rcpp - 在 Rcpp 中声明一个变量作为引用

Question

在 C++ 中，我们可以声明一个变量作为引用。

int a = 10;
int& b = a;

如果我们设置 b=15 ， a 也会改变。

我想在 Rcpp 中做类似的事情。

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

我想通过向其向量之一插入一个值来更新 R 中名为 X 的对象。上面的代码不起作用，所以我尝试了这个:

IntegerVector& x_i = X[index];

并收到一个错误。

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

这个问题在不同的变体中被问到很多。

以下是一些流行的答案:

Within C++ functions, how are Rcpp objects passed to other functions (by reference or by copy)?

Update Rcpp::NumericMatrix passed by reference using RcppArmadillo submat()

Rcpp pass by reference vs. by value

Passing by reference a data.frame and updating it with rcpp

Rcpp Update matrix passed by reference and return the update in R

Passing large matrices to RcppArmadillo function without creating copy (advanced constructors)

Rcpp: Inconsistent behavior with proxy model

Why is my Rcpp implementation for finding the number of unique items slower than base R?

更多细节...

从我写的常见问题条目 Rcpp changed the (const) object I passed by value :

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.

因此， & 只是可视化的糖，因为 Rcpp 对象已经作为引用。

因此，以下将显示结论:

#include <Rcpp.h>

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

例子:

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

我创建的以下 Rcpp 代理模型幻灯片应该进一步说明正在发生的事情







资料来源: https://twitter.com/axiomsofxyz/status/938881541396197377