std::unique_lock<std::mutex> or std::lock_guard<std::mutex>?

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 condition variable because one of the threads will wait on content to be stored into the queue by the other thread.

For use case A I see code example using std::lock_guard<>. For use case B I see code example using std::unique_lock<>.

What is the difference between the two and which one should I use in which use case?

The difference is that you can lock and unlock a std::unique_lock. std::lock_guard will be locked only once on construction and unlocked on destruction.

So for use case B you definitely need a std::unique_lock for the condition variable. In case A it depends whether you need to relock the guard.

std::unique_lock has other features that allow it to e.g.: be constructed without locking the mutex immediately but to build the RAII wrapper (see here).

std::lock_guard also provides a convenient RAII wrapper, but cannot lock multiple mutexes safely. It can be used when you need a wrapper for a limited scope, e.g.: a member function:

class MyClass{
    std::mutex my_mutex;
    void member_foo() {
        std::lock_guard<mutex_type> lock(this->my_mutex);            
        /*
         block of code which needs mutual exclusion (e.g. open the same 
         file in multiple threads).
        */

        //mutex is automatically released when lock goes out of scope
    }           
};

To clarify a question by chmike, by default std::lock_guard and std::unique_lock are the same. So in the above case, you could replace std::lock_guard with std::unique_lock. However, std::unique_lock might have a tad more overhead.

Note that these days (since, C++17) one should use std::scoped_lock instead of std::lock_guard.