Modify contents of object with "call by reference" - r

Question

I am trying to modify the contents of an object defined by a self-written class with a function that takes two objects of this class and adds the contents.
setClass("test",representation(val="numeric"),prototype(val=1))
I know that R not really works with "call by reference" but can mimic that behaviour with a method like this one:
setGeneric("value<-", function(test,value) standardGeneric("value<-"))
setReplaceMethod("value",signature = c("test","numeric"),
definition=function(test,value) {
test#val <- value
test
})
foo = new("test") #foo#val is 1 per prototype
value(foo)<-2 #foo#val is now set to 2
Until here, anything I did and got as result is consitent with my research here on stackexchange,
Call by reference in R (using function to modify an object)
and with this code from a lecture (commented and written in German)
What I wish to achieve now is a similar result with the following method:
setGeneric("add<-", function(testA,testB) standardGeneric("add<-"))
setReplaceMethod("add",signature = c("test","test"),
definition=function(testA,testB) {
testA#val <- testA#val + testB#val
testA
})
bar = new("test")
add(foo)<-bar #should add the value slot of both objects and save the result to foo
Instead I get the following error:
Error in `add<-`(`*tmp*`, value = <S4 object of class "test">) :
unused argument (value = <S4 object of class "test">)
The function call works with:
"add<-"(foo,bar)
But this does not save the value into foo. Using
foo <- "add<-"(foo,bar)
#or using
setMethod("add",signature = c("test","test"), definition= #as above... )
foo <- add(foo,bar)
works but this is inconsistent with the modifying method value(foo)<-2
I have the feeling that I am missing something simple here.
Any help is very much appreciated!

Answer 1

I do not remember why, but for <- functions, the last argument must be named 'value'.
So in your case:
setGeneric("add<-", function(testA,value) standardGeneric("add<-"))
setReplaceMethod("add",signature = c("test","test"),
definition=function(testA,value) {
testA#val <- testA#val + value#val
testA
})
bar = new("test")
add(foo)<-bar
You may also use a Reference class ig you want to avoid the traditional arguments as values thing.