Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is an older version of Boost and was released in 2019. The current version is 1.89.0.

posix::basic_descriptor::impl_

detail::io_object_impl< detail::reactive_descriptor_service, Executor > impl_;