message::MessageRequest::MessageRequest req;
req.set_msg("yoyoyo");

message::MessageRequest req;
std::vector<message::Pair> pairs;
for (auto& v: pairs) {
  //type: message::MessageRequest::field*  
  auto pair = req.add_pairs();
  pair->set_key('kk');
  pair->set_value('vv');
}

mutable_xx()->set_xx(/*xx*/);

// this is OK
std::string to_string(int i);
std::string to_string(bool b);

std::string si = to_string(0);
std::string sb = to_string(true);

// this is not OK
int from_string(std::string_view s);
bool from_string(std::string_view s);

int i = from_string("7");
bool b = from_string("false");

struct to_string_t
{
    std::string_view s;

    operator int() const;  // int  from_string(std::string_view s);
    operator bool() const; // bool from_string(std::string_view s);
};

int i = to_string_t{"7"};
bool b = to_string_t{"true"};

// base template, specialize and provide a static from_string method
template <class T, class = void>
struct to_string_impl 
{
};

namespace detail // hide impl detail
{
template <class T>
auto has_from_string(int) -> decltype(
    to_string_impl<T>::from_string(std::declval<std::string_view>()), 
    std::true_type{});

template <class T>
std::false_type has_from_string(char);
}

// check if T has a from_string
template <class T>
constexpr bool has_from_string = decltype(detail::has_from_string<T>(0))::value;

// return-type overload mechanism
struct to_string_t
{
    std::string_view s;

    template <class T>
    operator T() const 
    {
        static_assert(has_from_string<T>, "conversion to T not supported");
        return to_string_impl<T>::from_string(s); 
    }
};

// convenience wrapper to provide a "return-type overloaded function"
to_string_t from_string(std::string_view s) { return to_string_t{s}; }



//各种类型的实现自己拼好就可以了
template <>
struct to_string_impl<int>
{
    static int from_string(std::string_view s);
};

template <>
struct to_string_impl<bool>
{
    static bool from_string(std::string_view s);
};

//自定义实现
template <class T>
struct my_range { /* ... */ };

template <class T>
struct to_string_impl<my_range<T>, std::enable_if_t<has_from_string<T>>>
{
    static my_range<T> from_string(std::string_view s);
};

//cpu 优先级

    if (cpu_priority < current_cpu_priority) {
      TEST_SYNC_POINT_CALLBACK("ThreadPoolImpl::BGThread::BeforeSetCpuPriority",
                               &current_cpu_priority);
      // 0 means current thread.
      port::SetCpuPriority(0, cpu_priority);
      current_cpu_priority = cpu_priority;
      TEST_SYNC_POINT_CALLBACK("ThreadPoolImpl::BGThread::AfterSetCpuPriority",
                               &current_cpu_priority);
    }
//IO优先级
#ifdef OS_LINUX
    if (decrease_io_priority) {
#define IOPRIO_CLASS_SHIFT (13)
#define IOPRIO_PRIO_VALUE(class, data) (((class) << IOPRIO_CLASS_SHIFT) | data)
      // Put schedule into IOPRIO_CLASS_IDLE class (lowest)
      // These system calls only have an effect when used in conjunction
      // with an I/O scheduler that supports I/O priorities. As at
      // kernel 2.6.17 the only such scheduler is the Completely
      // Fair Queuing (CFQ) I/O scheduler.
      // To change scheduler:
      //  echo cfq > /sys/block/<device_name>/queue/schedule
      // Tunables to consider:
      //  /sys/block/<device_name>/queue/slice_idle
      //  /sys/block/<device_name>/queue/slice_sync
      syscall(SYS_ioprio_set, 1,  // IOPRIO_WHO_PROCESS
              0,                  // current thread
              IOPRIO_PRIO_VALUE(3, 0));
      low_io_priority = true;
    }
#else
    (void)decrease_io_priority;  // avoid 'unused variable' error
#endif

#include <condition_variable>  
#include <functional>
#include <list>
#include <mutex>  
#include <string>
#include <thread> 
#include <vector>

class ThreadPool {
 private:
  const int num_workers_;
  std::list<std::function<void()> > tasks_;
  std::mutex mutex_;
  std::condition_variable condition_;
  std::condition_variable capacity_condition_;
  bool waiting_to_finish_ = false;
  bool waiting_for_capacity_ = false;
  bool started_ = false;
  int queue_capacity_ = 2e9;
  std::vector<std::thread> all_workers_;

  void RunWorker(void* data) {
    ThreadPool* const thread_pool = reinterpret_cast<ThreadPool*>(data);
    std::function<void()> work = thread_pool->GetNextTask();
    while (work != NULL) {
      work();
      work = thread_pool->GetNextTask();
    }

 public:
  ThreadPool(const std::string& prefix, int num_workers)
      : num_workers_(num_workers) {}

