static pointer in C++ inheritance

Question

I'm getting confused why p->a() is calling B::a()?. Is there a paragraph somewhere in the C++ documentation/standard that describes this behavior well?

#include <iostream>
using namespace std;

class A {
  public:
  A() { cout << "A ctor" << endl; a_instance = this; }
  static A *get_instance() { return a_instance; }
  static A *a_instance;
  void virtual a() { cout << "From base class" << endl; }
};

class B : public A {
public:
  B() { cout << "B ctor" << endl; b_instance = this; }
  static B *get_instance() { return b_instance; }
  static B *b_instance;
  void virtual a() { cout << "From derived class" << endl; }
};

A *A::a_instance = 0;
B *B::b_instance = 0;

main()
{
    cout << "Create A" << endl;
    A ab;
    cout << "Create B" << endl;
    B abc;
    B *ptr = B::get_instance();
    A *p = A::get_instance();

    cout << "Called from A object type" << endl;
    if (p) {
       p->a();
    }
}

Answer 1

When you create the variable abc, A's constructor sets a_instance to that instance. Despite p being a pointer to an A, since the instance is pointing to a B, it's correctly calling B::a().

To fix this behaviour, you could use the following:

A* A::get_instance()
{
    static A a;
    return &a;
}

B* B::get_instance()
{
    static B b;
    return &b;
}  

and remove all code that has to do with a_instance and b_instance.