#include <iostream> 
using namespace std; 
               
class Base 
{ 
public: 
  virtual Base& operator=(const Base& rhs) //重载操作符可设为virtual 
  { 
    cout << "Base" << endl; 
    return *this; 
  } 
}; 
               
class Derived : public Base 
{ 
public: 
  //与基类的operator=完全不同，不是重新定义，不会动态绑定。 
  //如果不定义该操作符，会自动合成一个，并自动调用基类的operator=，不会动态绑定 
  Derived& operator=(const Derived& rhs)  
  { 
    cout << "Derived_D" << endl; 
    return *this; 
  } 
               
  //重新定义基类的operator=，会动态绑定 
  //virtual Base& operator=(const Base& rhs) //返回值两种都可以 
  virtual Derived& operator=(const Base& rhs)  
  { 
    cout << "Derived_B" << endl; 
    return *this; 
  } 
}; 
               
class Derived2 : public Derived 
{ 
  //此类需要3个operator= 
  //可定义private的copy函数，由3个operator=调用 
  //可使用dynamic_cast将基类引用参数转为子类，并捕获异常。 
  //如果未发生异常，则调用copy，发生异常则不需赋值 
}; 
               
int main() 
{ 
  Base b1, b2; 
  Derived d1, d2; 
               
  Derived &rd = d1; 
               
  Base &rb1 = b1; //动态类型为Base 
  Base &rb2 = d2; //动态类型为Derived 
                 
  rb1 = d1; //输出"Base" 
  rb2 = d2; //输出"Derived_B" 
               
  rb1 = rb2; //输出"Base" 
  rb2 = rb1; //输出"Derived_B" 
               
  rd = d1; //输出"Derived_D" 
  rd = b1; //输出"Derived_B" 
                 
  getchar(); 
  return 0; 
}