From ee3d36f78a59ebde397962d97fde4a4137f3aada Mon Sep 17 00:00:00 2001
From: Julian Eisel <julian@blender.org>
Date: Wed, 7 Sep 2022 12:05:56 +0200
Subject: Outliner: Initial (half working) port to new element storage

Basicaly the idea is to have a C++ class to better manage the runtime
tree element storage and access.

This branch doesn't even compile yet, it's just an early experiment so
far that I don't only want to keep locally.
---
 .../editors/space_outliner/outliner_dragdrop.cc    |  12 +-
 .../editors/space_outliner/outliner_draw.cc        | 145 ++++----
 .../editors/space_outliner/outliner_intern.hh      | 148 +++++++-
 .../editors/space_outliner/outliner_tree.cc        | 387 +++++++++------------
 .../editors/space_outliner/outliner_utils.cc       |  27 +-
 .../editors/space_outliner/space_outliner.cc       |   5 +-
 .../blender/editors/space_outliner/tree/common.cc  |  31 +-
 .../blender/editors/space_outliner/tree/common.hh  |   4 +-
 .../editors/space_outliner/tree/tree_display.hh    |  35 +-
 .../space_outliner/tree/tree_display_data.cc       |   6 +-
 .../space_outliner/tree/tree_display_libraries.cc  |  33 +-
 .../space_outliner/tree/tree_display_orphaned.cc   |   9 +-
 .../tree_display_override_library_hierarchies.cc   |  40 +--
 .../tree_display_override_library_properties.cc    |  31 +-
 .../space_outliner/tree/tree_display_scenes.cc     |   9 +-
 .../space_outliner/tree/tree_display_sequencer.cc  |  11 +-
 .../space_outliner/tree/tree_display_view_layer.cc |  56 ++-
 .../editors/space_outliner/tree/tree_element.cc    |  71 ++++
 .../editors/space_outliner/tree/tree_element.hh    |  26 +-
 .../space_outliner/tree/tree_element_anim_data.cc  |   8 +-
 .../space_outliner/tree/tree_element_driver.cc     |   3 +-
 .../editors/space_outliner/tree/tree_element_id.cc |   3 +-
 .../space_outliner/tree/tree_element_id_scene.cc   |  12 +-
 .../space_outliner/tree/tree_element_nla.cc        |   5 +-
 .../space_outliner/tree/tree_element_overrides.cc  |  11 +-
 .../space_outliner/tree/tree_element_rna.cc        |  16 +-
 .../tree/tree_element_scene_objects.cc             |   4 +-
 .../space_outliner/tree/tree_element_seq.cc        |   6 +-
 .../space_outliner/tree/tree_element_view_layer.cc |   2 +-
 29 files changed, 646 insertions(+), 510 deletions(-)

diff --git a/source/blender/editors/space_outliner/outliner_dragdrop.cc b/source/blender/editors/space_outliner/outliner_dragdrop.cc
index 4a0e00b8bf1..4f27af6bcea 100644
--- a/source/blender/editors/space_outliner/outliner_dragdrop.cc
+++ b/source/blender/editors/space_outliner/outliner_dragdrop.cc
@@ -64,9 +64,9 @@ static TreeElement *outliner_dropzone_element(TreeElement *te,
     }
   }
   /* Not it.  Let's look at its children. */
-  if (children && (TREESTORE(te)->flag & TSE_CLOSED) == 0 && (te->subtree.first)) {
-    LISTBASE_FOREACH (TreeElement *, te_sub, &te->subtree) {
-      TreeElement *te_valid = outliner_dropzone_element(te_sub, fmval, children);
+  if (children && (TREESTORE(te)->flag & TSE_CLOSED) == 0) {
+    for (TreeElement &te_sub : te->child_elements) {
+      TreeElement *te_valid = outliner_dropzone_element(&te_sub, fmval, children);
       if (te_valid) {
         return te_valid;
       }
@@ -139,14 +139,14 @@ static TreeElement *outliner_drop_insert_find(bContext *C,
 
     if (view_mval[1] < (te_hovered->ys + margin)) {
       if (TSELEM_OPEN(TREESTORE(te_hovered), space_outliner) &&
-          !BLI_listbase_is_empty(&te_hovered->subtree)) {
+          !te_hovered->child_elements.is_empty()) {
         /* inserting after a open item means we insert into it, but as first child */
-        if (BLI_listbase_is_empty(&te_hovered->subtree)) {
+        if (te_hovered->child_elements.is_empty()) {
           *r_insert_type = TE_INSERT_INTO;
           return te_hovered;
         }
         *r_insert_type = TE_INSERT_BEFORE;
-        return static_cast<TreeElement *>(te_hovered->subtree.first);
+        return &*te_hovered->child_elements.begin();
       }
       *r_insert_type = TE_INSERT_AFTER;
       return te_hovered;
diff --git a/source/blender/editors/space_outliner/outliner_draw.cc b/source/blender/editors/space_outliner/outliner_draw.cc
index 3f99b19cd16..acddf2b2b1d 100644
--- a/source/blender/editors/space_outliner/outliner_draw.cc
+++ b/source/blender/editors/space_outliner/outliner_draw.cc
@@ -81,22 +81,22 @@ namespace blender::ed::outliner {
  * \{ */
 
 static void outliner_tree_dimensions_impl(SpaceOutliner *space_outliner,
-                                          ListBase *lb,
+                                          SubTree &subtree,
                                           int *width,
                                           int *height)
 {
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
-    *width = MAX2(*width, te->xend);
+  for (TreeElement &te : subtree) {
+    *width = MAX2(*width, te.xend);
     if (height != nullptr) {
       *height += UI_UNIT_Y;
     }
 
