\documentclass{article}
\begin{document}
Hi there!
<<hello>>=
    # Say hello
    hello <- setClass("hello", slots = c(a="character", b="character"))
    sayHi <- hello(a="Hello", b="World")
    paste(sayHi@a, sayHi@b)
    rm(hello)

    hello <- list()
    hello <- sayHi@a
@
Hi again!
\end{document}