Latches and Barriers

Latches and barriers are simple thread synchronisation mechanisms which enable some threads to block until a counter becomes zero. At first, don’t confuse the new barriers with memory barriers, also known as fences. In C++20 we get latches and barriers in two variations: std::latch, and std::barrier.

First, there are two questions:

1. What are the differences between these two mechanisms to synchronise threads? You can use a std::latch only once, but you can use a std::barrier more than once. A std::latch is useful for managing one task by multiple threads; a std::barrier is useful for managing repeated tasks by multiple threads. Additionally, a std::barrier enables you to execute a function in the so-called completion step. The completion step is the state when the counter becomes zero.
2. What use cases do latches and barriers support that cannot be done in C++11 and C++14 with futures, threads, or condition variables in combination with locks? Latches...