Attributes and classes

basic vector classes(IntegerVector, NumericVector, LogicalVector, CharacterVector)
their scalar(int, double, bool, String)
matrix(IntegerMatrix, NumericMatrix, LogicalMatrix, CharacterMatrix)

All R objects have attributes, which can be queried and modified with .attr(). Rcpp also provides .names() as an alias for the name attribute. The following code snippet illustrates these methods. Note the use of ::create(), a class method. This allows you to create an R vector from C++ scalar values:

// [[Rcpp::export]]
NumericVector attribs() {
  NumericVector out = NumericVector::create(1, 2, 3);
  
  out.names() = CharacterVector::create("a", "b", "c");
  out.attr("my-attr") = "my-value";
  out.attr("class") = "my-class";
  
  return out;
}

For S4 objects, .slot() plays a similar role to .attr().

Lists and data frames

Rcpp also provides classes List and DataFrame.(more useful for output than input)

#include <Rcpp.h>
using namespace Rcpp;

//[[Rcpp::export]]
double mpe(List mod){
  if(!mod.inherits("lm")) stop("Input must be a linear model");
  
  NumericVector resid = as<NumericVector>(mod["residuals"]);
  NumericVector fitted = as<NumericVector>(mod["fitted.values"]);
  
  int n = resid.size();
  double err = 0;
  for(int i = 0; i < n; ++i){
    err += resid[i] / (fitted[i] + resid[i]);
  }
  return err / n;
}

Function

You can put R functions in an object of type Function.
This makes calling an R function from C++ straightforward.

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
RObject callWithOne(Function f){
  return f(1);
}

sourceCpp('callWithOne.cpp')
> callWithOne(function(x) x + 1)
[1] 2
> callWithOne(paste)
[1] "1"

What type of object does an R function return? We don't know, so we use the catchall type(包羅萬象的類型) RObject.
An alternative is to return a List.

example

#include <Rcpp.h>
using namespace Rcpp;


// [[Rcpp::export]]
List lapply1(List input, Function f){
  int n = input.size();
  List out(n);
  
  for(int i = 0; i < n; i++){
    out[i] = f(input[i]);
  }
  return out;
}