-    TreeStoreElem *tselem = TREESTORE(te);
+    TreeStoreElem *tselem = TREESTORE(&te);
     if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_tree_dimensions_impl(space_outliner, &te->subtree, width, height);
+      outliner_tree_dimensions_impl(space_outliner, te.child_elements, width, height);
     }
     else {
-      outliner_tree_dimensions_impl(space_outliner, &te->subtree, width, nullptr);
+      outliner_tree_dimensions_impl(space_outliner, te.child_elements, width, nullptr);
     }
   }
 }
@@ -105,7 +105,8 @@ void outliner_tree_dimensions(SpaceOutliner *space_outliner, int *r_width, int *
 {
   *r_width = 0;
   *r_height = 0;
-  outliner_tree_dimensions_impl(space_outliner, &space_outliner->tree, r_width, r_height);
+  outliner_tree_dimensions_impl(
+      space_outliner, space_outliner->runtime->root_elements, r_width, r_height);
 }
 
 /**
@@ -1030,7 +1031,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                        ViewLayer *view_layer,
                                        ARegion *region,
                                        SpaceOutliner *space_outliner,
-                                       ListBase *lb,
+                                       SubTree &subtree,
                                        RestrictPropertiesActive props_active_parent)
 {
   /* Get RNA properties (once for speed). */
@@ -1105,16 +1106,16 @@ static void outliner_draw_restrictbuts(uiBlock *block,
   /* Create buttons. */
   uiBut *bt;
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
-    TreeStoreElem *tselem = TREESTORE(te);
+  for (TreeElement &te : subtree) {
+    TreeStoreElem *tselem = TREESTORE(&te);
     RestrictPropertiesActive props_active = props_active_parent;
 
-    if (te->ys + 2 * UI_UNIT_Y >= region->v2d.cur.ymin && te->ys <= region->v2d.cur.ymax) {
+    if (te.ys + 2 * UI_UNIT_Y >= region->v2d.cur.ymin && te.ys <= region->v2d.cur.ymax) {
       if (tselem->type == TSE_R_LAYER &&
           ELEM(space_outliner->outlinevis, SO_SCENES, SO_VIEW_LAYER)) {
         if (space_outliner->show_restrict_flags & SO_RESTRICT_RENDER) {
           /* View layer render toggle. */
-          ViewLayer *layer = static_cast<ViewLayer *>(te->directdata);
+          ViewLayer *layer = static_cast<ViewLayer *>(te.directdata);
 
           bt = uiDefIconButBitS(block,
                                 UI_BTYPE_ICON_TOGGLE_N,
@@ -1122,7 +1123,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_RESTRICT_RENDER_OFF,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.render),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &layer->flag,
@@ -1136,18 +1137,18 @@ static void outliner_draw_restrictbuts(uiBlock *block,
           UI_but_drawflag_enable(bt, UI_BUT_ICON_REVERSE);
         }
       }
-      else if (((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) &&
-               (te->flag & TE_CHILD_NOT_IN_COLLECTION)) {
+      else if (((tselem->type == TSE_SOME_ID) && (te.idcode == ID_OB)) &&
+               (te.flag & TE_CHILD_NOT_IN_COLLECTION)) {
         /* Don't show restrict columns for children that are not directly inside the collection. */
       }
-      else if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
+      else if ((tselem->type == TSE_SOME_ID) && (te.idcode == ID_OB)) {
         PointerRNA ptr;
         Object *ob = (Object *)tselem->id;
         RNA_id_pointer_create(&ob->id, &ptr);
 
         if (space_outliner->show_restrict_flags & SO_RESTRICT_HIDE) {
-          Base *base = (te->directdata) ? (Base *)te->directdata :
-                                          BKE_view_layer_base_find(view_layer, ob);
+          Base *base = (te.directdata) ? (Base *)te.directdata :
+                                         BKE_view_layer_base_find(view_layer, ob);
           if (base) {
             PointerRNA base_ptr;
             RNA_pointer_create(&scene->id, &RNA_ObjectBase, base, &base_ptr);
@@ -1156,7 +1157,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                     0,
                                     0,
                                     (int)(region->v2d.cur.xmax - restrict_offsets.hide),
-                                    te->ys,
+                                    te.ys,
                                     UI_UNIT_X,
                                     UI_UNIT_Y,
                                     &base_ptr,
@@ -1183,7 +1184,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.select),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1208,7 +1209,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.viewport),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1233,7 +1234,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.render),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1253,7 +1254,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
         }
       }
       else if (tselem->type == TSE_CONSTRAINT) {
-        bConstraint *con = (bConstraint *)te->directdata;
+        bConstraint *con = (bConstraint *)te.directdata;
 
         PointerRNA ptr;
         RNA_pointer_create(tselem->id, &RNA_Constraint, con, &ptr);
@@ -1264,7 +1265,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.hide),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1282,7 +1283,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
         }
       }
       else if (tselem->type == TSE_MODIFIER) {
-        ModifierData *md = (ModifierData *)te->directdata;
+        ModifierData *md = (ModifierData *)te.directdata;
 
         PointerRNA ptr;
         RNA_pointer_create(tselem->id, &RNA_Modifier, md, &ptr);
@@ -1293,7 +1294,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.viewport),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1316,7 +1317,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.render),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1335,7 +1336,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
       }
       else if (tselem->type == TSE_POSE_CHANNEL) {
         PointerRNA ptr;
-        bPoseChannel *pchan = (bPoseChannel *)te->directdata;
+        bPoseChannel *pchan = (bPoseChannel *)te.directdata;
         Bone *bone = pchan->bone;
         Object *ob = (Object *)tselem->id;
         bArmature *arm = static_cast<bArmature *>(ob->data);
@@ -1348,7 +1349,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                   0,
                                   0,
                                   (int)(region->v2d.cur.xmax - restrict_offsets.viewport),
-                                  te->ys,
+                                  te.ys,
                                   UI_UNIT_X,
                                   UI_UNIT_Y,
                                   &ptr,
@@ -1372,7 +1373,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_RESTRICT_SELECT_OFF,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.select),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &(bone->flag),
@@ -1389,7 +1390,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
       }
       else if (tselem->type == TSE_EBONE) {
         bArmature *arm = (bArmature *)tselem->id;
-        EditBone *ebone = (EditBone *)te->directdata;
+        EditBone *ebone = (EditBone *)te.directdata;
 
