#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class Object
{
protected:
       string mName;
       string mInfo;
public:
        Object()
       {
              mName="Object";
              mInfo="";             
       }
       string name()
       {
              return mName;
       }
       string info()
       {
              return mInfo;
       }
};

class Point      : public Object
{
protected:
       int x;                     //坐標
       int y;

public:
       Point(int x=0,int y=0)
       {
              ostringstream s;
              this->x=x;
              this->y=y;
              s<<"P("<<x<<","<<y<<")";  //坐標信息
              mName="Point";
              mInfo=s.str();
       } 
};

class Line : public Object
{
private:
       Point mP1;
       Point mP2;
public:
       Line(Point p1,Point p2)
       {
              ostringstream s;
              mP1 =p1;
              mP2 =p2;
              s<<"Line from " <<p1.info() <<" to "<<p2.info();      //線的信息
              mName ="Line";
              mInfo =s.str();
       }
};

int main()
{
       Point p1(2,3); 
       Point p2(6,3);
       Line  L(p1,p2);

       
       cout<<p1.name()<<":"<<endl;
       cout<<p1.info()<<endl;

       
       cout<<L.name()<<":"<<endl;
       cout<<L.info()<<endl;
       return 0;
}

文章來源：https://www.cnblogs.com/lifexy/p/8698104.html

22.C++- 繼承與組合,protected訪問級別