[Rcpp-devel] Wrapping std::vector when Rcpp::wrap is user-overloaded for T

[Nathanael Fillmore]

Hi all,

Thanks for the great library, which I have been using very heavily lately.
I have been struggling to get Rcpp::wrap(x) to work when x is a
std::vector of a type with a user-defined wrap function ("non-intrusive
extension"). Specifically, I wrote a Rcpp::wrap overload for std::pair<T,
S>, and I want to wrap an std::vector<std::pair<T, S>>, but haven&#146;'t
been
able to get this to work.

Following the instructions at &#147;Extending Rcpp&#148;,
https://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-extending.pdf,
I wrote the following wrapper for std::pair<T, S>:

#include <map>
#include <string>
#include <utility>
#include <vector>
#include <RcppCommon.h>

// Overloads of "wrap" and "as" need to be in between RcppCommon.h and Rcpp.h
// or RcppArmadillo.h.
namespace Rcpp
{
template<typename T, typename S>
SEXP wrap(std::pair<T, S> pair)
{
 std::map<std::string, SEXP> ret {
   std::make_pair(std::string("first"), Rcpp::wrap(pair.first)),
   std::make_pair(std::string("second"), Rcpp::wrap(pair.second))
 };
 return Rcpp::wrap(ret);
}
}

#include <Rcpp.h>

I then wrote the following test function, which compiles and works as
expected:

RcppExport SEXP TestPair()
{
std::pair<int, std::string> p = std::make_pair(10, "hi");
return Rcpp::wrap(p);
}

I expected that the following function (which is along the lines of what I
wanted to use in my real project) would also work:

RcppExport SEXP TestVectorPair()
{
std::vector<std::pair<int, std::string>> p {
 std::make_pair(10, "hi"),
 std::make_pair(12, "hello"),
 std::make_pair(14, "ok")
};
return Rcpp::wrap(p);
}

However, compilation fails with the error messages pasted at the bottom of
the mail. My questions are as follows: Am I making an obvious mistake here
somewhere? Or, if not, is there some way to achieve this? Obviously, a
simple thing to do would be to write a Rcpp::wrap overloaded for
std::vector<std::pair<T, S>>, but that seems a bit inelegant.

Many thanks,
Nate

Platform info:
Linux
gcc (GCC) 5.1.0
R 3.2.0
Rcpp 0.12.1

Error message:
g++ -I/u/deweylab/sw/R-3.2.0/arch/x86_64-redhat-linux-gnu/lib64/R/include
-DNDEBUG  -I/usr/local/include   -std=c++11 -g -UNDEBUG -std=c++11
-I/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include -fpic
-g -O2  -c test.cpp -o test.o
In file included from
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/RcppCommon.h:179:0,
              from test.cpp:5:
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:
In instantiation of &#145;SEXPREC*
Rcpp::internal::wrap_dispatch_unknown_iterable(const T&,
Rcpp::traits::false_type) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;:
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:732:39:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch_unknown(const T&,
Rcpp::traits::false_type) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:770:30:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch_eigen(const T&,
Rcpp::traits::false_type) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:787:28:
 required from &#145;SEXPREC*
Rcpp::internal::wrap_dispatch_unknown_importable(const T&,
Rcpp::traits::false_type) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:807:41:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch(const T&,
Rcpp::traits::wrap_type_unknown_tag) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap_end.h:30:38:
 required from &#145;SEXPREC* Rcpp::wrap(const T&) [with T = std::pair<int,
std::__cxx11::basic_string<char> >; SEXP = SEXPREC*]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:188:37:
 [ skipping 8 instantiation contexts, use -ftemplate-backtrace-limit=0 to
disable ]
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:732:39:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch_unknown(const T&,
Rcpp::traits::false_type) [with T = std::vector<std::pair<int,
std::__cxx11::basic_string<char> > >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:770:30:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch_eigen(const T&,
Rcpp::traits::false_type) [with T = std::vector<std::pair<int,
std::__cxx11::basic_string<char> > >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:787:28:
 required from &#145;SEXPREC*
Rcpp::internal::wrap_dispatch_unknown_importable(const T&,
Rcpp::traits::false_type) [with T = std::vector<std::pair<int,
std::__cxx11::basic_string<char> > >; SEXP = SEXPREC*;
Rcpp::traits::false_type = Rcpp::traits::integral_constant<bool,
false>]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:807:41:
 required from &#145;SEXPREC* Rcpp::internal::wrap_dispatch(const T&,
Rcpp::traits::wrap_type_unknown_tag) [with T = std::vector<std::pair<int,
std::__cxx11::basic_string<char> > >; SEXP = SEXPREC*]&#146;
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap_end.h:30:38:
 required from &#145;SEXPREC* Rcpp::wrap(const T&) [with T =
std::vector<std::pair<int, std::__cxx11::basic_string<char> > >; SEXP =
SEXPREC*]&#146;
test.cpp:37:22:   required from here
/ua/nathanae/R/x86_64-unknown-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:522:2:
error: static assertion failed: cannot convert type to SEXP
static_assert( !sizeof(T), "cannot convert type to SEXP" ) ;
^

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