[Rcpp-devel] operator<< issues

[Tim Keitt]

http://www.keittlab.org/

On Wed, Mar 14, 2018 at 12:51 PM, Iñaki Úcar <i.ucar86 at gmail.com> wrote:

> Hi all,
>
> I'm not sure whether this is a bug or not, so I think this is the
> right place to start with. Consider the following code:
>
> Rcpp::sourceCpp(code='
>   #include <Rcpp.h>
>   using namespace Rcpp;
>
>   // [[Rcpp::export]]
>   void print_fun(Function x) {
>     Rcout << x << std::endl;
>   }
>
>   // [[Rcpp::export]]
>   void print_env(Environment x) {
>     Rcout << x << std::endl;
>   }
> ')
>
> print_fun(function() {})
> print_env(environment())
>
> It compiles and the output from the functions are two addresses. So
> far, so good. However, if we try the same for a data frame, the
> compilation fails, so we need to define the operator<< as follows:
>
> Rcpp::sourceCpp(code='
>   #include <Rcpp.h>
>   using namespace Rcpp;
>
>   inline std::ostream& operator<<(std::ostream& out, const DataFrame& df) {
>     out << "data.frame";
>     return out;
>   }
>
>   // [[Rcpp::export]]
>   void print_df(DataFrame x) {
>     Rcout << x << std::endl;
>   }
> ')
>
> print_df(data.frame(x=1))
>
> Now, it compiles and produces the output we defined. Once more, so
> far, so good. Now the problem comes when we try to merge the two
> examples above, that is:
>
> Rcpp::sourceCpp(code='
>   #include <Rcpp.h>
>   using namespace Rcpp;
>
>   inline std::ostream& operator<<(std::ostream& out, const DataFrame& df) {
>     out << "data.frame";
>     return out;
>   }
>
>   // [[Rcpp::export]]
>   void print_df(DataFrame x) {
>     Rcout << x << std::endl;
>   }
>
>   // [[Rcpp::export]]
>   void print_fun(Function x) {
>     Rcout << x << std::endl;
>   }
>
>   // [[Rcpp::export]]
>   void print_env(Environment x) {
>     Rcout << x << std::endl;
>   }
> ')
>
> The compilation fails again due to an ambiguous overload for Function
> and Environment types, so that we need to define the operator<< for
> these classes too in order to disambiguate and fix this. I suppose it
> may happen for other classes too. Is this... expected? Desirable? At
> the very least, it is confusing from my point of view.
>

Why not something like:

Rcpp::sourceCpp(code='
  #include <Rcpp.h>
   using Rcpp::Rcout;

   // [[Rcpp::export]]
   void print_addr(SEXP x){
                Rcout << static_cast<void*>(x) << std::endl;
 }')

I'm not sure why one would expect Rcpp types to automatically yield a
pointer appropriate for printing.

THK



>
> Regards,
> Iñaki
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> Rcpp-devel at lists.r-forge.r-project.org
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