What the heck is this?

gistfile1.cpp

#include <iostream>

struct bar {
	int i;
};

struct foo {
	int bar::* p;
};

int main() {
	bar b;
	b.i = 42;

	foo f;
	f.p = &bar::i;

	int bar::* foo::* ptr = &foo::p;
	std::cout << (b.*(f.*ptr));
}