Testing an object for membership in a container

Because node storage is a property of the objects that exist in containers, the objects themselves can be examined in order to check to see if the object is currently contained with in a container or not. This check is always a quick O(1) operation.

This section of the guide will show you various ways to test whether your object are currently a member of any of the containers they may currently be contained in.

Testing single-container objects using mix-ins

All node state classes provide a public InContainer() method, which may be used to test to see if the node state instance is currently a member of its container, as do the Listable or Containable mix-ins that objects typically derive from. Because the objects derive from the mix-in, they also expose this method, so testing an object that uses mix-ins, and can only be a member of single container, for container membership is as simple as calling InContainer() on the object.

class Obj : public fbl::SinglyLinkedLisable<Obj*> { /* ... */ };

Obj* the_obj = new Obj();
fbl::SinglyLinkedList<Obj*> the_list;
ASSERT(the_obj->InContainer() == false);

ASSERT(the_obj->InContainer() == true);

ASSERT(the_obj->InContainer() == false);

delete the_obj;

Multiple-container objects using mix-ins, without ContainableBaseClasses

When an object can be a member of multiple containers because it derives from multiple mix-ins, things can become a bit more complicated. The object itself now has multiple implementations of InContainer(), which it inherited from the mix-ins, so callers need to be specific about which one they want to call. While this is certainly possible, the syntax is awkward, it may benefit from being wrapped up in custom methods on the object. For example:

class Obj : public fbl::DoublyLinkedListable<Obj*>,
            public fbl::WAVLTreeContainable<Obj*> {
   // ...
   bool InList() const { return this->DoublyLinkedListable<Obj*>::InContainer(); }
   bool InTree() const { return this->WAVLTreeContainable<Obj*>::InContainer(); }
   // ...

void test(const Obj& obj) {
  bool in_list, in_tree;

  // The hard way
  in_list = obj.DoublyLinkedListable<Obj*>::InContainer();
  in_tree = obj.WAVLTreeContainable<Obj*>::InContainer();

  // The slightly easier way (the class still needs to implement the hard way)
  in_list = obj.InList();
  in_tree = obj.InTree();

Multiple container objects using mix-ins, with ContainableBaseClasses

Using the ContainableBaseClasses for existing in multiple containers simultaneously can make this a lot easier as it allows tags to be used in order to select the container that you want to test for membership. fbl:: provides a standalone InContainer function, which may be used along with tags to more easily test for membership. Let's look at the previous example, but this time using ContainableBaseClasses.

struct ListTag {};
struct TreeTag {};

class Obj
  : public fbl::ContainableBaseClasses<fbl::TaggedDoublyLinkedListable<Obj*, ListTag>,
                                       fbl::TaggedWAVLTreeContainable<Obj*, TreeTag>> { /* ... */ };

void test(const Obj& obj) {
  bool in_list = fbl::InContainer<ListTag>(obj);
  bool in_tree = fbl::InContainer<TreeTag>(obj);

Testing by directly using the NodeState object

Finally, in the event that NodeState objects and custom traits are being used, container membership can still be tested, but you need to ask the NodeState instance directly.

class Obj {
  // Obj impl here

  bool InFooList() const { return foo_list_node_.InContainer(); }
  bool InBarList() const { return bar_list_node_.InContainer(); }

  struct FooListTraits {
    static auto& node_state(Obj& obj) {
      return obj.foo_list_node_;

  struct BarListTraits {
    static auto& node_state(Obj& obj) {
      return obj.bar_list_node_;

  friend struct FooListTraits;
  friend struct BarListTraits;

  fbl::DoublyLinkedListNodeState<Obj*> foo_list_node_;
  fbl::DoublyLinkedListNodeState<fbl::RefPtr<Obj>> bar_list_node_;

  using FooList = fbl::DoublyLinkedListCustomTraits<Obj*, FooListTraits>;
  using BarList = fbl::DoublyLinkedListCustomTraits<fbl::RefPtr<Obj>, BarListTraits>;