Creating S3 class in an R Package

Question

I have written code that very simply creates an S3 class for a package. I seek to create a new S3 class so that I can develop, e.g., custom print() methods.

I have tested the code in a simple R script, but as soon as the function is wrapped into a package, the functionality breaks and the S3 class is no longer created. I cannot provide reproducible code for the creation of the package, but a simplified version of the function I would like to build into the package is introduced below.

My code works perfectly when written either raw or within a function.

    x <- 1:10
    class(x)
    class(x) <- append(class(x),"newS3class")

class(x) will return c("integer", "newS3class")

Likewise, now, declare a function that does the same thing. This also works fine. In reality, my function foo() first performs some action on the input, and then returns the output with a new class.

    foo <- function(y) {
      y <- y + 3
      class(y) <- append(class(y), "newS3class")
      y
    }

class(1:5) returns "integer". class(foo(1:5)) returns c("integer", "newS3class"). This is as expected.

But, when I develop a package, e.g., mypkg, and then run mypkg::foo(), the functionality is broken. I.e., (mypkg::foo(1:5)) returns "integer" rather than c("integer", "newS3class").

Is something happening regarding scoping, in the process of building the package from its constituent functions, that is preventing this from working?

Answer 1

The mistake I made here was that I did not properly export the class. To fix this I added the following codeblock, including the export line, before using the roxygen package to build my documentation and NAMESPACE.

#' @export summary.objectclass
makeobjectclass <- function(x) {
    class(x) <- c("objectclass", class(x))
    x
}