#include <iostream>

using namespace std;

class B
{
public:
  B ( void );
  virtual ~B ( void );
  virtual void f ( void ) = 0;
protected:
  B &operator = ( B const &rhs );  //遵循1
private:
  int kind;
};
B::B ( void ) : kind ( 0 )
{
}
B::~B ( void )
{
}
B & B::operator = ( B const &rhs )
{
  kind = rhs.kind;
  return ( *this );
}

class D : public B
{
public:
  D ( void );
  virtual ~D ( void );
  virtual void f ( void ) { };
  D &operator = ( D const &rhs );
private:
  int member;
};
D::D ( void ) : B ( ), member ( 0 )
{
}
D::~D ( void )
{
}
D & D::operator = ( D const &rhs )
{
  member = rhs.member;
  return ( *this );
}

int main ( void )
{
  D d1;
  D d2;
  B &b1 = d1;
  B &b2 = d2;

  b1 = b2;       //由于采取了保护，此用法被禁止，编译报错

  return ( 0 );
}