Two workarounds to get access to the c level connections api from c++.

gistfile1.cpp

#include <Rcpp.h>

// thanks to Simon for the macro magic
// problem #1: R internals uses "private" and "class" as 
//             member names. So a C++ compiler is confused
//             when we include Connections.h
//             this workaround uses the preprocessor to rename
//             class as class_name and private as private_ptr
#define class class_name
#define private private_ptr
#include <R_ext/Connections.h>
#undef class
#undef private

// problem #2: getConnection is not part of the R exported API
//             but it is not hidden, so we can cheat and get it 
//             like this. Not sure crandalf won't "you shall not pass"
//             that kind of workaround
extern "C" {
    extern Rconnection getConnection(int) ;
}

using namespace Rcpp ;

// [[Rcpp::export]]
std::string foo(SEXP con_){
    Rconnection con = getConnection( as<int>(con_) ) ;
    return con->class_name ;
}

/*** R
    con <- textConnection( "foobar" ) 
    f   <- file( "con.cpp" );  
    
    foo( con )
    # [1] "textConnection"
    foo( f )
    # [1] "file"
*/

Yes, Simon and I exchanged on that. He is the one who came up with the preprocessor trick.

I was wondering if you had done more work in this direction. Thanks!