问关于C++多态性用法的调查

EN

#include<iostream>

class base{
  public:
  virtual void run(){};
  protected:
  ~base(){std::cout<<"destructor for base"<<std::endl;};
};

class derived : public base {
   public:
   void run(){};
   ~derived(){std::cout<<"destructor for derived"<<std::endl;};
};

void get_type ( std::shared_ptr<base> b ){
    b.reset ( new derived );
    std::cout<<"end of get_type"<<std::endl;
}

int main(){
  std::shared_ptr<base> b;
  get_type ( b ) ;
  std::cout<<"out of get_type"<<std::endl;
  b->run();
}