source: doc/papers/concurrency/examples/Fib2.cpp @ a6cfb4d

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

#include <experimental/coroutine>

template<typename T>
struct cor_range {
        struct promise_type {
                T _value;

                cor_range get_return_object() {
                        return cor_range(std::experimental::coroutine_handle<promise_type>::from_promise(*this));
                }

                auto initial_suspend() { return std::experimental::suspend_always(); }
                auto final_suspend()   { return std::experimental::suspend_always(); }

                void return_value(T value) {
                        _value = value;
                }

                auto yield_value(T value) {
                        _value = value;
                        return std::experimental::suspend_always();
                }

                void unhandled_exception() {}
        };

        std::experimental::coroutine_handle<promise_type> _coroutine = nullptr;

        explicit cor_range(std::experimental::coroutine_handle<promise_type> coroutine)
                : _coroutine(coroutine)
        {}

        ~cor_range() {
                if(_coroutine) { _coroutine.destroy(); }
        }

        cor_range() = default;
        cor_range(cor_range const &) = delete;
        cor_range& operator=(cor_range const &) = delete;

        cor_range(cor_range&& other) {
                std::swap(_coroutine, other._coroutine);
        }

        cor_range& operator=(cor_range&& other) {
                if(&other != this) {
                        _coroutine = other._coroutine;
                        other._coroutine = nullptr;
                }
                return *this;
        }

        T next() {
                _coroutine.resume();
                return _coroutine.promise()._value;
        }

        struct iterator : std::iterator<std::input_iterator_tag, T> {
                std::experimental::coroutine_handle<promise_type> _coroutine = nullptr;

                iterator() = default;
                explicit iterator(std::experimental::coroutine_handle<promise_type> coroutine)
                        : _coroutine(coroutine)
                {}

                iterator& operator++() {
                        _coroutine.resume();
                        return *this;
                }

                T const & operator*() const {
                        return _coroutine.promise()._value;
                }
        };

        iterator begin() {
                if(_coroutine) {
                        _coroutine.resume();
                        if(_coroutine.done()) { return end(); }
                }

                return iterator{ _coroutine };
        }

        iterator end() { return iterator{}; }
};

cor_range<int> fib() {
        int fn1 = 0, fn = 1;
        for(;;) {
                co_yield fn1;
                int temp = fn1;
                fn1 = fn;
                fn = temp + fn;
        }
}

int main() {
        {
                auto f1 = fib();
                auto f2 = fib();
                for(int i = 0; i < 10; i++) {
                        std::cout << f1.next() << " " << f2.next() << std::endl;
                }
        }

        {
                auto f1 = fib();
                std::vector<int> fibs;
                std::copy_n(f1.begin(), 10, std::back_inserter(fibs));

                for(auto i : fibs) {
                        std::cout << i << std::endl;
                }
        }
}