe : fatal error LNK1120: 2 unresolved externals

#include "iostream.h"
class Body{
public:
    virtual double getv();           //求“体”的体积
    virtual double gets();           //求“体”的表面积
};
class Cylinder:public Body{          //圆柱体
private:
    double r;
    double h;
public:
    Cylinder(double rr,double hh):r(rr),h(hh){}
    double getv();
    double gets();
};
class Cube:public Body{             //正方体
private:
    double l;
public:
    Cube(double ll):l(ll){}
    double getv();
    double gets();
};
class Sphere:public Body{           //球体
private:
    double rad;
public:
    Sphere(double rr):rad(rr){}
    double getv();
    double gets();
};
double Cylinder::getv()
{
    return 3.14*r*r*h;
}
double Cylinder::gets()
{
    return 3.14*r*r*2+h*3.14*2*r;
}
double Cube::getv()
{
    return l*l*l;
}
double Cube::gets()
{
    return l*l*6;
}
double Sphere::getv()
{
    return 4/3*3.14*rad*rad*rad;
}
double Sphere::gets()
{
    return 4*3.14*rad*rad;
}
void main()
{
    Body *p=new Cylinder(1,1);
    cout<<"圆柱体的体积是"<<p->getv()<<endl;
    cout<<"圆柱体的表面积是"<<p->gets()<<endl;
    p=new Cube(1);
    cout<<"正方体的体积是"<<p->getv()<<endl;
    cout<<"正方体的表面积是"<<p->gets()<<endl;
    p=new Sphere(1);
    cout<<"球体的体积是"<<p->getv()<<endl;
    cout<<"球体的表面积是"<<p->gets()<<endl;
    delete p;
}

Answer 1

class Body{
public:
    virtual double getv() = 0;           //求“体”的体积
    virtual double gets() = 0;           //求“体”的表面积
};

虚函数，需要写 = 0，因为没有函数体