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 2018. The current version is 1.89.0.

ssl_stream::impl_struct

Structure for use with deprecated impl_type.

using impl_struct = typename ssl_stream_type::impl_struct;