note about pseudo-coroutine in C++

pseudocoro.cpp

#include <iostream>
#include <boost/optional.hpp>

boost::optional<int> coro (int const a)
{
#define yield(arg) ___lino = __LINE__;return boost::optional<int>(arg);case __LINE__:
#define yield_return ___lino = 0;return boost::optional<int>();case __LINE__:
	static int ___lino = 0;

	//decl of vars
	//--------------------
	static int i;
	//--------------------

	switch(___lino){
	case 0:

		//----init of vars
		i = 0;
		//------------------

		for(i=0; i < 42; i++){
			if(i & 1){
				yield(i);
			} else {
				yield(i*2);
			}
		}
		yield_return;
	}
	return boost::optional<int>();
#undef yield
#undef yield_return
}

int main()
{
	while(boost::optional<int> i = coro(0)){
		std::cout << *i << std::endl;
	}
	while(boost::optional<int> i = coro(0)){
		std::cout << *i * 2  << std::endl;
	}
	return 0;
}