R devel - Dec 2025 - S4 generic functions don't respect S3 class hierarchies.

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<russell.g.almond at gmail.com>
> 
> 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]]

Thanks.? That's helpful.? It also works in my use case with R6 classes, 
even if I don't include the "R6Class" in the setOldClass
definition.

 ??? --Russell


On 12/12/25 08:49, Mikael Jagan wrote:
> 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<russell.g.almond at gmail.com>
>>
>> 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]]
>
-- 
Russell Almond
https://ralmond.net/

	[[alternative HTML version deleted]]