为什么虚函数在编译期间无法确定下来

struct Base
{
  virtual void func()
  { std::cout << "Base::func()" << std::endl; }
};

struct Derived : Base
{
  virtual void func()
  { std::cout << "Derived::func()" << std::endl; }
};

int main()
{
  Base *bp = 0;

  std::string input;
  std::cin >> input;

  if (input == "base")
    bp = new Base;
  else
    bp = new Derived;

  /* The compiler cannot decide which
     function is called here: */
  bp->func();

  return 0;
}