         if (space_outliner->show_restrict_flags & SO_RESTRICT_VIEWPORT) {
           bt = uiDefIconButBitI(block,
@@ -1398,7 +1399,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_RESTRICT_VIEW_OFF,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.viewport),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &(ebone->flag),
@@ -1420,7 +1421,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_RESTRICT_SELECT_OFF,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.select),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &(ebone->flag),
@@ -1437,7 +1438,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
       }
       else if (tselem->type == TSE_GP_LAYER) {
         ID *id = tselem->id;
-        bGPDlayer *gpl = (bGPDlayer *)te->directdata;
+        bGPDlayer *gpl = (bGPDlayer *)te.directdata;
 
         if (space_outliner->show_restrict_flags & SO_RESTRICT_HIDE) {
           bt = uiDefIconButBitS(block,
@@ -1446,7 +1447,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_HIDE_OFF,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.hide),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &gpl->flag,
@@ -1467,7 +1468,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                 0,
                                 ICON_UNLOCKED,
                                 (int)(region->v2d.cur.xmax - restrict_offsets.select),
-                                te->ys,
+                                te.ys,
                                 UI_UNIT_X,
                                 UI_UNIT_Y,
                                 &gpl->flag,
@@ -1480,17 +1481,17 @@ static void outliner_draw_restrictbuts(uiBlock *block,
           UI_but_flag_enable(bt, UI_BUT_DRAG_LOCK);
         }
       }
-      else if (outliner_is_collection_tree_element(te)) {
+      else if (outliner_is_collection_tree_element(&te)) {
         PointerRNA collection_ptr;
         PointerRNA layer_collection_ptr;
 
         if (outliner_restrict_properties_collection_set(
-                scene, te, &collection_ptr, &layer_collection_ptr, &props, &props_active)) {
+                scene, &te, &collection_ptr, &layer_collection_ptr, &props, &props_active)) {
 
           LayerCollection *layer_collection = (tselem->type == TSE_LAYER_COLLECTION) ?
-                                                  static_cast<LayerCollection *>(te->directdata) :
+                                                  static_cast<LayerCollection *>(te.directdata) :
                                                   nullptr;
-          Collection *collection = outliner_collection_from_tree_element(te);
+          Collection *collection = outliner_collection_from_tree_element(&te);
 
           if (layer_collection != nullptr) {
             if (space_outliner->show_restrict_flags & SO_RESTRICT_ENABLE) {
@@ -1499,7 +1500,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                       0,
                                       0,
                                       (int)(region->v2d.cur.xmax) - restrict_offsets.enable,
-                                      te->ys,
+                                      te.ys,
                                       UI_UNIT_X,
                                       UI_UNIT_Y,
                                       &layer_collection_ptr,
@@ -1519,7 +1520,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                       0,
                                       0,
                                       (int)(region->v2d.cur.xmax - restrict_offsets.hide),
-                                      te->ys,
+                                      te.ys,
                                       UI_UNIT_X,
                                       UI_UNIT_Y,
                                       &layer_collection_ptr,
@@ -1548,7 +1549,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                       0,
                                       0,
                                       (int)(region->v2d.cur.xmax - restrict_offsets.holdout),
-                                      te->ys,
+                                      te.ys,
                                       UI_UNIT_X,
                                       UI_UNIT_Y,
                                       &layer_collection_ptr,
@@ -1578,7 +1579,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                   0,
                   0,
                   (int)(region->v2d.cur.xmax - restrict_offsets.indirect_only),
-                  te->ys,
+                  te.ys,
                   UI_UNIT_X,
                   UI_UNIT_Y,
                   &layer_collection_ptr,
@@ -1610,7 +1611,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                     0,
                                     0,
                                     (int)(region->v2d.cur.xmax - restrict_offsets.viewport),
-                                    te->ys,
+                                    te.ys,
                                     UI_UNIT_X,
                                     UI_UNIT_Y,
                                     &collection_ptr,
@@ -1647,7 +1648,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                     0,
                                     0,
                                     (int)(region->v2d.cur.xmax - restrict_offsets.render),
-                                    te->ys,
+                                    te.ys,
                                     UI_UNIT_X,
                                     UI_UNIT_Y,
                                     &collection_ptr,
@@ -1682,7 +1683,7 @@ static void outliner_draw_restrictbuts(uiBlock *block,
                                     0,
                                     0,
                                     (int)(region->v2d.cur.xmax - restrict_offsets.select),
-                                    te->ys,
+                                    te.ys,
                                     UI_UNIT_X,
                                     UI_UNIT_Y,
                                     &collection_ptr,
@@ -1713,16 +1714,16 @@ static void outliner_draw_restrictbuts(uiBlock *block,
         }
       }
     }
-    else if (outliner_is_collection_tree_element(te)) {
+    else if (outliner_is_collection_tree_element(&te)) {
       PointerRNA collection_ptr;
       PointerRNA layer_collection_ptr;
       outliner_restrict_properties_collection_set(
-          scene, te, &collection_ptr, &layer_collection_ptr, &props, &props_active);
+          scene, &te, &collection_ptr, &layer_collection_ptr, &props, &props_active);
     }
 
     if (TSELEM_OPEN(tselem, space_outliner)) {
       outliner_draw_restrictbuts(
-          block, scene, view_layer, region, space_outliner, &te->subtree, props_active);
+          block, scene, view_layer, region, space_outliner, te.child_elements, props_active);
     }
   }
 }
@@ -1797,7 +1798,7 @@ static void outliner_draw_userbuts(uiBlock *block,
 static void outliner_draw_overrides_rna_buts(uiBlock *block,
                                              const ARegion *region,
                                              const SpaceOutliner *space_outliner,
-                                             const ListBase *lb,
+                                             const SubTree &subtree,
                                              const int x)
 {
   const float pad_x = 2.0f * UI_DPI_FAC;
@@ -1805,17 +1806,17 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
   const float item_max_width = round_fl_to_int(OL_RNA_COL_SIZEX - 2 * pad_x);
   const float item_height = round_fl_to_int(UI_UNIT_Y - 2.0f * pad_y);
 
