cpp/src/common/thread_pool.h

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * License); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
#ifndef COMMON_THREAD_POOL_H
#define COMMON_THREAD_POOL_H

#include <condition_variable>
#include <functional>
#include <future>
#include <mutex>
#include <queue>
#include <thread>
#include <vector>

namespace common {

class ThreadPool {
   public:
    explicit ThreadPool(size_t num_threads) : stop_(false) {
        for (size_t i = 0; i < num_threads; i++) {
            workers_.emplace_back([this] {
                for (;;) {
                    std::function<void()> task;
                    {
                        std::unique_lock<std::mutex> lock(mutex_);
                        cv_.wait(lock,
                                 [this] { return stop_ || !tasks_.empty(); });
                        if (stop_ && tasks_.empty()) return;
                        task = std::move(tasks_.front());
                        tasks_.pop();
                    }
                    task();
                }
            });
        }
    }

    ~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(mutex_);
            stop_ = true;
        }
        cv_.notify_all();
        for (auto& w : workers_) w.join();
    }

    template <typename F>
    std::future<typename std::result_of<F()>::type> submit(F&& f) {
        using RetType = typename std::result_of<F()>::type;
        auto task = std::make_shared<std::packaged_task<RetType()>>(
            std::forward<F>(f));
        std::future<RetType> result = task->get_future();
        {
            std::unique_lock<std::mutex> lock(mutex_);
            tasks_.emplace([task]() { (*task)(); });
        }
        cv_.notify_one();
        return result;
    }

   private:
    std::vector<std::thread> workers_;
    std::queue<std::function<void()>> tasks_;
    std::mutex mutex_;
    std::condition_variable cv_;
    bool stop_;
};

}  // namespace common

#endif  // COMMON_THREAD_POOL_H