关于#c++#的问题，如何解决？

Question

我想编辑一个关于点 圆 柱体的三个类，然后我在编译圆的时候定义了一个r半径，但编译器一直说[Error] invalid use of non-static data member 'Circle::r'是因为非静态成员变量的原因吗，哪位大佬帮我这个菜鸟解读一下，还要就是显示函数问题我也没搞懂哭了

#include<iostream>
using namespace std;
class Point{
    friend ostream& operator<<(ostream &output,const Point &c){
        cout<<c.x<<","<<c.y<<endl;return output;
    }
    protected:
        int x,y;
    public:
        Point(int n=0,int i=0){setPoint(n,i);}
        setPoint(int p,int q){x=p;y=q;}
};
class Circle:public Point{
    protected:
        int r;
    public:
        Circle(int n,int i,int j):Point(n,i){setCircle(j);}
        setCircle(int j){r=j;}
        int Area()const{return r*r*3.14;}
        friend ostream& operator<<(ostream &output,const Circle &c){
        cout<<"Center:"<<"["<<c.x<<","<<c.y<<"]"<<endl;
        cout<<"The r="<<r<<endl;
        cout<<"area:"<<Area();
        return output;
    }
};
class Cylinder:public Circle{
    protected:
        int h;
    public:
        friend ostream& operator<<(ostream &output,const Cylinder &c){
        cout<<"Center:"<<"["<<c.x<<","<<c.y<<"]"<<endl;
        cout<<"The r="<<r<<endl;
        cout<<"area:"<<Area();
        cout<<"volume:"<<volume();
        return output;
    }
    Cylinder(int i,int j,int k,int e):Circle(i,j,k){setHigh(e);}
    setHigh(int n){h=n;}
    int volume()const{return Area()*h;}
};
int main(){
    Cylinder obj1(1,2,3,4);
    cout<<obj1<<endl;
}

Answer 1

帮你修改如下：

#include<iostream>
using namespace std;
class Point{
    friend ostream& operator<<(ostream &output,const Point &c){
        cout<<c.x<<","<<c.y<<endl;return output;
    }
    protected:
        int x,y;
    public:
        Point(int n=0,int i=0){setPoint(n,i);}
        void setPoint(int p,int q){x=p;y=q;}  // 返回值 void
};
class Circle:public Point{
    protected:
        int r;
    public:
        Circle(int n,int i,int j):Point(n,i){setCircle(j);}
        void setCircle(int j){r=j;} // 返回值 void
        int Area()const{return r*r*3.14;}
        friend ostream& operator<<(ostream &output,const Circle &c){
            cout<<"Center:"<<"["<<c.x<<","<<c.y<<"]"<<endl;
            cout<<"The r="<<c.r<<endl; // 修改
            cout<<"area:"<<c.Area(); // 修改
            return output;
        }
};
class Cylinder:public Circle{
    protected:
        int h;
    public:
        friend ostream& operator<<(ostream &output,const Cylinder &c){
            cout<<"Center:"<<"["<<c.x<<","<<c.y<<"]"<<endl;
            cout<<"The r="<<c.r<<endl; // 修改
            cout<<"area:"<<c.Area(); // 修改
            cout<<"volume:"<<c.volume(); // 修改
            return output;
        }
        Cylinder(int i,int j,int k,int e):Circle(i,j,k){setHigh(e);}
        void setHigh(int n){h=n;} // 返回值 void
        int volume()const{return Area()*h;}
};
int main(){
    Cylinder obj1(1,2,3,4);
    cout<<obj1<<endl;
}