-  LISTBASE_FOREACH (const TreeElement *, te, lb) {
-    const TreeStoreElem *tselem = TREESTORE(te);
+  for (const TreeElement &te : subtree) {
+    const TreeStoreElem *tselem = TREESTORE(&te);
     if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_overrides_rna_buts(block, region, space_outliner, &te->subtree, x);
+      outliner_draw_overrides_rna_buts(block, region, space_outliner, te.child_elements, x);
     }
 
-    if (!outliner_is_element_in_view(te, &region->v2d)) {
+    if (!outliner_is_element_in_view(&te, &region->v2d)) {
       continue;
     }
     TreeElementOverridesProperty *override_elem = tree_element_cast<TreeElementOverridesProperty>(
-        te);
+        &te);
     if (!override_elem) {
       continue;
     }
@@ -1826,7 +1827,7 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
                             0,
                             override_elem->rna_path.c_str(),
                             x + pad_x,
-                            te->ys + pad_y,
+                            te.ys + pad_y,
                             item_max_width,
                             item_height,
                             nullptr,
@@ -1840,14 +1841,14 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
     }
 
     if (const TreeElementOverridesPropertyOperation *override_op_elem =
-            tree_element_cast<TreeElementOverridesPropertyOperation>(te)) {
+            tree_element_cast<TreeElementOverridesPropertyOperation>(&te)) {
       StringRefNull op_label = override_op_elem->getOverrideOperationLabel();
       uiDefBut(block,
                UI_BTYPE_LABEL,
                0,
                op_label.c_str(),
                x + pad_x,
-               te->ys + pad_y,
+               te.ys + pad_y,
                item_max_width,
                item_height,
                nullptr,
@@ -1870,7 +1871,7 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
                                     (prop_type == PROP_ENUM) ? nullptr : "",
                                     ICON_NONE,
                                     x + pad_x,
-                                    te->ys + pad_y,
+                                    te.ys + pad_y,
                                     item_max_width,
                                     item_height);
     /* Added the button successfully, nothing else to do. Otherwise, cases for multiple buttons
@@ -1883,7 +1884,7 @@ static void outliner_draw_overrides_rna_buts(uiBlock *block,
       /* TODO what if the array is longer, and doesn't fit nicely? What about multi-dimension
        * arrays? */
       uiDefAutoButsArrayR(
-          block, ptr, prop, ICON_NONE, x + pad_x, te->ys + pad_y, item_max_width, item_height);
+          block, ptr, prop, ICON_NONE, x + pad_x, te.ys + pad_y, item_max_width, item_height);
     }
   }
 }
@@ -1906,19 +1907,20 @@ static void outliner_draw_overrides_restrictbuts(Main *bmain,
                                                  uiBlock *block,
                                                  const ARegion *region,
                                                  const SpaceOutliner *space_outliner,
-                                                 const ListBase *lb,
+                                                 const SubTree &subtree,
                                                  const int x)
 {
-  LISTBASE_FOREACH (const TreeElement *, te, lb) {
-    const TreeStoreElem *tselem = TREESTORE(te);
+  for (const TreeElement &te : subtree) {
+    const TreeStoreElem *tselem = TREESTORE(&te);
     if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_overrides_restrictbuts(bmain, block, region, space_outliner, &te->subtree, x);
+      outliner_draw_overrides_restrictbuts(
+          bmain, block, region, space_outliner, te.child_elements, x);
     }
 
-    if (!outliner_is_element_in_view(te, &region->v2d)) {
+    if (!outliner_is_element_in_view(&te, &region->v2d)) {
       continue;
     }
-    TreeElementID *te_id = tree_element_cast<TreeElementID>(te);
+    TreeElementID *te_id = tree_element_cast<TreeElementID>(&te);
     if (!te_id) {
       continue;
     }
@@ -1939,7 +1941,7 @@ static void outliner_draw_overrides_restrictbuts(Main *bmain,
                                WM_OP_EXEC_DEFAULT,
                                icon,
                                x,
-                               te->ys,
+                               te.ys,
                                UI_UNIT_X,
                                UI_UNIT_Y,
                                "");
@@ -3946,12 +3948,13 @@ void draw_outliner(const bContext *C)
     if (space_outliner->lib_override_view_mode == SO_LIB_OVERRIDE_VIEW_PROPERTIES) {
       UI_block_emboss_set(block, UI_EMBOSS);
       UI_block_flag_enable(block, UI_BLOCK_NO_DRAW_OVERRIDDEN_STATE);
-      outliner_draw_overrides_rna_buts(block, region, space_outliner, &space_outliner->tree, x);
+      outliner_draw_overrides_rna_buts(
+          block, region, space_outliner, space_outliner->runtime->root_elements, x);
       UI_block_emboss_set(block, UI_EMBOSS_NONE_OR_STATUS);
     }
     else if (space_outliner->lib_override_view_mode == SO_LIB_OVERRIDE_VIEW_HIERARCHIES) {
       outliner_draw_overrides_restrictbuts(
-          mainvar, block, region, space_outliner, &space_outliner->tree, x);
+          mainvar, block, region, space_outliner, space_outliner->runtime->root_elements, x);
     }
   }
   else if (right_column_width > 0.0f) {
@@ -3963,7 +3966,7 @@ void draw_outliner(const bContext *C)
                                tvc.view_layer,
                                region,
                                space_outliner,
-                               &space_outliner->tree,
+                               space_outliner->runtime->root_elements,
                                props_active);
   }
 
diff --git a/source/blender/editors/space_outliner/outliner_intern.hh b/source/blender/editors/space_outliner/outliner_intern.hh
index ad5d653949c..c67f7c9ff93 100644
--- a/source/blender/editors/space_outliner/outliner_intern.hh
+++ b/source/blender/editors/space_outliner/outliner_intern.hh
@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <iterator>
+#include <list>
 #include <memory>
 
 #include "RNA_types.h"
@@ -40,10 +42,132 @@ namespace blender::ed::outliner {
 
 class AbstractTreeDisplay;
 class AbstractTreeElement;
+struct SpaceOutliner_Runtime;
+struct TreeElement;
 
 namespace treehash = blender::bke::outliner::treehash;
 
