Rcpp modules: Validator function for exposed constructors with same number of parameters

Question

I would like to expose a C++ class with Rcpp modules that has two constructors with the same number of parameters. As documented in the Rcpp modules vignette, this can be achieved by using a validator function as the second argument to .constructor with the type

typedef bool (*ValidConstructor)(SEXP*,int);

Could somebody provide an example of how it is supposed to be used? In the vignette, there is only a TODO note that an example should be included there.

Answer 1

With the disclaimer that I haven't personally used this before, the following appears to work:

#include <Rcpp.h>
using namespace Rcpp;

class Example {
public:
    Example(int x_, int y_)
        : x(x_), y(y_)
    {
        Rcout << __PRETTY_FUNCTION__ << "\n";
    }

    Example(std::string x_, std::string y_)
        : x(x_.size()), y(y_.size())
    {
        Rcout << __PRETTY_FUNCTION__ << "\n";
    }

    int add() const
    { return x + y; }

private:
    int x, y;
};

bool validate_int_int(SEXP* args, int nargs)
{ return TYPEOF(args[0]) == INTSXP && TYPEOF(args[1]) == INTSXP; }

bool validate_string_string(SEXP* args, int nargs)
{ return TYPEOF(args[0]) == STRSXP && TYPEOF(args[1]) == STRSXP; }

RCPP_MODULE(ExampleMod) {

    class_<Example>("Example")
        .constructor<int, int>(
            "(int, int) constructor",
            validate_int_int
        )
        .constructor<std::string, std::string>(
            "(string, string) constructor",
            validate_string_string
        )
        .method("add", &Example::add)
    ;
}

---

Testing from R,

ex.int <- new(Example, 1L, 2L)
# Example::Example(int, int)

ex.string <- new(Example, "one", "two")
# Example::Example(std::string, std::string)

ex.int$add()
# [1] 3

ex.string$add()
# [1] 6 

---

In each of validate_int_int and validate_string_string we are just testing the input SEXP types against the signatures of their corresponding constructors.