Extended Futures

Tasks in the form of promises and futures have an ambivalent reputation in C++11. On the one hand, they are a lot easier to use than threads or condition variables; on the other hand, they have a significant deficiency. They cannot be composed. C++20/23 overcomes this deficiency.

I have written about tasks in the form of std::async, std::packaged_task, or std::promise and std::future. The details are here: tasks. With C++20/23 we may get extended futures.

Concurrency TS v1

std::future

The name extended futures is quite easy to explain. First, the interface of the C++11 std::future was extended; second, there are new functions for creating special futures that are composable. I start with my first point.

The extended future has three new methods:

• The unwrapping constructor that unwraps the outer future of a wrapped future (future<future<T>>).
• The predicate is_ready that returns if a shared state is available.
• The method then that attaches...