-struct TreeElement;
+/**
+ * Container to own a number of tree elements.
+ */
+class SubTree {
+  /** The tree element owning this sub-tree. Remains null for root level elements. */
+  TreeElement *parent_ = nullptr;
+
+  std::list<std::unique_ptr<TreeElement>> elements_;
+
+  /* TODO all this boilerplate... make this a generic indirect-iterator? */
+  template<typename IterT, typename NestedIterT, typename ElemT> class IteratorBase {
+   public:
+    NestedIterT iter_;
+
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = ElemT;
+    using difference_type = ptrdiff_t;
+    using pointer = ElemT *;
+    using reference = ElemT &;
+
+    explicit IteratorBase(NestedIterT iter) : iter_(iter)
+    {
+    }
+
+    ElemT &operator*()
+    {
+      return **iter_;
+    }
+    ElemT *operator->()
+    {
+      return &**iter_;
+    }
+    IterT &operator++()
+    {
+      ++iter_;
+      return *static_cast<IterT *>(this);
+    }
+    IterT &operator--()
+    {
+      --iter_;
+      return *static_cast<IterT *>(this);
+    }
+    IterT operator++(int)
+    {
+      IterT tmp = *this;
+      ++iter_;
+      return tmp;
+    }
+    IterT operator--(int)
+    {
+      IterT tmp = *this;
+      --iter_;
+      return tmp;
+    }
+    friend bool operator==(const IterT &a, const IterT &b)
+    {
+      return a.iter_ == b.iter_;
+    }
+    friend bool operator!=(const IterT &a, const IterT &b)
+    {
+      return a.iter_ != b.iter_;
+    }
+  };
+
+  class Iterator : public IteratorBase<Iterator, decltype(elements_)::iterator, TreeElement> {
+    using IteratorBase::IteratorBase;
+  };
+  class ConstIterator : public IteratorBase<ConstIterator,
+                                            decltype(elements_)::const_iterator,
+                                            const TreeElement> {
+    using IteratorBase::IteratorBase;
+  };
+
+ public:
+  using iterator = Iterator;
+  using const_iterator = Iterator;
+
+  Iterator begin()
+  {
+    return Iterator{elements_.begin()};
+  }
+  std::reverse_iterator<Iterator> rbegin()
+  {
+    return std::reverse_iterator<Iterator>(Iterator{elements_.end()});
+  }
+  Iterator end()
+  {
+    return Iterator{elements_.end()};
+  }
+  std::reverse_iterator<Iterator> rend()
+  {
+    return std::reverse_iterator<Iterator>(Iterator{elements_.begin()});
+  }
+  ConstIterator begin() const
+  {
+    return ConstIterator{elements_.begin()};
+  }
+  ConstIterator end() const
+  {
+    return ConstIterator{elements_.end()};
+  }
+
+  explicit SubTree(TreeElement &parent);
+  explicit SubTree(SpaceOutliner_Runtime &root);
+
+  void add_back(std::unique_ptr<TreeElement> elem);
+  void insert_before(Iterator pos, std::unique_ptr<TreeElement> elem);
+  /** Detaches the given element from this sub-tree, and moves ownership of it to the calling
+   * scope (by returning its unique pointer). This way the caller can decide to keep it still,
+   * for example to reinsert it elsewhere. If the caller doesn't use it (e.g. by ignoring the
+   * return value entirely), the element will be destructed and freed cleanly. */
+  std::unique_ptr<TreeElement> remove(TreeElement &element);
+  /** Erase and destruct all elements in the container. */
+  void clear();
+
+  bool has_child(const TreeElement &needle) const;
+
+  bool is_empty() const;
+  /** Get the element owning these children. Will return null in case of root level elements. */
+  TreeElement *parent() const;
+};
 
 struct SpaceOutliner_Runtime {
   /** Object to create and manage the tree for a specific display type (View Layers, Scenes,
@@ -53,7 +177,11 @@ struct SpaceOutliner_Runtime {
   /* Hash table for tree-store elements, using `(id, type, index)` as key. */
   std::unique_ptr<treehash::TreeHash> tree_hash;
 
-  SpaceOutliner_Runtime() = default;
+  /** Entry point for the outliner tree. This is essentially a sorted vector of #TreeElement's,
+   * whereby each can have its own #SubTree containing the same. */
+  SubTree root_elements;
+
+  SpaceOutliner_Runtime();
   /** Used for copying runtime data to a duplicated space. */
   SpaceOutliner_Runtime(const SpaceOutliner_Runtime &);
   ~SpaceOutliner_Runtime() = default;
@@ -76,6 +204,12 @@ enum TreeTraversalAction {
 
 typedef TreeTraversalAction (*TreeTraversalFunc)(TreeElement *te, void *customdata);
 
+/**
+ * Legacy representation of an item in the Outliner tree. The new representation is
+ * #AbstractTreeElement, efforts should be put into slowly transitioning to it. However, since not
+ * all tree element types support this yet (#TreeElement.abstract_element will be null), things are
+ * split between the too, and the situation is rather chaotic.
+ */
 struct TreeElement {
   TreeElement *next, *prev, *parent;
 
@@ -88,6 +222,9 @@ struct TreeElement {
   std::unique_ptr<AbstractTreeElement> abstract_element;
 
   ListBase subtree;
+
+  SubTree child_elements;
+
   int xs, ys;                /* Do selection. */
   TreeStoreElem *store_elem; /* Element in tree store. */
   short flag;                /* Flag for non-saved stuff. */
@@ -96,6 +233,10 @@ struct TreeElement {
   short xend;                /* Width of item display, for select. */
   const char *name;
   void *directdata; /* Armature Bones, Base, ... */
+
+  TreeElement() : child_elements(*this)
+  {
+  }
 };
 
 struct TreeElementIcon {
@@ -254,7 +395,6 @@ enum TreeItemSelectAction {
 
 /* outliner_tree.c ----------------------------------------------- */
 
-void outliner_free_tree(ListBase *tree);
 void outliner_cleanup_tree(struct SpaceOutliner *space_outliner);
 /**
  * Free \a element and its sub-tree and remove its link in \a parent_subtree.
@@ -262,7 +402,7 @@ void outliner_cleanup_tree(struct SpaceOutliner *space_outliner);
  * \note Does not remove the #TreeStoreElem of \a element!
  * \param parent_subtree: Sub-tree of the parent element, so the list containing \a element.
  */
-void outliner_free_tree_element(TreeElement *element, ListBase *parent_subtree);
+void outliner_free_tree_element(TreeElement *element, SubTree &parent_subtree);
 
