#include <iostream>

using namespace std;

struct s 
{
	int b;
	int a;
};

struct s *get_s_ptr(int *a_ptr)
{
	cout<< (struct s*)0 << endl;
	//cout<< ((struct s*)0)->a << endl;	//this will cause an access violation
	cout<< &((struct s*)0)->a << endl;

	return (struct s*)((char*)a_ptr - (int)&((struct s*)0)->a); 
}

int main()
{
	struct s s_var;
	struct s* s_var_ptr = &s_var;	
	
	int *a_ptr = &s_var.a;	//pointer to a
	s *s_ptr = get_s_ptr(a_ptr);	//pointer to s

	cout<< "address directly from s pointer: " << s_var_ptr <<endl;
	cout<< "address from subroutine: " << s_ptr<< endl;

	cin.get();

	return 0;
}

0
0x4
address directly from s pointer: 0xbfa2acd0
address from subroutine: 0xbfa2acd0