C++笔试题之实现一个定时器

一.定时器（timer）的需求

1.执行定时任务的时，主线程不阻塞，所以timer必须至少持有一个线程用于执行定时任务
2.考虑到timer线程资源的合理利用，一个timer需要能够管理多个定时任务，所以timer要支持增删任务，通过容器储存任务
3.当timer空闲时（即没有任务或执行任务的时刻未到），timer中的线程不应该空转来占用资源，可通过条件变量实现
4.支持重复任务和非重复任务

二.定时器（timer）的实现

#include <algorithm>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <functional>
#include <map>
#include <mutex>
#include <thread>
#include <iostream>
#include <iomanip>
#include <sstream>namespace CC
{
using TaskFunc = std::function<void()>;struct Task
{uint64_t id;uint64_t period;bool repeated;TaskFunc func;bool removed;Task(uint64_t id, uint64_t period, bool repeated, TaskFunc func): id(id), period(period), repeated(repeated), func(func), removed(false){}
};class Timer
{
public:Timer() : m_stop(false){m_worker = std::thread(&Timer::run, this);}~Timer(){m_stop.store(true);m_condition.notify_all();m_worker.join();}uint64_t add(uint64_t period_ms, bool repeated, TaskFunc func){uint64_t when = now() + period_ms;Task task(m_cur_id, period_ms, repeated, func);{std::lock_guard<std::mutex> lock(m_tasks_mutex);m_tasks.insert({when, task});}m_condition.notify_all();return m_cur_id++;}// Timer::remove并没有真正的将定时任务删除，仅仅是将removed标志位设置为true，删除操作实际是在Timer::run中进行的。// 为什么要这么做？如果在这里如果由Timer::remove来执行m_tasks.erase(it)，那么有可能删除的是Timer::run里正在执行的那个任务，这是明显不对的。// 所以才采用将removed标志位设置为true的这种做法。bool remove(uint64_t id){bool flag = false;std::lock_guard<std::mutex> lock(m_tasks_mutex);std::multimap<uint64_t, Task>::iterator it =std::find_if(m_tasks.begin(), m_tasks.end(),[id](const std::pair<uint64_t, Task> &item) -> bool { return item.second.id == id; });if (it != m_tasks.end()){it->second.removed = true;flag = true;}return flag;}private:std::thread m_worker;std::atomic<bool> m_stop;std::multimap<uint64_t, Task> m_tasks;std::mutex m_tasks_mutex;std::condition_variable m_condition;uint64_t m_cur_id;// m_condition.wait之后继续向下执行，此时如果m_stop是true，那么表明timer要被停止了，那线程也要结束，所以一个break跳出最开始的while (true)循环，让线程执行结束。// 如果m_stop是false，那表明现有可能有定时任务需要执行了。取出第一个任务m_tasks.begin()，也是按时间排序最靠前的任务。用任务的时刻和当前时刻对比：// 如果“时辰已到”，那就执行。执行的之后需要注意的是，要将锁释放lock.unlock()，因为继续持有没有任何意义，反而会阻塞住对m_tasks的一些操作。// 如果“时辰未到”，那就执行m_condition.wait_for，让当前线程休眠，直到std::chrono::milliseconds(task_time - cur_time)这段时间过去或者被唤醒。void run(){while (true){std::unique_lock<std::mutex> lock(m_tasks_mutex);m_condition.wait(lock, [this]() -> bool { return !m_tasks.empty() || m_stop; });if (m_stop){break;}uint64_t cur_time = now();std::multimap<uint64_t, Task>::iterator it = m_tasks.begin();uint64_t task_time = it->first;if (cur_time >= task_time){Task &cur_task = it->second;if (!cur_task.removed){lock.unlock();cur_task.func();lock.lock();if (cur_task.repeated && !cur_task.removed){uint64_t when = cur_time + cur_task.period;Task new_task(cur_task.id, cur_task.period, cur_task.repeated, cur_task.func);m_tasks.insert({when, new_task});}}m_tasks.erase(it);}else{m_condition.wait_for(lock, std::chrono::milliseconds(task_time - cur_time));}}}uint64_t now() //ms{auto now = std::chrono::system_clock::now();auto duration = now.time_since_epoch();return std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();}
};} // namespace CC// 格式化时间，精确到毫秒.
std::string getTimeString()
{auto now = std::chrono::system_clock::now();auto duration = now.time_since_epoch();auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();std::time_t time = std::chrono::system_clock::to_time_t(now);std::tm *tm = std::localtime(&time);std::stringstream ss;ss << std::put_time(tm, "%Y-%m-%d %H:%M:%S") << "." << std::setw(3) << std::setfill('0') << millis % 1000;return ss.str();
}// 待执行的任务
void theTask(int id)
{std::cout << getTimeString() << " id = " << id << std::endl;
}int main()
{CC::Timer *timer = new CC::Timer();timer->add(3000, false, std::bind(theTask, 1));uint64_t id = timer->add(2000, true, std::bind(theTask, 2));timer->add(1000, true, std::bind(theTask, 3));std::this_thread::sleep_for(std::chrono::seconds(3));timer->remove(id);std::this_thread::sleep_for(std::chrono::seconds(1));delete timer;std::this_thread::sleep_for(std::chrono::seconds(1));return 0;
}