 /**
  * Main entry point for building the tree data-structure that the outliner represents.
diff --git a/source/blender/editors/space_outliner/outliner_tree.cc b/source/blender/editors/space_outliner/outliner_tree.cc
index 070df284c6f..42ead451354 100644
--- a/source/blender/editors/space_outliner/outliner_tree.cc
+++ b/source/blender/editors/space_outliner/outliner_tree.cc
@@ -140,7 +140,7 @@ static void outliner_storage_cleanup(SpaceOutliner *space_outliner)
 }
 
 static void check_persistent(
-    SpaceOutliner *space_outliner, TreeElement *te, ID *id, short type, short nr)
+    SpaceOutliner *space_outliner, TreeElement &te, ID *id, short type, short nr)
 {
   if (space_outliner->treestore == nullptr) {
     /* if treestore was not created in readfile.c, create it here */
@@ -156,7 +156,7 @@ static void check_persistent(
    * (note that there may be multiple unused elements in case of linked objects) */
   TreeStoreElem *tselem = space_outliner->runtime->tree_hash->lookup_unused(type, nr, id);
   if (tselem) {
-    te->store_elem = tselem;
+    te.store_elem = tselem;
     tselem->used = 1;
     return;
   }
@@ -168,7 +168,7 @@ static void check_persistent(
   tselem->id = id;
   tselem->used = 0;
   tselem->flag = TSE_CLOSED;
-  te->store_elem = tselem;
+  te.store_elem = tselem;
   space_outliner->runtime->tree_hash->add_element(*tselem);
 }
 
@@ -178,31 +178,20 @@ static void check_persistent(
 /** \name Tree Management
  * \{ */
 
-void outliner_free_tree(ListBase *tree)
-{
-  LISTBASE_FOREACH_MUTABLE (TreeElement *, element, tree) {
-    outliner_free_tree_element(element, tree);
-  }
-}
-
 void outliner_cleanup_tree(SpaceOutliner *space_outliner)
 {
-  outliner_free_tree(&space_outliner->tree);
+  space_outliner->runtime->root_elements.clear();
   outliner_storage_cleanup(space_outliner);
 }
 
-void outliner_free_tree_element(TreeElement *element, ListBase *parent_subtree)
+void outliner_free_tree_element(TreeElement *element, SubTree &parent_subtree)
 {
-  BLI_assert(BLI_findindex(parent_subtree, element) > -1);
-  BLI_remlink(parent_subtree, element);
-
-  outliner_free_tree(&element->subtree);
+  BLI_assert(parent_subtree.has_child(*element));
+  parent_subtree.remove(*element);
 
   if (element->flag & TE_FREE_NAME) {
     MEM_freeN((void *)element->name);
   }
-  element->abstract_element = nullptr;
-  MEM_delete(element);
 }
 
 /* ********************************************************* */
@@ -218,29 +207,22 @@ bool outliner_requires_rebuild_on_select_or_active_change(const SpaceOutliner *s
 }
 
 /* special handling of hierarchical non-lib data */
-static void outliner_add_bone(SpaceOutliner *space_outliner,
-                              ListBase *lb,
-                              ID *id,
-                              Bone *curBone,
-                              TreeElement *parent,
-                              int *a)
+static void outliner_add_bone(
+    SpaceOutliner *space_outliner, ID *id, Bone *curBone, TreeElement *parent, int *a)
 {
-  TreeElement *te = outliner_add_element(space_outliner, lb, id, parent, TSE_BONE, *a);
+  TreeElement *te = outliner_add_element(space_outliner, id, parent, TSE_BONE, *a);
 
   (*a)++;
   te->name = curBone->name;
   te->directdata = curBone;
 
   LISTBASE_FOREACH (Bone *, child_bone, &curBone->childbase) {
-    outliner_add_bone(space_outliner, &te->subtree, id, child_bone, te, a);
+    outliner_add_bone(space_outliner, id, child_bone, te, a);
   }
 }
 
 #ifdef WITH_FREESTYLE
-static void outliner_add_line_styles(SpaceOutliner *space_outliner,
-                                     ListBase *lb,
-                                     Scene *sce,
-                                     TreeElement *te)
+static void outliner_add_line_styles(SpaceOutliner *space_outliner, Scene *sce, TreeElement *te)
 {
   ViewLayer *view_layer;
   FreestyleLineSet *lineset;
@@ -261,7 +243,7 @@ static void outliner_add_line_styles(SpaceOutliner *space_outliner,
           continue;
         }
         linestyle->id.tag &= ~LIB_TAG_DOIT;
-        outliner_add_element(space_outliner, lb, linestyle, te, TSE_SOME_ID, 0);
+        outliner_add_element(space_outliner, linestyle, te, TSE_SOME_ID, 0);
       }
     }
   }
@@ -275,18 +257,17 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
                                          Object *ob)
 {
   if (outliner_animdata_test(ob->adt)) {
-    outliner_add_element(space_outliner, &te->subtree, ob, te, TSE_ANIM_DATA, 0);
+    outliner_add_element(space_outliner, ob, te, TSE_ANIM_DATA, 0);
   }
 
   /* FIXME: add a special type for this. */
-  outliner_add_element(space_outliner, &te->subtree, ob->poselib, te, TSE_SOME_ID, 0);
+  outliner_add_element(space_outliner, ob->poselib, te, TSE_SOME_ID, 0);
 
-  outliner_add_element(space_outliner, &te->subtree, ob->data, te, TSE_SOME_ID, 0);
+  outliner_add_element(space_outliner, ob->data, te, TSE_SOME_ID, 0);
 
   if (ob->pose) {
     bArmature *arm = static_cast<bArmature *>(ob->data);
-    TreeElement *tenla = outliner_add_element(
-        space_outliner, &te->subtree, ob, te, TSE_POSE_BASE, 0);
+    TreeElement *tenla = outliner_add_element(space_outliner, ob, te, TSE_POSE_BASE, 0);
     tenla->name = IFACE_("Pose");
 
