问多态性和STL containers...are指针是否不必要？

EN

#include <vector>
#include <iostream>
#include <functional>

namespace
{
struct A
{
  virtual void operator()(void) { std::cout << "in A" << std::endl; }
};

struct B : public A
{
  virtual void operator()(void) { std::cout << "in B" << std::endl; }
};
}

int main(void)
{
  A a;
  B b;
  std::vector<std::reference_wrapper<A>> v;
  v.push_back(a);
  v.push_back(b);
  v[0]();
  v[1]();
  return 0;
}