[Rcpp-devel] Rcpp::Dimension initialized from an SEXP

[Douglas Bates]

The enclosed section of code using Rcpp::Dimension fails to compile
because of the const qualifiers for the simple::nrow and simple::ncol
method functions.  If you omit those const qualifiers then it will
compile and behave as desired.  Of course, I would prefer to have
those method functions use the const qualifier.  Does anyone have
suggestions on how to modify this code so I can use the const
qualifiers?

My guess is that this is related to the Rcpp::Dimension::operator[]()
returning an int& and not an int so somehow there is a delayed
evaluation going on - but I haven't learned enough C++ to be able to
decide exactly where the problem originates.
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library(inline)
cdef <- '
class simple {
    Rcpp::Dimension dd;
public:
    simple(SEXP xp) : dd(xp) {}
    int nrow() const { return dd[0]; }
    int ncol() const { return dd[1]; }
};
'
cpp <- '
simple ss(ia);
return Rcpp::wrap(ss.nrow());
'
ff <- cfunction(signature(ia = "integer"), cpp, Rcpp = TRUE, includes = cdef)