使用xptr存储和检索存储器中的矩阵 [英] store and retreive matrices in memory using xptr

问题描述

我想能够将在R中创建的矩阵存储在内存中并返回指针。然后稍后使用指针从内存中取回矩阵。我运行R版本3.0.1（2013-05-16） - 好运动在Ubuntu 13.01和Rcpp版本0.10.6。谢谢。我试过...

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". Thanks. 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))

getMemObject(x)
Error: not compatible with REALSXP

推荐答案

您输入 XPtr 这里是 writeMemObject 的本地变量的地址。很自然，你有未定义的行为。

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对象的事物，并且 NumericMatrix 是R对象，所以看起来错了。

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，你真的想要一个外部指针 NumericMatrix ，那么你可以这样做：

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 超出了 writeMemObject 函数的作用域。

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

此外，请在您的版本中查看 getMemObject 中的更改：

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

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

您不能取消引用指针 wrap 只是一个身份，并返回外部指针，而不是pointee，我猜你在寻找。

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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