Question

Can you pass by reference with “R” ?
for example, in the following code:

setClass("MyClass",
    representation(
    name="character"
    ))


instance1 <-new("MyClass",name="Hello1")
instance2 <-new("MyClass",name="Hello2")

array = c(instance1,instance2)

instance1
array

instance1@name="World!"

instance1
array

the output is

> instance1
An object of class “MyClass”
Slot "name":
[1] "World!"

> array
[[1]]
An object of class “MyClass”
Slot "name":
[1] "Hello1"


[[2]]
An object of class “MyClass”
Slot "name":
[1] "Hello2"

but I wish it was

> instance1
An object of class “MyClass”
Slot "name":
[1] "World!"

> array
[[1]]
An object of class “MyClass”
Slot "name":
[1] "World!"


[[2]]
An object of class “MyClass”
Slot "name":
[1] "Hello2"

is it possible ?

Answer 1

No.

Objects in assignment statements are immutable. R will copy the object not just the reference.

> v = matrix(1:12, nrow=4)
> v
           [,1] [,2] [,3]
     [1,]    1    5    9
     [2,]    2    6   10
     [3,]    3    7   11
     [4,]    4    8   12
> v1 = v
> v1[,1]     # fetch the first column 
     [1] 1 2 3 4

(proviso: the statement above is true for R primitives, e.g., vectors, matrices), and also for functions; I cannot say for certain whether it’s true for all R objects–just most of them, as well as the vast majority of the ones most often used.)

If you don’t like this behavior you can opt out of it with the help from an R Package. E.g., there is an R Package called R.oo that allows you to mimic pass-by-reference behavior; R.oo is available on CRAN.