r - 为环境分配属性会导致意外？

Question

根据R语言定义:

Unlike most other R objects, environments are not copied when passed to functions or used in assignments. Thus, if you assign the same environment to several symbols and change one, the others will change too. In particular, assigning attributes to an environment can lead to surprises.

> e = new.env()
> attr(e, "mynumber", TRUE)
NULL
> attr(e, "mynumber") = 555
> attr(e, "mynum")
[1] 555

究竟有哪些惊喜？

Answer 1

例如，我们将 2 分配给 e 的 at 属性和 f 的 at 属性也会自动更改:

e <- new.env()
attr(e, "at") <- 1
f <- e
attr(e, "at") <- 2
attributes(f)
## $at
## [1] 2