[Rcpp-devel] Extract a function from a list and create a call

[Douglas Bates]

I must be missing something horribly obvious but I have now spent
several hours trying to find a way to extract a character string from
a list.  I am more-or-less certain that I am copying a construction in
the unitTests/runit.CharacterVector.R but the compiler just keeps
spitting out error messages at me.

The background is that a glm "family" object in R is a named list (or
should be, it has an S3 class but that doesn't mean anything about the
structure).  The standard ones look like

> str(poisson())
List of 12
 $ family    : chr "poisson"
 $ link      : chr "log"
 $ linkfun   :function (mu)
 $ linkinv   :function (eta)
 $ variance  :function (mu)
 $ dev.resids:function (y, mu, wt)
 $ aic       :function (y, n, mu, wt, dev)
 $ mu.eta    :function (eta)
 $ initialize:  expression({     if (any(y < 0))
stop("negative values not allowed for the Poisson family")     n <-
rep.int(1, nobs)     mustart <- y + 0.1 })
 $ validmu   :function (mu)
 $ valideta  :function (eta)
 $ simulate  :function (object, nsim)
 - attr(*, "class")= chr "family"

All I want to do is to take such a list and extract the "family" and
"link" strings as std::string.  If the name of the Rcpp::List object
in the C++ code is lst, I can get as far as

StringVector fam = lst["family"];

but after that it all turns to custard.  In the class I am defining
the family member is declared as a std::string.  In the constructor
for the class from a List object I try

family = Rcpp::as<std::string>(fam[0]);

or even, mimicking a couple of the tests in unitTests/runit.CharacterVector.R,

family += fam[0];

but I have not been able to construct anything that my compiler will accept.

So, how do I get the value of one of the elements of a CharacterVector
(or StringVector, I think they are synonyms) as a std::string, short
of using std::string(CHAR(STRING_ELT(fam, 0))