Linux API is accessible from C++ the same way it is accessible from C: for example you can #include <sys/inotify.h> in a C++ program and call inotify_init() or inotify_add_watch() directly.
(granted, to use it sanely, you may want to wrap it into a boost::asio::posix::stream_descriptor and spend a bit of time on a good read handler.. but that's something that a library, not the OS, is meant for)
However, there are C++ libraries with make things easier to use reliably, or use things in portable way. These libraries are avaliable in a ad-hoc manner rather than in a systematic way.
Boost is a defacto standard C++ library whose emphasis is portablilty (and efficienty?). It provides a lot of commonly used, useful stuff.
Many add on packages provide C++ bindings, such as GTK+ for the Gnome Toolkit, ...