Function when_all

getting-started/gke/index_gcs.cc:212–243

template <typename T>
future<std::vector<future<T>>> when_all(std::vector<future<T>> w) {
  class Accumulator : public std::enable_shared_from_this<Accumulator> {
   public:
    Accumulator() : unsatisfied_(0) {}

    future<std::vector<future<T>>> Start(std::vector<future<T>> all) {
      auto self = this->shared_from_this();
      unsatisfied_ = all.size();
      accumulator_.resize(all.size());
      std::size_t i = 0;
      for (auto& f : all) {
        cont_.push_back(f.then([index = i++, self](future<T> g) {
          self->OnCompletion(index, std::move(g));
        }));
      }
      return done_.get_future();
    }

   private:
    void OnCompletion(std::size_t index, future<T> g) {
      accumulator_[index] = std::move(g);
      if (--unsatisfied_ == 0) done_.set_value(std::move(accumulator_));
    }
    std::atomic<std::size_t> unsatisfied_;
    std::vector<future<T>> accumulator_;
    std::vector<future<void>> cont_;
    google::cloud::promise<std::vector<future<T>>> done_;
  };

  auto accumulator = std::make_shared<Accumulator>();
  return accumulator->Start(std::move(w));
}

template <class... Ts>
struct overloaded : Ts... {