The Rcpp package provides R functions as well as C++ classes which
offer a seamless integration of R and C++. Many R data types and
objects can be mapped back and forth to C++ equivalents which
facilitates both writing of new code as well as easier integration of
third-party libraries.
Documentation about Rcpp is provided by several vignettes included in
this package, via the Rcpp Gallery site at http://gallery.rcpp.org,
the paper by Eddelbuettel and Francois (2011, JSS), and the book by
Eddelbuettel (2013, Springer).
See citation("Rcpp") for details on the last two.