Lesson 06 · An object graph with a shared node

An object graph with a shared node

Everything so far comes together into an object graph. Today you build a small graph where one object is referenced by two parents - a shared node - and walk a path through it, proving the model can express real sharing.

The goal

Wire a graph with a shared child and read a value across two hops.

Start here - the target

TO DO

Scenario: A shared object is reachable through two different parents

Givenobjects r = New(2), a = New(1), b = New(1), s = New(0) (ids 0, 1, 2, 3), with r.field0 = a, r.field1 = b, a.field0 = s, and b.field0 = s

Whenthe graph is walked from r down field0 twice, and from r down field1 then field0

ThenGetField(GetField(r, 0), 0) returns 3 and GetField(GetField(r, 1), 0) returns 3 - both paths reach s

AndAddRoot(r) makes Roots() return [0], with s reachable only through a or b, never directly

Background

The whole point of reference fields is that objects form a graph, not a tree. Here s is a shared node: both a and b hold its id in a field, so there are two distinct paths from the root r down to s. Nothing about the model treats the second reference specially - s is just an id, and any number of fields may hold it.

Sharing is exactly what makes garbage collection interesting. When you later free things, you must not reclaim s while either a or b still points at it, and when you copy the graph you must copy s once and update both references - not copy it twice. Walking r.field0.field0 and r.field1.field0 and getting the same id 3 both times confirms the graph is wired correctly. With the model complete, the next chapter answers the central question: starting from the roots, which objects can you reach?

Make it work

r, _ := h.New(2); a, _ := h.New(1); b, _ := h.New(1); s, _ := h.New(0)
h.SetField(r, 0, a); h.SetField(r, 1, b)
h.SetField(a, 0, s); h.SetField(b, 0, s) // s is shared by a and b
h.AddRoot(r)
// GetField(GetField(r,0),0) == GetField(GetField(r,1),0) == s

CheckpointDONE

You can build and traverse an object graph with a shared node. Commit and stop here.