When to use Mutex C ++ 11, lock, unique_lock, shared_lock, etc.

Question

• What is the difference between shared_lock and shared_mutex.lock_shared (), except that the shared_lock destructor unlocks the associated mutex?
• Is shared_mutex the only mutex class that I can use with shared_lock?
• Why does anyone want to use lock_guard instead of unique_lock?
• If I have many threads that constantly block reading (shared_lock) of a variable, and I have a variable that tries to lock it for writing (unique_lock), will this message flow take precedence over others?
• Is there a chance for a dead end for # 4?