std::unique_lock or std::lock_guard?

Question

I have two use cases.

A. I want to synchronise access to a queue for two threads.

B. I want to synchronise access to a queue for two threads and use a conditio

Answer 1

There are certain common things between lock_guard and unique_lock and certain differences.

But in the context of the question asked, the compiler does not allow using a lock_guard in combination with a condition variable, because when a thread calls wait on a condition variable, the mutex gets unlocked automatically and when other thread/threads notify and the current thread is invoked (comes out of wait), the lock is re-acquired.

This phenomenon is against the principle of lock_guard. lock_guard can be constructed only once and destructed only once.

Hence lock_guard cannot be used in combination with a condition variable, but a unique_lock can be (because unique_lock can be locked and unlocked several times).