The fibonacci example from Gor’s presentation (PDF, YouTube) is the subject of this post. generator<T> implements the Range Concept so it works with existing algorithms from STL and Rangev3 and the range-for feature. It can be found in the header experimental/generator. A function that returns generator<T> is allowed to use the yield_value keyword (which evaluates to await generator<T>::promise_type::yield_value(T)).

// yield.cpp : Defines the entry point for the console application.

#include <iostream>

#include <experimental/resumable>
#include <experimental/generator>
namespace ex = std::experimental;

ex::generator<int> fibonacci(int n) {
    int a = 0;
    int b = 1;

    while (n-- > 0) {
        __yield_value a;
        auto next = a + b; a = b;
        b = next;

int wmain() {
    for (auto v : fibonacci(35)) {
        if (v > 10)
        std::cout << v << ' ';


PlantUML SVG diagram

The fibonacci() function uses yield_value to interleave the fibonacci() loop and the caller loop on the current thread. The begin() and operator++() functions resume the fibonacci() loop while yield_value resumes the caller loop. generator<T> uses this interleaving to pass the value to the caller as a const reference to the value inside the fibonacci() loop. This avoids a copy while preventing the caller from modifiying the state of the fibonacci() loop.

try it out

After installing Visual Studio 2015 Preview clone await and open the await solution. This code is in the yield project in the solution.