Lesson 07 · An object's children - Build a Garbage Collector

An object's children

Tracing means following references, so first we need the references leaving a single object. Today you build children - the ids an object points at through its non-nil fields - the one step every trace repeats.

The goal

Return the ids an object references, skipping nil fields, in field order.

Start here - the target

TO DO

Scenario: An object reports the objects it directly references

Givenan object a = New(3) with a.field0 = 5, a.field1 = Nil, a.field2 = 2

WhenChildren(a) is queried

Thenit returns [5, 2] - the non-nil fields in field order, with the nil field skipped

Andfor an object with every field nil, Children returns an empty list

Background

A trace is just this one move repeated: stand on an object, look at everything it references, step to each of those, and keep going. Children is that one move - it returns the ids in an object’s fields, skipping any field that holds the nil reference because nil points at nothing. Preserving field order keeps results deterministic and easy to assert.

This tiny helper is the seam between the object model and every algorithm to come. The reachable-set trace calls it; the mark phase greys the children it returns; the copying collector forwards each child. Keeping “which objects does this one point at” in one place means the rest of the collector never touches the field layout directly. Next lesson you follow these edges from a root to compute a whole reachable set.

Make it work

// the objects reachable in one step from obj: its non-nil fields, in order
func (h *Heap) Children(obj Ref) []Ref {
  var out []Ref
  for _, f := range h.slots[obj].fields {
    if f != Nil { out = append(out, f) } // skip the dead-end fields
  }
  return out
}

CheckpointDONE

An object can report the objects it directly references. Commit and stop here.