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

The type of allocator associated with this object.

using allocator_type = net::associated_allocator_t< Handler, Allocator >;

If a class derived from boost::beast::async_base is a completion handler, then the associated allocator of the derived class will be this type.