Inheritance - why is this illegal?

Question

I'm going through a C++ quiz. And came across the following code - it's illegal, but I can't understand why. Can anyone explain why this line:

Box* b1 = s1->duplicate();

generates the compiler error, "cannot convert from Shape* to Box"? I assumed that s1->duplicate() is calling Box::duplicate() because s1 actually points to a Box - but from the compiler error it looks like it's calling Shape::duplicate().

#include <iostream>

struct Shape
{
  virtual Shape* duplicate()
  {
    return new Shape;
  }

  virtual ~Shape() {}
};

struct Box : public Shape
{
  virtual Box* duplicate()
  {
    return new Box;
  }

};

int main(int argc, char** argv) 
{ 
  Shape* s1 = new Box;

  Box* b1 = s1->duplicate();

  delete s1;
  delete b1;
  return 0; 
}

Answer 1

C++ language is statically typed. The decisions about the legality of your call are made at compile time. The compiler, obviously, cannot know that s1->duplicate() returns a pointer to a Box object. Under these circumstances, it would be illogical to expect it to accept your code.

Yes, s1->duplicate() indeed calls Box::duplicate in your example, but how do you expect the compiler to know this? One can say that it is "obvious" from your specific example, but the specification of this language feature makes no exception for such "obvious" cases.