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 2022. The current version is 1.89.0.

http::header::is_request

Indicates if the header is a request or response.

using is_request = std::integral_constant< bool, isRequest >;