#include <iostream>

using namespace std;

class  Base
{  public:
Base(int i=0):x(i){}
virtual int sum( ) const
{     return x;     }
private:
    int x;  };
   
    class Derived: public Base
    { public:
    Derived(int i=0,int j=0):Base(i),y(j){}
    int sum( )
    {      return Base::sum()+y;     }
    private:
        int y;  };
        void Call ( Base &b )
  {   cout<<"Sum="<<b.sum()<<endl;   }  

        void main()
        {     Base b(10);
        Derived  d(10,40);
        Call (b);
        Call (d);  }

为什么不加红笔的const，运行结果是：Sum=10 Sum=50 ，而加上红笔的const之后，运行结果是: Sum = 10 Sum =10

常成员函数   只能访问类的数据成员  不能改变数据成员
是为了增加程序的健壮性而加的

virtual int sum( ) const
    int sum( )
不能重载，不符合重载条件。
应写成：
#include <iostream>

using namespace std;

class  Base
{
public:
    Base(int i=0):x(i)
    {}
    virtual int sum( ) const
    {
        return x;
    }
private:
    int x;
};
   
class Derived: public Base
{
public:
    Derived(int i=0,int j=0):Base(i),y(j)
    {}
    int sum( )const
    {
        return Base::sum()+y;     
    }
private:
    int y;  
};
void Call ( Base &b )
{
    cout<<"Sum="<<b.sum()<<endl;   
}  

void main()
{
    Base b(10);
    Derived  d(10,40);
    Call (b);
    Call (d);  
}