S4 is "formal", so you need to declare inheritance:
setOldClass(c("SubClass", "SuperClass"))
This usage (first argument of length greater than one) is documented by
help("setOldClass") including among the examples where analogously I see
setOldClass(c("mlm", "lm"))
and there are more examples in the list of old classes defined by 'methods',
exported as '.OldClassesList'.
Mikael
Date: Thu, 11 Dec 2025 17:10:28 -0500
From: Russell Almond<[email protected]>
I noticed this problem while working with R6 objects, but I can
reproduce it using S3 classes.
If I define a method on the S3 superclass, the subclass doesn't inherit it.
``` r
library(methods)
SuperClass <- function(n) {
result <- list(n=n)
class(result) <- "SuperClass"
result
}
setOldClass("SuperClass")
setGeneric("sampleSize",function(x) {standardGeneric("sampleSize")})
#> [1] "sampleSize"
setMethod("sampleSize","SuperClass", function(x) {x$n})
SubClass <- function (n) {
result <- SuperClass(n)
class(result) <- c("SubClass",class(result))
result
}
setOldClass("SubClass")
example <- SubClass(7)
class(example)
#> [1] "SubClass" "SuperClass"
is(example,"SuperClass")
#> [1] TRUE
methods("sampleSize")
#> [1] sampleSize,SuperClass-method
#> see '?methods' for accessing help and source code
sampleSize(example)
#> Error: unable to find an inherited method for function 'sampleSize'
for signature 'x = "SubClass"'
```
<sup>Created on 2025-12-11 with [reprex
v2.1.1](https://reprex.tidyverse.org)</sup>
I'm using R 4.5.2 and the same version of methods.
Is this a bug?
--Russell
-- Russell Almond https://ralmond.net/ [[alternative HTML version deleted]]
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