Question

I'm trying to understand what the c++ sizeof does when operating on an RCpp vector. As an example:

library(Rcpp)
cppFunction('int size_of(NumericVector a) {return(sizeof a);}')
size_of(1.0)
# [1] 16

this returns the value 16 for any numeric or integer vector passed to it.

As also does

cppFunction('int size_of(IntegerVector a) {return(sizeof a);}')
size_of(1)
# [1] 16

I thought that numerics in R were 8 bytes and integers 4 bytes. So what is going on here? The motivation is to use memcpy on RCpp vectors, for which the size needs to be known.

Answer 1

Everything 我们从R传递给C（++）并返回SEXP类型 - 指针到S表达式。

因此，如果我们推广您的函数并实际让SEXP进入，我们可以看到一些有趣的事情：

R> Rcpp::cppFunction('int size_of(SEXP a) {return(sizeof a);}')
R> size_of(1L)    ## single integer -- still a pointer
[1] 8 
R> size_of(1.0)   ## single double -- still a pointer
[1] 8
R> size_of(seq(1:100))   ## a sequence ...
[1] 8
R> size_of(help)         ## a function
[1] 8
R> size_of(globalenv)    ## an environment
[1] 8
R>

简而言之，您遇到了编译时C ++类型分析运算符（sizeof）和运行时功能，即所有内容都变为SEXP类型。对于实际向量，您可能需要size()或length()成员函数等等。

Answer 2

You would have to get into how NumericVector and IntegerVector are implemented to discover why they statically take up a certain number of bytes.

Based on your observation of the size of a "numeric" or "integer" in this context, it is likely that the value 16 accounts for any/all of the following:

Pointer to [dynamically-allocated?] data
Current logical size of container (number of elements)
Any other metadata

Ideally, don't use memcpy to transfer the state of one object to another, unless you are absolutely certain that it is a trivial object with only members of built-in type. If I have correctly guessed the layout of a NumericVector, using memcpy on it will violate its ownership semantics and thus be incorrect. There are other ways to copy R vectors.

what does sizeof return for RCpp vectors

2 个答案: