Converting from Rcpp

Vectors

Note that each cpp11 vector class has a read-only and writeable version. The default classes, e.g. cpp11::doubles are read-only classes that do not permit modification. If you want to modify the data you or create a new vector, use the writeable variant.

Another major difference in Rcpp and cpp11 is how vectors are grown. Rcpp vectors have a push_back() method, but unlike std::vector() no additional space is reserved when pushing. This makes calling push_back() repeatably very expensive, as the entire vector has to be copied each call. In contrast cpp11 vectors grow efficiently, reserving extra space. See https://cpp11.r-lib.org/articles/motivations.html#growing-vectors for more details.

Rcpp also allows very flexible implicit conversions, e.g. if you pass a REALSXP to a function that takes a Rcpp::IntegerVector() it is implicitly converted to a INTSXP. These conversions are nice for usability, but require (implicit) duplication of the data, with the associated runtime costs. cpp11 throws an error in these cases. If you want the implicit coercions you can add a call to as.integer() or as.double() as appropriate from R when you call the function.

Other objects

Functions

Note that cpp11::stop() and cpp11::warning() are thin wrappers around Rf_stop() and Rf_warning(). These are simple C functions with a printf() API, so they do not understand C++ objects like std::string. Therefore you need to call obj.c_str() when passing string data to them.

R functions

Calling R functions from C++ is similar to using Rcpp.

// Rcpp -----------------------------------------------
Rcpp::Function as_tibble("as_tibble", Rcpp::Environment::namespace_env("tibble"));
as_tibble(x, Rcpp::Named(".rows", num_rows), Rcpp::Named(".name_repair", name_repair));

// cpp11 -----------------------------------------------
using namespace cpp11::literals; // so we can use ""_nm syntax

auto as_tibble = cpp11::package("tibble")["as_tibble"];
as_tibble(x, ".rows"_nm = num_rows, ".name_repair"_nm = name_repair);

Unsupported Rcpp features

• None of Modules
• None of Sugar
• Some parts of Attributes
  • No dependencies
  • No random number generator restoration
  • No support for roxygen2 comments
  • No interfaces

RNGs

Rcpp includes calls to GetRNGstate() and PutRNGstate() around the wrapped function. This ensures that if any C++ code calls the R API functions unif_rand(), norm_rand(), exp_rand(), or R_unif_index() the random seed state is set accordingly. cpp11 does not do this, so you must include the calls to GetRNGstate() and PutRNGstate() yourself if you use any of those functions in your C++ code. See R-exts 6.3 - Random number generation for details on these functions.

One convenient way to do safely is to use a simple class:

class local_rng {
public:
  local_rng() {
    GetRNGstate();
  }

  ~local_rng(){
    PutRNGstate();
  }
};

void foo() {
  local_rng rng_state;
  /* my code using the RNG */
}

STL includes

Rcpp.h includes a number of STL headers automatically, notably <string> and <vector>, however the cpp11 headers generally do not. If you have errors like

error: no type named 'string' in namespace 'std'

You will need to include the appropriate STL header, in this case <string>.

Strict headers

If you see something like this:

 In file included from file.cpp:1:
 In file included from path/cpp11/include/cpp11.hpp:3:
 path/cpp11/include/cpp11/R.hpp:12:9: warning: 'STRICT_R_HEADERS' macro redefined [-Wmacro-redefined]
 #define STRICT_R_HEADERS

Make sure to remove PKG_CPPFLAGS=-DSTRICT_R_HEADERS from src/Makevars.

R API includes

cpp11 conflicts with macros declared by some R headers unless the macros R_NO_REMAP and STRICT_R_HEADERS are defined. If you include cpp11.hpp (or, at a minimum, cpp11/R.hpp) before any R headers these macros will be defined appropriately, otherwise you may see errors like

R headers were included before cpp11 headers and at least one of R_NO_REMAP or STRICT_R_HEADERS was not defined.

Which indicate that you must either change your include order or add preprocessor definitions for R_NO_REMAP and STRICT_R_HEADERS. Note that transitive includes of R headers (for example, those included by Rcpp.h) can also introduce the conflicting macros.

Type aliases

If you use typedefs for cpp11 types or define custom types you will need to define them in a pkgname_types.hpp file so that cpp_register() can include it in the generated code.

Logical vector construction

If you are constructing a length 1 logical vector you may need to explicitly use a r_bool() object in the initializer list rather than TRUE, FALSE or NA_INTEGER. This issue only occurs with the clang compiler, not gcc. When constructing vectors with more than one element this is not an issue

// bad
cpp11::writable::logicals({FALSE});

// good
cpp11::writable::logicals({r_bool(FALSE)});

// good
cpp11::writable::logicals({FALSE, NA_LOGICAL});