Slightly different take on http://dunkels.com/adam/pt/expansion.html (2)

coro.cpp

#include <functional>
#include <iostream>

template<typename T>
struct coro {
    using fn = std::function<T(T)>;
    coro(fn f) : m_lc(0), m_done(false), m_fn(f) {}
    bool done() const { return m_done; }
    T resume(T arg) { return m_fn(arg); }

    unsigned int m_lc;
    bool m_done;

private:
    fn m_fn;
};

#define CORO_BEGIN(co) \
    switch (co.m_lc) { case 0:

#define CORO_END(co, res) \
    } co.m_lc = 0; co.m_done = true; return res

#define CORO_YIELD(co, res) \
    co.m_lc = __LINE__; return res; case __LINE__:

int main()
{
    coro<int> co([&](int n) {
        static int i;

        CORO_BEGIN(co);

        for (i = 0; i < 5; i++) {
            CORO_YIELD(co, n + i);
        }

        CORO_END(co, n + 5);
    });

    int n = 100;

    while (!co.done()) {
        n = co.resume(n);
        std::cout << "Back in main with n = " << n << '\n';
    }
}