Thread里面的CountDownLatch的作用是什么？

[yinghaoyu]

仅仅是为了new thread first 吗？

[chenshuo]

https://www.zhihu.com/question/294270506/answer/489820748

为了保证 Thread::start() 之后 Thread::tid() 立刻能拿到正确的值。
如果没有 latch_，会有 race condition，即调用 Thread::tid() 的时候线程还没有启动，结果返回初值 0。

struct ThreadData
{
  void runInThread()
  {
    *tid_ = muduo::CurrentThread::tid();
    tid_ = NULL;
    latch_->countDown();
    latch_ = NULL;

使用方法：

map<pid_t, unique_ptr<Thread>> threads;
Thread* thr = new Thread(...);
thr->start();
threads[thr->tid()].reset(thr);

[yinghaoyu]

It make sence.