# AbstractTrees parent child circular reference

**URL:** <https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [December 21, 2022, 2:50pm UTC](https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959 "2022-12-21T14:50:52Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Lighfe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lighfe/32/38831_2.png) [@Lighfe](https://discourse.julialang.org/u/Lighfe)\
**Post date:** [December 21, 2022, 2:50pm UTC](https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959/1 "2022-12-21T14:50:52Z")

</div>

I’m trying to implement a fairly simple tree.

```julia
mutable struct TreeNode
    parent::Union{Nothing,TreeNode}
    name::String
    children::Vector{TreeNode}
    size::Int
end
mutable struct Tree
    nodes::Vector{TreeNode}
end

Tree() = Tree([TreeNode(nothing, "root", Vector{TreeNode}(),0)])

AbstractTrees.nodevalue(n::TreeNode) = n.size
AbstractTrees.children(n::TreeNode) = n.children
AbstractTrees.ParentLinks(::Type{<:TreeNode}) = StoredParents()
AbstractTrees.parent(n::TreeNode) = get_parent(n)

function addchild(tree::Tree, parent::TreeNode, name::String, size::Int64)
    new_node = TreeNode(parent, name, Vector{}(),size)
    push!(parent.children, new_node)
end

```

But obviously I get a circular reference. What’s the correct way to store both parent and children in a node? I guess I’m mainly lost in understanding how to implement the children function.

Other resources for trees in Julia would be appreciated too!

---

<div class="post-metadata">

**Author:** ![skleinbo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/skleinbo/32/36080_2.png) [@skleinbo](https://discourse.julialang.org/u/skleinbo)\
**Post date:** [December 21, 2022, 3:20pm UTC](https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959/2 "2022-12-21T15:20:36Z")

</div>

This seems correct. Though I don’t believe `get_parent` is defined.  
You probably want

```julia
AbstractTrees.parent(n::TreeNode) = n.parent

```

Where is the circular reference?

---

<div class="post-metadata">

**Author:** ![Lighfe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lighfe/32/38831_2.png) [@Lighfe](https://discourse.julialang.org/u/Lighfe)\
**Post date:** [December 21, 2022, 3:40pm UTC](https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959/3 "2022-12-21T15:40:58Z")

</div>

I get a circular reference in the children Vector.  
When executing this

```julia
tr = Tree()
current = tr.nodes[1]
new_node = TreeNode(current, "name", Vector{TreeNode}(),0)
push!(current.children, new_node)

```

it returns:  
1-element Vector{TreeNode}:  
TreeNode(TreeNode(nothing, “root”, TreeNode[#= circular reference @-3 =#], 0), “name”, TreeNode, 0)

---

<div class="post-metadata">

**Author:** ![skleinbo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/skleinbo/32/36080_2.png) [@skleinbo](https://discourse.julialang.org/u/skleinbo)\
**Post date:** [December 21, 2022, 3:59pm UTC](https://discourse.julialang.org/t/abstracttrees-parent-child-circular-reference/91959/4 "2022-12-21T15:59:32Z")

</div>

Ah, that is an “artifact” of the way `show` is implemented. By trying to print the fields of any `children`, the `parent` is indeed referenced again, thus the output.

You can overload `show(io::IO, n::TreeNode)`, i.e.

```julia
import Base: show

function show(io::IO, n::TreeNode)
   str_parent = isnothing(n.parent) ? "no parent" : "with parent"
   str_children = "$(length(children(n))) children"
   print(io, "TreeNode($str_parent, $(n.name), $(nodevalue(n)), $str_children)")
end

julia> current
TreeNode(no parent, root, 0, 1 children)

julia> tr
Tree(TreeNode[TreeNode(no parent, root, 0, 1 children)])

```