In case you use promise and future to synchronize threads, they have much in common with condition variables. But most of the time, tasks are the better choice.

The parent of a thread has to take care of their child. The parent can wait until his child is done or detach himself from his child. But that is not really new. But that will not hold for std::async. The big charm of std::async is that the parent has not taken care of his child.

With std::promise and std::future, you have full control over the task.

std::packaged_task enables you to write a simple wrapper for a callable, which you can invoke later.

std:.async feels like an asynchronous function call. Under the hood std::async is a task. One, which is extremely easy to use.

Tasks were one of the latest additions to the C++11 standard. They give you a better abstraction than threads. In the general case, they should be your first choice. 

Condition variables allow us to synchronize threads via notifications. So, you can implement workflows like sender/receiver or producer/consumer. In such a workflow, the receiver is waiting for the sender's notification. If the receiver gets the notification, it continues its work.

By using the keyword thread_local, you define the thread local data. Thread-local can easily be explained in a few words.

In case the data is not modified when shared between threads, the story is simple. The data has only to be initialized in the thread safe way. It is not necessary to use an expensive lock for each access.

With C++14 came reader-writer locks. The idea is straightforward and promising. Arbitrary reading threads can access the critical region at the same time, but only one thread is allowed to write.