Here the short story. I have a listener on a queue which sends me several jobs. Each job is launched in a thread and takes different times to finish. I have two main constraints :

  • I want to retrieve the result of the job when it finishes (hence the use of std::async rather than std::thread)
  • I want to have a collection of threads in the same kind of way as boost:thread_group but I cannot use a join_all/get_all method since I do not know when the job finishes and I do not want to be blocked.

So the idea chosen was to use the clean_ready_futures method which goes through the vector periodically and remove the ready futures such as proposed on this SO thread.


#pragma once

#include <vector>
#include <future>
#include <chrono>
#include <algorithm>
#include <mutex>

template<typename R>
bool is_future_ready(std::future<R> const& f) { 
    return f.wait_for(std::chrono::seconds(0)) == std::future_status::ready; 

template <typename T>
struct async_group {
    std::vector<std::future<T>> futures;

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

    // not working yet
    // template<class Function, class... Args>
    // void create_future(Function&& f, Args&&... args)  { 
    //     futures.emplace_back(std::async(
    //         std::launch::async,
    //         std::forward<Function>(f),
    //         std::forward<Args>(args)...
    //     ));
    // }
    void add_future(std::future<T>&& ret) { 
        std::lock_guard<std::mutex> lock{mutex}; 

    std::size_t size() const { return futures.size(); }

    // Use this to have a similar behaviour as join_all()
    std::vector<T> get_all() {
        auto result = std::vector<T>{};
        for (auto & f : futures) {
        std::lock_guard<std::mutex> lock{mutex};
        return result;

    // Use this periodicall if you don't want to call get() and "detach" the asyncs
    std::vector<T> clean_ready_futures() {
        std::lock_guard<std::mutex> lock{mutex};
        // erase from futures vector and store ready futures in finished results
        auto finished_results = std::vector<T>{};
                [&finished_results](auto& f) -> bool {
                    if (is_future_ready(f)) { // future is ready then
                        finished_results.emplace_back(f.get()); // store the ready future in results
                        return true; // erase it from pending futures
                    return false;

        return finished_results;

    std::mutex mutex;

illustative usage

#include <iostream>
#include "async_group.hpp"

std::string foo() {
    return "foo";

std::string bar() {
    return "bar";

async_group<std::string>  pending_futures;

void launch_jobs() {
    pending_futures.add_future(std::async(std::launch::async, foo));
    pending_futures.add_future(std::async(std::launch::async, bar));

void clean_ready_jobs() {
    auto results = pending_futures.clean_ready_futures();
    std::cout << "Clean ready jobs: ";
    for (auto const& r : results) {
        std::cout << r << ' ';
    std::cout << std::endl;

void get_all_jobs() {
    auto results = pending_futures.get_all();
    std::cout << "Clean ready jobs: ";
    for (auto const& r : results) {
        std::cout << r << ' ';
    std::cout << std::endl;

int main() {
    get_all_jobs(); // both foo and bar should be done
    clean_ready_jobs(); // bar should be done
    clean_ready_jobs(); // foo should be done

Demo on Wandbox

Have I missed something ? How can this be improved ? I am looking forward to your insights.

  • \$\begingroup\$ I'm using Xcode 8.2 and the line if (is_future_ready(f)) { // future is ready then doesn't compile, while this may be a problem in my compiler, auto isn't guaranteed to work in lambda expressions arguments until C++14 according to everything I can find on the web. Example stackoverflow.com/questions/7709894/…. \$\endgroup\$ – pacmaninbw Jun 2 '17 at 14:19
  • 1
    \$\begingroup\$ Yes I will precise the C++14 tag. If you put [&finished_results](std::future<T>& f) instead of [&finished_results](auto& f) it will work \$\endgroup\$ – coincoin Jun 2 '17 at 14:19

We are assuming this is a multi threaded environemnt.

Thus all methods need to guard against access by a different thread and lock access to members.

// There is no lock here:
std::size_t size() const { return futures.size(); }

// There is no lock here:
std::vector<T> get_all() {
| improve this answer | |
  • \$\begingroup\$ So adding std::lock_guard<std::mutex> lock{mutex};at the top scope of the method definition will do the work right ? \$\endgroup\$ – coincoin Jun 2 '17 at 20:03
  • \$\begingroup\$ @coincoin: Yes. \$\endgroup\$ – Martin York Jun 2 '17 at 21:51

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