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 2013. The current version is 1.90.0.

ssl::basic_context::impl_type

The native implementation type of the locking dispatcher.

typedef service_type::impl_type impl_type;