Use std::lock_guard with try_lock

A basic design invariant of lock_guard is that it always holds the lock. This minimizes the overhead since its destructor can unconditionally call unlock(), and it doesn’t have to store extra state.

If you need the try-to-lock behavior, use unique_lock:

std::unique_lock<std::mutex> lock(_mutex, std::try_to_lock);
if(!lock.owns_lock()){
    // mutex wasn't locked. Handle it.
}