A thread safe queue that blocks pop when no elements are available

asyncqueue.hpp

/* Copyright 2015, Richard Wiedenhöft <[email protected]>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *   * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *   * Neither the name of the copyright holder nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#pragma once

#include<queue>
#include<mutex>
#include<condition_variable>
#include<utility>
#include<stdexcept>
#include<type_traits>

namespace alpha {

	/**
	 * @brief Thrown when calling modifying methods on AsyncQueue after it has been closed
	 */
	class AsyncQueueClosedError : public std::runtime_error {
		public:
			AsyncQueueClosedError() : std::runtime_error("The AsyncQueue has been closed.") {}
	};

	/**
	 * @brief A queue that may be accessed from different threads
	 */
	template<typename T>
	class AsyncQueue {
		private:
			std::queue<T> queue;
			std::mutex mutex;
			std::condition_variable cond;
			bool _closed = false;

		public:
			/* Big 5 */
			AsyncQueue(const AsyncQueue& other) = delete;
			AsyncQueue(AsyncQueue&& other) = delete;
			AsyncQueue& operator=(const AsyncQueue& other) = delete;
			AsyncQueue& operator=(AsyncQueue&& other) = delete;
			virtual ~AsyncQueue() = default;

			/**
			 * @brief Default constructor
			 */
			AsyncQueue() {
			}

			/**
			 * @brief Push a value to the async queue
			 */
			template<typename U>
			void push(U&& object) {
				static_assert(
					std::is_convertible<U, T>::value,
					"Argument is push is incompatible to template type of AsyncQueue."
				);
				{
					std::lock_guard<std::mutex> lock(this->mutex);
					if(this->_closed) {
						throw AsyncQueueClosedError();
					}
					this->queue.emplace(std::forward<U>(object));
				}
				this->cond.notify_one();
			}

			/**
			 * @brief Pop a value from the queue
			 */
			T pop() {
				std::unique_lock<std::mutex> lock(this->mutex);
				while(this->queue.empty() && !this->_closed) {
					this->cond.wait(lock);
				}
				if(this->queue.empty()) {
					/* If the queue is empty at this point it was closed. */
					throw AsyncQueueClosedError();
				}

				T object = std::move(this->queue.front());
				this->queue.pop();
				return object;
			}

			/**
			 * @brief Returns if the queue is empty
			 */
			bool empty() {
				std::lock_guard<std::mutex> lock(this->mutex);
				return this->queue.empty();
			}

			/**
			 * @brief Close the queue while allowing to pop already inserted elements
			 */
			void close() {
				{
					std::lock_guard<std::mutex> lock(this->mutex);
					this->_closed = true;
				}
				this->cond.notify_all();
			}

			/**
			 * @brief Returns whether the queue is closed
			 */
			bool closed() {
				std::lock_guard<std::mutex> lock(this->mutex);
				return this->_closed;
			}
	};
}