My initial thought was to have C++ source blocks that can be normally executed with babel, making functions defined in them available to R through cpp11. Muddling through the errors I encountered, I discovered a couple things I needed to do:

1. Tell the compiler where the cpp11 R library’s headers were.

   cpp11 defines all of its functionality in header files, which come with the R library when it is installed. These live in the standard location for “extra” stuff required in an R packakge, the inst/ directory, in an include/ subfolder. This directory is unpacked to the top level library directory on installation

        list.files(system.file("include", package = "cpp11"), recursive = T)
   

       [1] "cpp11.hpp"                  "cpp11/altrep.hpp"          
       [3] "cpp11/as.hpp"               "cpp11/attribute_proxy.hpp" 
       [5] "cpp11/data_frame.hpp"       "cpp11/declarations.hpp"    
       [7] "cpp11/doubles.hpp"          "cpp11/environment.hpp"     
       [9] "cpp11/external_pointer.hpp" "cpp11/function.hpp"        
      [11] "cpp11/integers.hpp"         "cpp11/list_of.hpp"         
      [13] "cpp11/list.hpp"             "cpp11/logicals.hpp"        
      [15] "cpp11/matrix.hpp"           "cpp11/named_arg.hpp"       
      [17] "cpp11/protect.hpp"          "cpp11/r_bool.hpp"          
      [19] "cpp11/r_string.hpp"         "cpp11/r_vector.hpp"        
      [21] "cpp11/R.hpp"                "cpp11/raws.hpp"            
      [23] "cpp11/sexp.hpp"             "cpp11/strings.hpp"         
      [25] "fmt/core.h"                 "fmt/format-inl.h"          
      [27] "fmt/format.h"
   

   These files must be on the include path of the compiler. The include path can be modified using the :flags argument of the babel src block. On my machine this looked like

        :flags -I/opt/homebrew/Cellar/r/4.2.1_3 -I/Users/ejneer/Library/R/arm64/4.2/library/cpp11/include
   

2. Stop babel from wrapping the C++ in a main method.

   Since I only intend define functions in C++ and use them from R, I don’t need a main method. If there is not main method defined in the C++ src blocks, default babel behavior is to wrap the block in a simple main method. From the docs:

   If a main method is not present in a code block then the entire block is wrapped in a trivial main function call.

   This behavior can be turned off with another header argument, :main no.

   With these settings in place, I no longer encountered cpp11 or babel related errors. I still encountered other errors that I think were related to using standard library functions, and ultimately prevented me from compiling the src blocks like I wanted. I don’t know if this is actually possible. An example error is below.

         /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_ctype.h:128:1: error: expected unqualified-id
         __BEGIN_DECLS
         ^
   

   I settled on another method to integrate cpp11 and babel that feels pretty natural to me.

It is possible to “tangle” src blocks, meaning the code in the block is extracted and put in to a file that you can optionally name. This is useful since the cpp11 library provides a way to “source” c++ files, much in the same way you can source an R script. The workflow then comprises two steps:

1. Tangle the C++ src block
2. Source the C++ file in an R block with cpp_source

By default, the tangled file is named the same as the org file, using a file extension appropriate for the src block type, here .cpp. Additionally, the :eval header arg can be set to never-export so babel does not try to execute the src block on export, which would error (as above) for C++ blocks. An example is taken from the cpp11 docs to show this workflow.

Tangle this block:

  #include "cpp11/doubles.hpp"
  using namespace cpp11;

  [[cpp11::register]]
  doubles pdist_cpp(double x, doubles ys) {
    int n = ys.size();
    writable::doubles out(n);
    for(int i = 0; i < n; ++i) {
      out[i] = sqrt(pow(ys[i] - x, 2.0));
    }
    return out;
  }

Source it here:

  library(cpp11)
  cpp_source("trying_out_cpp11_babel.cpp")

  pdist_cpp(6, runif(5))