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

ip::address_v6::to_v4

(Deprecated: Use make_address_v4().) Converts an IPv4-mapped or IPv4-compatible address to an IPv4 address.

address_v4 to_v4() const;