How to create an S4 class A that will have a list of S4 classes B as its attribute?

Question

I am quite new to R so my question might have an obvious answer.

I am trying to create something like this

A {
  list: B []
}

Where both A and B are S4 classes. How to achieve that?

EDIT

I actually do not care at this point whether list attribute is an array or an actual list. For my use case it is fairly irrelevant.

Answer 1

You define S4 classes with setClass, and within this you use representation to declare members and their types.

In this case, class A needs only contain a member of type list to house the collection of objects of S4 class B.

setClass("A", representation(List = "list"))
setClass("B", representation(value = "numeric"))

You declare new S4 objects with the function new, in which you specify the class name first and their members as named parameters:

my_object <- new("A", List = list(new("B", value = 1), new("B", value = 2)))
my_object
#> An object of class "A"
#> Slot "List":
#> [[1]]
#> An object of class "B"
#> Slot "value":
#> [1] 1
#> 
#> 
#> [[2]]
#> An object of class "B"
#> Slot "value":
#> [1] 2

We can get the List member using the @ operator:

my_object@List
#> [[1]]
#> An object of class "B"
#> Slot "value":
#> [1] 1
#> 
#> 
#> [[2]]
#> An object of class "B"
#> Slot "value":
#> [1] 2

From which we can access the list members, and their S4 slots directly:

my_object@List[[1]]
#> An object of class "B"
#> Slot "value":
#> [1] 1

my_object@List[[1]]@value
#> [1] 1

^{Created on 2020-03-16 by the reprex package (v0.3.0)}