Exporting and importing S3 method between packages

问题

Disclaimer: I found the solution to this problem when writing this question. But my question now is "How does it work?"

I am trying to export S3 method from one package (say pkg.from) and import it into another package (say pkg.to). To export the method I use roxygen2 comments.

#' @export
myclass <- function() {
  structure(NULL, class = 'myclass')
}

#' @export
print.myclass <- function(x, ...) {
  print('NULL with class myclass')
}

File NAMESPACE now contains both the constructor and method.

S3method(print,myclass)
export(myclass)

I believe that means both of them are exported. This supports the fact that I can call both after loading the package.

library(pkg.from)
methods(print)
# ...
# [130] print.myclass*
# ...
print(myclass())
# [1] "NULL with class myclass"

The problem is when I want to import this method into the other package using roxygen2.

#' @importFrom pkg.from print.myclass

During Build this message occurs:

Error : object 'print.myclass' is not exported by 'namespace:pkg.from'
ERROR: lazy loading failed for package 'pkg.to'

If I don't import print.myclass it works even if I don't have package pkg.from loaded. Hadley Wickham writes "you don’t need to do anything special for S3 methods". But I don't think importing a function is "anything special". Now I have more questions then I had when started writing this question.

Where does R stores the methods available for every generic?
How can one solve conflicts when he has installed more packages with the same method for a class?

来源：https://stackoverflow.com/questions/42952244/exporting-and-importing-s3-method-between-packages

标签

roxygen2