Function converter

inst/include/Rcpp/Module.h:564–567

    template <typename FROM, typename TO>
    void converter( const char* from, const char* to, TO (*fun)(FROM), const char* docstring = 0 ){
        std::string fun_name = internal::get_converter_name<FROM,TO>( from, to ) ;
        function( fun_name.c_str(), fun, docstring ) ;
    }

    class CppClass : public S4{
        typedef S4 Base;