Would you tell me why you use notify_one instead of notify_all here?

[CoinCheung]

thread-pool/include/ThreadPool.h

Line 94 in 99ae49b

m_conditional_lock.notify_one();

I am reading the code and try to learn from it, and I am puzzled that you use notify_one in this line rather than notify_all, would you please tell me why notify_one is better here ?