task_runner.cc

// g++ -std=c++14 task.cc -pthread
#include <iostream>
#include <functional>
#include <future>
#include <thread>
#include <chrono>
#include <mutex>
#include <queue>
#include <memory>
#include <atomic>

std::queue<std::unique_ptr<std::packaged_task<void()>>> tasks;
std::mutex task_list_mutex;
std::atomic_bool quit;

void
add_task(std::unique_ptr<std::packaged_task<void()>> task) {
	std::lock_guard<std::mutex> lock(task_list_mutex);
	tasks.push(std::move(task));
}

std::unique_ptr<std::packaged_task<void()>>
get_task() {
	std::lock_guard<std::mutex> lock(task_list_mutex);
	if (tasks.size() == 0) {
		return nullptr;
	}

	auto task_ptr = std::move(tasks.front());
	tasks.pop();
	return task_ptr;
}

void
printer() {
	std::cout << "printer()" << std::endl;
}

void
slow_function() {
	using namespace std::literals;
	std::cout << "starting slow function" << std::endl;
	std::this_thread::sleep_for(5s);
	std::cout << "finished slow function" << std::endl;
}

int
main() {
	std::thread task_runner([]() {
		while (true) {
			if (quit) {
				return;
			}

			auto taskptr = get_task();
			if (taskptr) {
				(*taskptr)();
			} else {
				using namespace std::literals;
				std::this_thread::sleep_for(100us);
			}
		}
	});

	add_task(std::make_unique<std::packaged_task<void()>>(printer));

	auto slow_task = std::make_unique<std::packaged_task<void()>>(slow_function);
	auto slow_task_future = (*slow_task).get_future();
	add_task(std::move(slow_task));

	slow_task_future.wait();
	quit = true;
	task_runner.join();
	return 0;
}