我试图在我的包中处理有类的向量，而不导入另一个包。我为类haven_labelled写了一个as.character方法。它仅供内部使用，所以我不导出它：

as.character.haven_labelled <- function(x, ...) {
    as.character.default(x)
}

我在导出的一个数据集中测试了这个向量：

> class(mtcars_lab$vs)
[1] "haven_labelled" "vctrs_vctr"     "double"

但是，当我加载包并将向量传递给as.character时，R分派给as.character.vctrs_vctr而不是我的方法as.character.haven_labelled，从而导致错误：

> devtools::load_all(".")

ℹ Loading sift

> as.character(mtcars_lab$vs)

Error in `as.character()`:
! Can't convert `x` <haven_labelled> to <character>.
Run `rlang::last_error()` to see where the error occurred.

> rlang::last_error()

<error/vctrs_error_cast>
Error in `as.character()`:
! Can't convert `x` <haven_labelled> to <character>.
---
Backtrace:
 1. base::as.character(mtcars_lab$vs)
 2. vctrs:::as.character.vctrs_vctr(mtcars_lab$vs)
Run `rlang::last_trace()` to see the full context.

> rlang::last_trace()

<error/vctrs_error_cast>
Error in `as.character()`:
! Can't convert `x` <haven_labelled> to <character>.
---
Backtrace:
     ▆
  1. ├─base::as.character(mtcars_lab$vs)
  2. ├─vctrs:::as.character.vctrs_vctr(mtcars_lab$vs)
  3. │ └─vctrs::vec_cast(x, character())
  4. └─vctrs (local) `<fn>`()
  5.   └─vctrs::vec_default_cast(...)
  6.     ├─base::withRestarts(...)
  7.     │ └─base (local) withOneRestart(expr, restarts[[1L]])
  8.     │   └─base (local) doWithOneRestart(return(expr), restart)
  9.     └─vctrs::stop_incompatible_cast(...)
 10.       └─vctrs::stop_incompatible_type(...)
 11.         └─vctrs:::stop_incompatible(...)
 12.           └─vctrs:::stop_vctrs(...)
 13.             └─rlang::abort(message, class = c(class, "vctrs_error"), ..., call = vctrs_error_call(call))