使用xptr在内存中存储和检索矩阵 [英] store and retrieve matrices in memory using xptr

问题描述

我希望能够在内存中存储在R中创建的矩阵并返回指针.然后，稍后使用指针从内存中取回矩阵.我正在Ubuntu 3.01和Rcpp版本"0.10.6"上运行R版本3.0.1(2013-05-16)-"Good Sport".我已经尝试过...

I would like to be able to store a matrix created in R in memory and return the pointer. And then later use the pointer to fetch back the matrix from memory. I am running R version 3.0.1 (2013-05-16) -- "Good Sport" on Ubuntu 13.01 and Rcpp version "0.10.6". I have tried ...

// [[Rcpp::export]]
SEXP writeMemObject(NumericMatrix mat)
{
  XPtr<NumericMatrix> ptr(&mat, true);
  return ptr;
}

// [[Rcpp::export]]
NumericMatrix getMemObject(SEXP ptr)
{
  XPtr<NumericMatrix> out(ptr);
  return wrap(out);
}

# This returns a pointer
x <- writeMemObject(matrix(1.0))

但是这失败了，当我再次尝试时会崩溃R

But this fails and crashes R when I try again

getMemObject(x)
Error: not compatible with REALSXP

推荐答案

此处提供给XPtr的指针是writeMemObject本地变量的地址.很自然，您有未定义的行为.

The pointer you feed to XPtr here is the address of a variable that is local to writeMemObject. Quite naturally you have undefined behavior.

此外，外部指针通常用于不是R对象的对象，而NumericMatrix是R对象，因此看起来是错误的.

Also, external pointers are typically used for things that are not R objects, and a NumericMatrix is an R object, so that looks wrong.

但是，如果由于某种原因，您确实想要指向NumericMatrix的外部指针，则可以执行以下操作:

If however, for some reason you really want an external pointer to a NumericMatrix then you could do something like this:

#include <Rcpp.h>
using namespace Rcpp ;

// [[Rcpp::export]]
SEXP writeMemObject(NumericMatrix mat){
  XPtr<NumericMatrix> ptr( new NumericMatrix(mat), true);
  return ptr;
}

// [[Rcpp::export]]
NumericMatrix getMemObject(SEXP ptr){
  XPtr<NumericMatrix> out(ptr);
  return *out ;
}

因此，由new创建的指针超出了writeMemObject函数的作用域.

So the pointer created by new outlives the scope of the writeMemObject function.

另外，请查看您所使用的版本中getMemObject中的更改:

Also, please see the changes in getMemObject, in your version you had:

XPtr<NumericMatrix> out(ptr);
return wrap(out);

您没有取消引用指针，wrap只是一个标识并返回外部指针，而不是我猜您正在寻找的指针.

You are not dereferencing the pointer, wrap would just be an identity and return the external pointer rather than the pointee I guess you were looking for.

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