The Streamulus template / header-only library by Irit Katriel provides a very nice C++ stream processing environment--a domain-specific embedded language for stream processing.
This package provides the Streamulus headers and uses Rcpp for a convenient bi-directional connection to the C++ layer.
The Streamulus and RcppStreams pages.
Dirk Eddelbuettel for this package
Irit Katriel for Streamulus
GPL (>= 3) just like Streamulus