In quite some user-space code the R side has something like

weightedLeastSquares(X, y, w)      # ie y ~ X given weights w

where the third argument is options, ie

weightedLeastSquares(X, y, NULL)

In that case we generally cannot have the third strongly typed as NumericMatrix or alike and have to keep it a SEXP in the interface. Nothing wrong with that but a bit of an eyesore.

Following some discussion with @dcdillon I have written a really simple wrapper class Nullable that contains a SEXP as well as a bool for state. With that we can do (stylized)

...., NumericVector & w) {
//...
if (!w.isNull()) {
    // do stuff with w
}

If there is no protest -- or someone reminds of something much better we already have ;-) -- then I'll fold that in.