     /* channels undefined in editmode, but we want the 'tenla' pose icon itself */
@@ -294,8 +275,7 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
       int const_index = 1000; /* ensure unique id for bone constraints */
       int a;
       LISTBASE_FOREACH_INDEX (bPoseChannel *, pchan, &ob->pose->chanbase, a) {
-        TreeElement *ten = outliner_add_element(
-            space_outliner, &tenla->subtree, ob, tenla, TSE_POSE_CHANNEL, a);
+        TreeElement *ten = outliner_add_element(space_outliner, ob, tenla, TSE_POSE_CHANNEL, a);
         ten->name = pchan->name;
         ten->directdata = pchan;
         pchan->temp = (void *)ten;
@@ -303,13 +283,13 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
         if (!BLI_listbase_is_empty(&pchan->constraints)) {
           /* Object *target; */
           TreeElement *tenla1 = outliner_add_element(
-              space_outliner, &ten->subtree, ob, ten, TSE_CONSTRAINT_BASE, 0);
+              space_outliner, ob, ten, TSE_CONSTRAINT_BASE, 0);
           tenla1->name = IFACE_("Constraints");
           /* char *str; */
 
           LISTBASE_FOREACH (bConstraint *, con, &pchan->constraints) {
             TreeElement *ten1 = outliner_add_element(
-                space_outliner, &tenla1->subtree, ob, tenla1, TSE_CONSTRAINT, const_index);
+                space_outliner, ob, tenla1, TSE_CONSTRAINT, const_index);
 #if 0 /* disabled as it needs to be reworked for recoded constraints system */
             target = get_constraint_target(con, &str);
             if (str && str[0]) {
@@ -349,14 +329,13 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
 
     /* Pose Groups */
     if (!BLI_listbase_is_empty(&ob->pose->agroups)) {
-      TreeElement *ten_bonegrp = outliner_add_element(
-          space_outliner, &te->subtree, ob, te, TSE_POSEGRP_BASE, 0);
+      TreeElement *ten_bonegrp = outliner_add_element(space_outliner, ob, te, TSE_POSEGRP_BASE, 0);
       ten_bonegrp->name = IFACE_("Bone Groups");
 
       int index;
       LISTBASE_FOREACH_INDEX (bActionGroup *, agrp, &ob->pose->agroups, index) {
         TreeElement *ten = outliner_add_element(
-            space_outliner, &ten_bonegrp->subtree, ob, ten_bonegrp, TSE_POSEGRP, index);
+            space_outliner, ob, ten_bonegrp, TSE_POSEGRP, index);
         ten->name = agrp->name;
         ten->directdata = agrp;
       }
@@ -364,18 +343,16 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
   }
 
   for (int a = 0; a < ob->totcol; a++) {
-    outliner_add_element(space_outliner, &te->subtree, ob->mat[a], te, TSE_SOME_ID, a);
+    outliner_add_element(space_outliner, ob->mat[a], te, TSE_SOME_ID, a);
   }
 
   if (!BLI_listbase_is_empty(&ob->constraints)) {
-    TreeElement *tenla = outliner_add_element(
-        space_outliner, &te->subtree, ob, te, TSE_CONSTRAINT_BASE, 0);
+    TreeElement *tenla = outliner_add_element(space_outliner, ob, te, TSE_CONSTRAINT_BASE, 0);
     tenla->name = IFACE_("Constraints");
 
     int index;
     LISTBASE_FOREACH_INDEX (bConstraint *, con, &ob->constraints, index) {
-      TreeElement *ten = outliner_add_element(
-          space_outliner, &tenla->subtree, ob, tenla, TSE_CONSTRAINT, index);
+      TreeElement *ten = outliner_add_element(space_outliner, ob, tenla, TSE_CONSTRAINT, index);
 #if 0 /* disabled due to constraints system targets recode... code here needs review */
       target = get_constraint_target(con, &str);
       if (str && str[0]) {
@@ -395,54 +372,36 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
   }
 
   if (!BLI_listbase_is_empty(&ob->modifiers)) {
-    TreeElement *ten_mod = outliner_add_element(
-        space_outliner, &te->subtree, ob, te, TSE_MODIFIER_BASE, 0);
+    TreeElement *ten_mod = outliner_add_element(space_outliner, ob, te, TSE_MODIFIER_BASE, 0);
     ten_mod->name = IFACE_("Modifiers");
 
     int index;
     LISTBASE_FOREACH_INDEX (ModifierData *, md, &ob->modifiers, index) {
-      TreeElement *ten = outliner_add_element(
-          space_outliner, &ten_mod->subtree, ob, ten_mod, TSE_MODIFIER, index);
+      TreeElement *ten = outliner_add_element(space_outliner, ob, ten_mod, TSE_MODIFIER, index);
       ten->name = md->name;
       ten->directdata = md;
 
       if (md->type == eModifierType_Lattice) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((LatticeModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((LatticeModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eModifierType_Curve) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((CurveModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((CurveModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eModifierType_Armature) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((ArmatureModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((ArmatureModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eModifierType_Hook) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((HookModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((HookModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eModifierType_ParticleSystem) {
         ParticleSystem *psys = ((ParticleSystemModifierData *)md)->psys;
         TreeElement *ten_psys;
 
-        ten_psys = outliner_add_element(space_outliner, &ten->subtree, ob, te, TSE_LINKED_PSYS, 0);
+        ten_psys = outliner_add_element(space_outliner, ob, te, TSE_LINKED_PSYS, 0);
         ten_psys->directdata = psys;
         ten_psys->name = psys->part->id.name + 2;
       }
@@ -451,64 +410,45 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
 
