Pointer to private data member of a class

Question

Is it possible to declare a pointer to private data member of a class? If so, how do you do it?

Answer 1

Yes, and the same way you would create any other pointer. The catch is, of course, that since the member is private, you can only create the pointer inside the class, where you can see the member.

class A 
{
  public:
    int* getFooPtr()
    {
       return &foo;  // OK; Inside the class foo is visible
    }

  private:
    int foo;
};

int main()
{
   A a;

   int* p_foo1 = &a.foo; // Illegal; Outside the class, foo is private
   int* p_foo2 = a.getFooPtr(); // OK; getFooPtr() is a public member function
}

So it's possible to create pointers to private members, but only inside the class' member functions, and it is possible to return those created pointers from member functions. Whether or not it's a good idea to return pointers to private members is another question entirely (usually it's not a good idea).

Answer 2

You can access outside of the declaring class any non-static private member if you are exploiting the fact that C++ allows you to pass the address of the private member in explicit instantiation. However you won't be able to access static private members, since you have to use a pointer to member with this technique.

struct A
{
     A() : x("proof!") {}
private:
     char const* x;
};

template class stow_private<A_x,&A::x>;

int main()
{
        A a;

        // Use the stowed private member pointer
        std::cout << a.*stowed<A_x>::value << std::endl;
};

You can find the implementation of stowed<> and details here: http://bloglitb.blogspot.hu/2010/07/access-to-private-members-thats-easy.html and https://gist.github.com/dabrahams/1528856