TinyThreadPool.h

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

// originally from https://github.com/xiaoshengMr/TinyThreadPool but updated for C++17
// Changed 'result_of' to 'invoke_result_t' as it is not a member of 'std' in C++17

#include <vector>
#include <queue>
#include <memory>
#include <future>
#include <mutex>
#include <condition_variable>

class TinyThreadPool {
public:
    TinyThreadPool(size_t);
    template <class F, class... Args>
    auto enqueue(F&& f, Args&&... args) -> std::future<std::invoke_result_t<F, Args...>>;
    ~TinyThreadPool();

private:
    std::vector<std::thread> workers;
    std::queue<std::function<void()>> tasks;
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

inline TinyThreadPool::TinyThreadPool(size_t threads) : stop(false) {
    for (size_t i = 0; i < threads; i++) {
        workers.emplace_back([this] {
            for (;;) {
                std::function<void()> task;
                {
                    std::unique_lock<std::mutex> lock(this->queue_mutex);
                    this->condition.wait(lock, [this] { return this->stop || !this->tasks.empty(); });
                    if (this->stop && this->tasks.empty()) {
                        return;
                    }
                    task = std::move(this->tasks.front());
                    this->tasks.pop();
                }
                task();
            }
            });
    }
}

template <class F, class... Args>
auto TinyThreadPool::enqueue(F&& f, Args&&... args) -> std::future<std::invoke_result_t<F, Args...>> {
    using return_type = std::invoke_result_t<F, Args...>;

    auto task = std::make_shared<std::packaged_task<return_type()>>(std::bind(std::forward<F>(f), std::forward<Args>(args)...));

    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        if (stop) {
            throw std::runtime_error("enqueue on stopped ThreadPool");
        }
        tasks.emplace([task] { (*task)(); });
    }
    condition.notify_one();
    return res;
}

inline TinyThreadPool::~TinyThreadPool() {
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();
    for (std::thread& worker : workers) {
        worker.join();
    }
}

#endif