virtual function doubts c++

Question

Virtual function d() in the base class A is overidden by a private method in derived class B. When d() is invoked on an instance of B from outside, via a pointer of type A *, it is B::d() that runs, even though it is private to B.

Why does this happen? Shouldn't it be impossible for code outside B to invoke B's private methods?

Demo

#include<iostream>
using namespace std;

class A {
  public:
  virtual void d(){
    cout<<" gfgd ";
  }
};

class B :
  public A
  {
    private:
    void d(){
        cout<<"hytyhtht";
    }
  };

  int main() {
    A *a1;
    B b;
    a1=&b;
    a1->d();
    return 0;
  }

Output:

hytyhtht

Answer 1

The public/protected/private access-rules are enforced at compile-time, not at run-time, which means that they have to rely on logic that the compiler can enforce at compile-time.

In particular, the compiler knows that you are calling the d() method using a A* pointer, and that class A has declared virtual void d() to be a public method, so that means it's okay to call the method.

In general the compiler can't know at compile-time that your pointer is really pointing to a B object, so even if it wanted to it wouldn't be able to flag the call as a compile-time error. (Well, maybe in your particular example it could, but in many cases the compiler doesn't know what the type of the pointer-to object will be, it only knows the type of the pointer)

For example:

// in someotherfile.cpp
void MyFunction(A * a)
{
   a->d();  // should this compile, or not?
}