Why virtual function has to be implemented in superclass?

I am trying to compile the following code:

#include <iostream>
class X{
public:
    virtual void func();
};
class Y : public X{
public:
    virtual void func(){
        std::cout << "y" << std::endl;
    }
};
int main(){
    Y* y = new Y();
    y->func();
    return 0;
}

But building fails (on Xcode - C++11) with the following messages:

Undefined symbols for architecture x86_64:
  "typeinfo for X", referenced from:
      typeinfo for Y in c.o
  "vtable for X", referenced from:
      X::() in c.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

However, as soon as I add an implementation for func in X, it builds successfully. I am pretty sure, that virtual method is optional to be implemented in the superclass but I don't understand why is this happening. Also, if comment the code in main(), it builds successfully. I am assuming that the problem is calling the func() in main, but Xcode doesn't list it as runtime error, it only says build-time error.