The .then() method appears to be something on concurrency::task, which is part of the Microsoft's Parallel Pattern Library. This article says:

"we've started a new project called Casablanca to build a modern C++ library for cloud-based client-server communication, in which all concurrent operations are exposed as PPL tasks."

http://www.drdobbs.com/parallel/improving-futures-and-callba...

Link to Microsoft's C++ committee proposal (N3327): http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n332...