Rcpp 中 `STRING_ELT` 的含义
meaning of `STRING_ELT` in Rcpp
我搜索了 RCPP 的源代码，但找不到 STRING_ELT 的定义，有人可以指出一个参考，在那里我可以找到 RCPP 中宏之类的所有定义吗？
这是 R 内部的一部分，可通过以下方式访问：
#include <R.h>
#include <Rinternals.h>
参见5.9.7 Handling character data of Writing R Extensions：
R character vectors are stored as STRSXPs, a vector type like VECSXP
where every element is of type CHARSXP. The CHARSXP elements of
STRSXPs are accessed using STRING_ELT and SET_STRING_ELT.
CHARSXPs are read-only objects and must never be modified. In
particular, the C-style string contained in a CHARSXP should be
treated as read-only and for this reason the CHAR function used to
access the character data of a CHARSXP returns (const char *) (this
also allows compilers to issue warnings about improper use). Since
CHARSXPs are immutable, the same CHARSXP can be shared by any STRSXP
needing an element representing the same string. R maintains a global
cache of CHARSXPs so that there is only ever one CHARSXP representing
a given string in memory.
You can obtain a CHARSXP by calling mkChar and providing a
nul-terminated C-style string. This function will return a
pre-existing CHARSXP if one with a matching string already exists,
otherwise it will create a new one and add it to the cache before
returning it to you. The variant mkCharLen can be used to create a
CHARSXP from part of a buffer and will ensure null-termination.
Note that R character strings are restricted to 2^31 - 1 bytes, and
hence so should the input to mkChar be (C allows longer strings on
64-bit platforms).
我搜索了 RCPP 的源代码，但找不到 STRING_ELT 的定义，有人可以指出一个参考，在那里我可以找到 RCPP 中宏之类的所有定义吗？
这是 R 内部的一部分，可通过以下方式访问：
#include <R.h>
#include <Rinternals.h>
参见5.9.7 Handling character data of Writing R Extensions：
R character vectors are stored as STRSXPs, a vector type like VECSXP where every element is of type CHARSXP. The CHARSXP elements of STRSXPs are accessed using STRING_ELT and SET_STRING_ELT.
CHARSXPs are read-only objects and must never be modified. In particular, the C-style string contained in a CHARSXP should be treated as read-only and for this reason the CHAR function used to access the character data of a CHARSXP returns (const char *) (this also allows compilers to issue warnings about improper use). Since CHARSXPs are immutable, the same CHARSXP can be shared by any STRSXP needing an element representing the same string. R maintains a global cache of CHARSXPs so that there is only ever one CHARSXP representing a given string in memory.
You can obtain a CHARSXP by calling mkChar and providing a nul-terminated C-style string. This function will return a pre-existing CHARSXP if one with a matching string already exists, otherwise it will create a new one and add it to the cache before returning it to you. The variant mkCharLen can be used to create a CHARSXP from part of a buffer and will ensure null-termination.
Note that R character strings are restricted to 2^31 - 1 bytes, and hence so should the input to mkChar be (C allows longer strings on 64-bit platforms).