YAZ++ is an application programming interface (API) to YAZ which supports the development of Z39.50/SRU/Solr client and server applications using C++. Like YAZ, it supports Z39.50-2003 (version 3) as well as SRU version 1.1 thru 2.0 in both the client and server roles. YAZ++ includes an implementation of the ZOOM C++ binding and a generic client/server API based on the Observer/Observable design pattern. (taken from https://www.indexdata.com/resources/software/yazpp/) YAZ++-APU depends on the YZ Toolkit (dev-libs/yaz)
see also https://github.com/gentoo/gentoo/pull/9634