  ~ThreadPool() {
    if (started_) {
      std::unique_lock<std::mutex> mutex_lock(mutex_);
      waiting_to_finish_ = true;
      mutex_lock.unlock();
      condition_.notify_all();
      for (int i = 0; i < num_workers_; ++i) {
        all_workers_[i].join();
      }
    }
  }

  void SetQueueCapacity(int capacity) {
    queue_capacity_ = capacity;
  }

  void StartWorkers() {
    started_ = true;
    for (int i = 0; i < num_workers_; ++i) {
      all_workers_.push_back(std::thread(&RunWorker, this));
    }
  }

  std::function<void()> GetNextTask() {
    std::unique_lock<std::mutex> lock(mutex_);
    for (;;) {
      if (!tasks_.empty()) {
        std::function<void()> task = tasks_.front();
        tasks_.pop_front();
        if (tasks_.size() < queue_capacity_ && waiting_for_capacity_) {
          waiting_for_capacity_ = false;
          capacity_condition_.notify_all();
        }
        return task;
      }
      if (waiting_to_finish_) {
        return nullptr;
      } else {
        condition_.wait(lock);
      }
    }
    return nullptr;
  }

  void Schedule(std::function<void()> closure) {
    std::unique_lock<std::mutex> lock(mutex_);
    while (tasks_.size() >= queue_capacity_) {
      waiting_for_capacity_ = true;
      capacity_condition_.wait(lock);
    }
    tasks_.push_back(closure);
    if (started_) {
      lock.unlock();
      condition_.notify_all();
    }
  }
};

#include <condition_variable>
#include <mutex>
#include <thread>
#include <queue>
#include <list>

class DynamicThreadPool
{
public:
    explicit DynamicThreadPool(int reserve_threads)
      :shutdown_(false), reserve_threads_(reserve_threads), nthreads_(0), threads_waiting_(0){
          for (int i = 0; i < reserve_threads_; i++) {
              std::lock_guard<std::mutex> lock(mu_);
              nthreads_++;
              new DynamicThread(this);
          }
      }
  
    ~DynamicThreadPool() {
        std::unique_lock<std::mutex> lock_(mu_);
        shutdown_ = true;
        cv_.notify_all();

        while (nthreads_ != 0) {
            shutdown_cv_.wait(lock_);        
        }

        ReapThreads(&dead_threads_);    
    }

    void Add(const std::function<void()> &callback) {
        std::lock_guard<std::mutex> lock(mu_);

        // Add works to the callbacks list
        callbacks_.push(callback);

        // Increase pool size or notify as needed
        if (threads_waiting_ == 0) {
            // Kick off a new thread
            nthreads_++;
            new DynamicThread(this);
        } else {
            cv_.notify_one();
        }

        // Also use this chance to harvest dead threads
        if (!dead_threads_.empty()) {
            ReapThreads(&dead_threads_);
        }
    }

private:
    class DynamicThread {
    public:
        DynamicThread(DynamicThreadPool* pool):pool_(pool),thd_(new std::thread(&DynamicThreadPool::DynamicThread::ThreadFunc, this)){}
        ~DynamicThread() {
            thd_->join();
            thd_.reset();    
        }

    private:
        DynamicThreadPool* pool_;
        std::unique_ptr<std::thread> thd_;
        void ThreadFunc() {
          pool_->ThreadFunc();

          // Now that we have killed ourselves, we should reduce the thread count
          std::unique_lock<std::mutex> lock(pool_->mu_);
          pool_->nthreads_--;

          // Move ourselves to dead list
          pool_->dead_threads_.push_back(this);

          if ((pool_->shutdown_) && (pool_->nthreads_ == 0)) {
              pool_->shutdown_cv_.notify_one();
          }
      }
    };
    
    std::mutex mu_;
    std::condition_variable cv_;
    std::condition_variable shutdown_cv_;
    bool shutdown_;
    std::queue<std::function<void()>> callbacks_;
    int reserve_threads_;
    int nthreads_;
    int threads_waiting_;
    std::list<DynamicThread*> dead_threads_;

    void ThreadFunc() {
        for (;;) {
            std::unique_lock<std::mutex> lock(mu_);
            // Wait until work is available or we are shutting down.
            if (!shutdown_ && callbacks_.empty()) {
                // If there are too many threads waiting, then quit this thread
                if (threads_waiting_ >= reserve_threads_)
                    break;
                threads_waiting_++;
                cv_.wait(lock);
                threads_waiting_--;
            }

            // Drain callbacks before considering shutdown to ensure all work gets completed.
            if (!callbacks_.empty()) {
                auto cb = callbacks_.front();
                callbacks_.pop();
                lock.unlock();
                cb();            
            } else if (shutdown_)
                break;            

        }
    }
  
    static void ReapThreads(std::list<DynamicThread*>* tlist) {
        for (auto t = tlist->begin(); t != tlist->end(); t = tlist->erase(t)) {
            delete *t;
        }
    }
};

makeFutureWith(x)
  .via(exe1).then(y)
  .via(exe2).then(z);

void connection::do_read() 
{
  socket_.async_read_some(in_buffer_, 
    wrap(strand_, [this](error_code ec, size_t n)
      {
        if (!ec) do_read();
      }));
}
//strand_ 是asio中的概念，可以简单理解成executor，换成pool之类的也是可以的