I am writing a package with an S4 class that looks like a simple tree structure: the class, say S4node, has a slot nodes which is a list of child S4node (via a common virtual ancestor, say S4base). The class also has a slot parent of type S4base that links back up to the parent of the child nodes. That looks more-or-less like this (with barebones coding for clarity):

# Create the root node, slot parent is set to NA, slot id is set to 567
> root <- new_node(567) 

# Create some child nodes, slot parent is set to root, slot nodes has new node added
> root <- add_node(root, 5671)
> root <- add_node(root, 5672)

This works ok and all slots are set correctly. Traversing down the hierarchy works, as well as traversing up the hierarchy. But here is the problem: when I want to traverse up the hierarchy the nodes slot is set to NULL, which becomes obvious when traveling down some other branch. Like this:

> node5671 <- find_node(root, 5671)

# Now traverse 5671 -> 567 -> 5672
> node5672 <- node_travel(node5671, "../5672")
Error: path not found

On the console this becomes clearer:

> length(root@nodes)
[1] 2
> node5671@parent@id
[1] 567
> nodes5671@parent@nodes
NULL

# Best one yet: take the root node, get a child, get the parent (= root), get the nodes = NULL!!!
> root@nodes[[1]]@parent@nodes
NULL

It appears to me that the list with nodes is emptied automatically, but only when implicitly accessed: I can construct the list in the first place, only when referencing the list through a @parent lookup is the list emptied. Is this to avoid endless recursion? Is it a quirk of gc()? Is there a work-around for this?