Declare a variable as a reference in Rcpp

Question

In C++, we can declare a variable as a reference.

int a = 10;
int& b = a;

If we set b=15, a also changes.

I want to do the similar thing in Rcpp.

List X = obj_from_R["X"];
IntegerVector x_i = X[index];
x_i = value;

I want to update an object from R called X by inserting a value to one of its vector. The code above did not work, so I tried this:

IntegerVector& x_i = X[index];

and received an error.

error: non-const lvalue reference to type 'IntegerVector'
      (aka 'Vector<13>') cannot bind to a temporary of type 'Proxy' (aka 'generic_proxy<19>')

Answer 1

This question gets asked a lot in differing variants.

Here are some popular answers:

1. Within C++ functions, how are Rcpp objects passed to other functions (by reference or by copy)?
2. Update Rcpp::NumericMatrix passed by reference using RcppArmadillo submat()
3. Rcpp pass by reference vs. by value
4. Passing by reference a data.frame and updating it with rcpp
5. Rcpp Update matrix passed by reference and return the update in R
6. Passing large matrices to RcppArmadillo function without creating copy (advanced constructors)
7. Rcpp: Inconsistent behavior with proxy model
8. Why is my Rcpp implementation for finding the number of unique items slower than base R?

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More details...

From the FAQ entry Rcpp changed the (const) object I passed by value that I wrote:

Rcpp objects are wrappers around the underlying R objects' SEXP, or S-expression. The SEXP is a pointer variable that holds the location of where the R object data has been stored R:Internals. That is to say, the SEXP does not hold the actual data of the R object but merely a reference to where the data resides. When creating a new Rcpp object for an R object to enter C++, this object will use the same SEXP that powers the original R object if the types match otherwise a new SEXP must be created to be type safe. In essence, the underlying SEXP objects are passed by reference without explicit copies being made into C++. We refer to this arrangement as a proxy model.

So, the & is just visual sugar as Rcpp objects already act as references.

Thus, the following would show the conclusion:

#include <Rcpp.h>

// [[Rcpp::export]]
void show_references(Rcpp::List X, 
                     Rcpp::IntegerVector y, 
                     int index = 0) {
  X[index] = y;
}

Example:

y_vec = c(-1L, 8L, 12L)
X_list = list(a = c(0L, 2L, 3L), b = c(42L, 50L, 30L))
X_list
# $a
# [1] 0 2 3
#
# $b
# [1] 42 50 30

show_references(X_list, y_vec)
X_list
# $a
# [1] -1  8 12
#
# $b
# [1] 42 50 30

The following Rcpp proxy model slides I created should further illustrate what is happening

Source: https://twitter.com/axiomsofxyz/status/938881541396197377