C++多态中通过使用“指向派生类对象的基类指针”访问的正确姿势

先上结论：“指向派生类对象的基类指针”只能访问在抽象基类中声明（不非得要求在基类中定义过）的虚函数（virtual function），而不能访问派生类中自己新增的函数。

实验：

#include<iostream>
 using namespace std;
 
 //define abstract class
 class Shape
 {
  public:
    virtual float area() const {return 0;}//抽象基类中的三个虚函数，其中一个是纯虚函数。 
    virtual float volume() const {return 0;}
    virtual void shapeName() const=0;
  };
  
  class Point:public Shape
  {
    public:
      Point(float=0,float=0);
      void setPoint(float a,float b);
      
      
      float getX()const {return x;}
      float getY()const {return y;}
      virtual void shapeName() const {cout<<"Point:";}
      friend ostream& operator<< (ostream& o,const Point& p){
        o<<"["<<p.x<<","<<p.y<<"]";
        return o;
      }
  protected:
    float x,y;
  };
  
  Point::Point(float a,float b):x(a),y(b){
  }
  
  void Point::setPoint(float a,float b){
    x=a,y=b;
  }
  int main()
  {
    Point point(3.2,4.5);
    point.shapeName();
    cout<<point<<endl;
    
    cout<<"------------------------------------------"<<endl;
    
    Shape* p;
    p=&point;
    p->shapeName();  
    cout<<"x="<<p->getX()<<","<<"y="<<p->getY()<<endl;  //此处妄想通过p指针访问派生类的新函数 
    cout<<"area="<<p->area()<<"/n"
        <<"volume="<<p->volume()<<endl;
    
    return 0;
  }

在注释处已经指明。

错误如下：

C:\Users\11604\Desktop\my_cpp\Point.cpp In function 'int main()':
48  19
  
C:\Users\11604\Desktop\my_cpp\Point.cpp [Error] 'class Shape' has no member named 'getX'
48  41  

C:\Users\11604\Desktop\my_cpp\Point.cpp [Error] 'class Shape' has no member named 'getY'

修改：对于派生类新增的函数，使用对象名point.getX()访问。

#include<iostream>
 using namespace std;
 
 //define abstract class
 class Shape
 {
  public:
    virtual float area() const {return 0;}//抽象基类中的三个虚函数，其中一个是纯虚函数。 
    virtual float volume() const {return 0;}
    virtual void shapeName() const=0;
  };
  
  class Point:public Shape
  {
    public:
      Point(float=0,float=0);
      void setPoint(float a,float b);
      
      
      float getX()const {return x;}
      float getY()const {return y;}
      virtual void shapeName() const {cout<<"Point:";}
      friend ostream& operator<< (ostream& o,const Point& p){
        o<<"["<<p.x<<","<<p.y<<"]";
        return o;
      }
  protected:
    float x,y;
  };
  
  Point::Point(float a,float b):x(a),y(b){
  }
  
  void Point::setPoint(float a,float b){
    x=a,y=b;
  }
  int main()
  {
    Point point(3.2,4.5);
    point.shapeName();
    cout<<point<<endl;
    
    cout<<"------------------------------------------"<<endl;
    
    Shape* p;
    p=&point;
    p->shapeName();  
    //cout<<"x="<<p->getX()<<","<<"y="<<p->getY()<<endl;  //此处修改为通过point对象访问派生类的新函数 
    cout<<"x="<<point.getX()<<","<<"y="<<point.getY()<<endl;  //此处妄想通过p指针访问派生类的新函数 
  cout<<"area="<<point.area()<<"\n"   //此处继承下来的函数也改为point对象修改 
        <<"volume="<<p->volume()<<endl;
    
    return 0;
  }

编译通过，结果如下：

总结：对于所有函数无论是继承的还是新增的都可以通过对象访问，但是对于派生类新增的函数(没有在抽象基类中声明为虚函数的函数)只能通过 派生类对象访问。