I'm shortly before announcing such API for displaying maps and implementing location of POIs and routing based on OpenStreetMap data. The library is designed to work offline. That means you download a *.osm file for your region, process it, get some binary format and then can use the library to acess data.

The C++ library works on the Desktop in that it already shows maps drawn using Cairo and also already does simple routing.

It is however far from perfect and finished if your look at the details because of the complexity of OSM data structures in some areas (relations :-/), some likely performance problems and general code cleanup.

I'm also sure that there will some performance problems on the real device. That are the reasons that I plan to go public and request for help real soon now (need a small web page with some examples and some demos)...

Since libchamplain AFAIK has a plugin concept to integrate renderers it could be possible to join forces (havn't had time yet to take a look). Note however that my library does not focus on fany map displaying but focuses on map rendering and non-GUI data-layer handling for general navigation (but only for OSM).

If anybody is interested don't hesitate to contact me. Don't wait for the announcement.

You can find the sources at https://sourceforge.net/projects/libosmscout/.