Actually, there are two. One is the original, written by me (Jan Wielemaker), which is part of the standard SWI-Prolog packages. It consists of the file SWI-cpp.h and is documented in:
Now, I'm not much of a C++ programmer and the above interface is pretty simple. It mostly deals with automatic type conversion, mapping of exceptions (both ways) and scoping of Prolog artifacts such as query objects.
Volker Wysk has created another interface that is based on how an experienced C++ programmers looks at Prolog. Unfortunately, this interface is not maintained any more and requires updating for both the current C++ standards and recent SWI-Prolog versions. This pointer is maintained on this page to provide a starting point for anyone looking for a more advanced C++ interface.
Both interfaces are header only, which means that there is no accompanying library. Both interfaces can be freely mixed with each other and with calls to the plain C API documented in the SWI-Prolog reference manual. Have a look here if you are missing something as the C-interface provides more than the two C++ layers on top of it.