std::lock_guard
http://www.cplusplus.com/reference/mutex/lock_guard/
用于托管一个mutex 变量，负责对齐加锁解锁。
A lock guard is an object that manages a mutex object by keeping it always locked.
On construction, the mutex object is locked by the calling thread, and on destruction, the mutex is unlocked. It is the simplest lock, and is specially useful as an object with automatic duration that lasts until the end of its context. In this way, it guarantees the mutex object is properly unlocked in case an exception is thrown.
Note though that the lock_guard object does not manage the lifetime of the mutex object in any way: the duration of the mutex object shall extend at least until the destruction of the lock_guard that locks it.
MyNote:
@ 有变量std::mutex myMutex; 需要用其保护一段critical section, 则需要在前后使用myMutex.lock(), myMutex.unlock()函数。缺点是lock和unlock需要成对出现，如果critical section出现异常，则后面的unlock不会执行了，容易导致死锁。使用std::lock_gard<mutex> myLockGuard(myMutex) ， （lock_guard的构造函数将myMutex上锁）， 可以以并解决这些个缺点。
@ lock_guard object does not manage the lifetime of the mutex object in any way。 lock_guard的析构函数调用时，会unLock其托管的mutex变量，如果这时该变量已经消失了那就没有意义了。故，lock_guard保存的是其托管的mutex对象的引用/地址吧。
std::unique_lock
http://www.cplusplus.com/reference/mutex/unique_lock/
A unique lock is an object that manages a mutex object with unique ownership in both states: locked and unlocked.
On construction (or by move-assigning to it), the object acquires a mutex object, for whose locking and unlocking operations becomes responsible.
The object supports both states: locked and unlocked.
This class guarantees an unlocked status on destruction (even if not called explicitly). Therefore it is especially useful as an object with automatic duration, as it guarantees the mutex object is properly unlocked in case an exception is thrown.
Note though, that the unique_lock object does not manage the lifetime of the mutex object in any way: the duration of the mutex object shall extend at least until the destruction of the unique_lock that manages it.
MyNote:
@ unique_lock的功能比lock_guard更多，从成员函数就可以看出
例子1： try_lock_for ?
http://www.cplusplus.com/reference/mutex/unique_lock/try_lock_for/
Ref：
https://www.cnblogs.com/xudong-bupt/p/9194394.html