   /* Grease Pencil modifiers. */
   if (!BLI_listbase_is_empty(&ob->greasepencil_modifiers)) {
-    TreeElement *ten_mod = outliner_add_element(
-        space_outliner, &te->subtree, ob, te, TSE_MODIFIER_BASE, 0);
+    TreeElement *ten_mod = outliner_add_element(space_outliner, ob, te, TSE_MODIFIER_BASE, 0);
     ten_mod->name = IFACE_("Modifiers");
 
     int index;
     LISTBASE_FOREACH_INDEX (GpencilModifierData *, md, &ob->greasepencil_modifiers, index) {
-      TreeElement *ten = outliner_add_element(
-          space_outliner, &ten_mod->subtree, ob, ten_mod, TSE_MODIFIER, index);
+      TreeElement *ten = outliner_add_element(space_outliner, ob, ten_mod, TSE_MODIFIER, index);
       ten->name = md->name;
       ten->directdata = md;
 
       if (md->type == eGpencilModifierType_Armature) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((ArmatureGpencilModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((ArmatureGpencilModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eGpencilModifierType_Hook) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((HookGpencilModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((HookGpencilModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
       else if (md->type == eGpencilModifierType_Lattice) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((LatticeGpencilModifierData *)md)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((LatticeGpencilModifierData *)md)->object, ten, TSE_LINKED_OB, 0);
       }
     }
   }
 
   /* Grease Pencil effects. */
   if (!BLI_listbase_is_empty(&ob->shader_fx)) {
-    TreeElement *ten_fx = outliner_add_element(
-        space_outliner, &te->subtree, ob, te, TSE_GPENCIL_EFFECT_BASE, 0);
+    TreeElement *ten_fx = outliner_add_element(space_outliner, ob, te, TSE_GPENCIL_EFFECT_BASE, 0);
     ten_fx->name = IFACE_("Effects");
 
     int index;
     LISTBASE_FOREACH_INDEX (ShaderFxData *, fx, &ob->shader_fx, index) {
       TreeElement *ten = outliner_add_element(
-          space_outliner, &ten_fx->subtree, ob, ten_fx, TSE_GPENCIL_EFFECT, index);
+          space_outliner, ob, ten_fx, TSE_GPENCIL_EFFECT, index);
       ten->name = fx->name;
       ten->directdata = fx;
 
       if (fx->type == eShaderFxType_Swirl) {
-        outliner_add_element(space_outliner,
-                             &ten->subtree,
-                             ((SwirlShaderFxData *)fx)->object,
-                             ten,
-                             TSE_LINKED_OB,
-                             0);
+        outliner_add_element(
+            space_outliner, ((SwirlShaderFxData *)fx)->object, ten, TSE_LINKED_OB, 0);
       }
     }
   }
@@ -517,14 +457,12 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
   if (ELEM(ob->type, OB_MESH, OB_GPENCIL, OB_LATTICE)) {
     const ListBase *defbase = BKE_object_defgroup_list(ob);
     if (!BLI_listbase_is_empty(defbase)) {
-      TreeElement *tenla = outliner_add_element(
-          space_outliner, &te->subtree, ob, te, TSE_DEFGROUP_BASE, 0);
+      TreeElement *tenla = outliner_add_element(space_outliner, ob, te, TSE_DEFGROUP_BASE, 0);
       tenla->name = IFACE_("Vertex Groups");
 
       int index;
       LISTBASE_FOREACH_INDEX (bDeformGroup *, defgroup, defbase, index) {
-        TreeElement *ten = outliner_add_element(
-            space_outliner, &tenla->subtree, ob, tenla, TSE_DEFGROUP, index);
+        TreeElement *ten = outliner_add_element(space_outliner, ob, tenla, TSE_DEFGROUP, index);
         ten->name = defgroup->name;
         ten->directdata = defgroup;
       }
@@ -533,20 +471,19 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
 
   /* duplicated group */
   if (ob->instance_collection && (ob->transflag & OB_DUPLICOLLECTION)) {
-    outliner_add_element(
-        space_outliner, &te->subtree, ob->instance_collection, te, TSE_SOME_ID, 0);
+    outliner_add_element(space_outliner, ob->instance_collection, te, TSE_SOME_ID, 0);
   }
 }
 
 /* Can be inlined if necessary. */
 static void outliner_add_id_contents(SpaceOutliner *space_outliner,
-                                     TreeElement *te,
+                                     TreeElement &te,
                                      TreeStoreElem *tselem,
                                      ID *id)
 {
   /* tuck pointer back in object, to construct hierarchy */
   if (GS(id->name) == ID_OB) {
-    id->newid = (ID *)te;
+    id->newid = (ID *)&te;
   }
 
   /* expand specific data always */
@@ -556,19 +493,19 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       BLI_assert_msg(0, "ID type expected to be expanded through new tree-element design");
       break;
     case ID_OB: {
-      outliner_add_object_contents(space_outliner, te, tselem, (Object *)id);
+      outliner_add_object_contents(space_outliner, &te, tselem, (Object *)id);
       break;
     }
     case ID_ME: {
       Mesh *me = (Mesh *)id;
 
       if (outliner_animdata_test(me->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, me, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, me, &te, TSE_ANIM_DATA, 0);
       }
 
-      outliner_add_element(space_outliner, &te->subtree, me->key, te, TSE_SOME_ID, 0);
+      outliner_add_element(space_outliner, me->key, &te, TSE_SOME_ID, 0);
       for (int a = 0; a < me->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, me->mat[a], te, TSE_SOME_ID, a);
+        outliner_add_element(space_outliner, me->mat[a], &te, TSE_SOME_ID, a);
       }
       /* could do tfaces with image links, but the images are not grouped nicely.
        * would require going over all tfaces, sort images in use. etc... */
@@ -578,11 +515,11 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       Curve *cu = (Curve *)id;
 
       if (outliner_animdata_test(cu->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, cu, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, cu, &te, TSE_ANIM_DATA, 0);
       }
 
       for (int a = 0; a < cu->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, cu->mat[a], te, TSE_SOME_ID, a);
+        outliner_add_element(space_outliner, cu->mat[a], &te, TSE_SOME_ID, a);
       }
       break;
     }
@@ -590,40 +527,40 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       MetaBall *mb = (MetaBall *)id;
 
       if (outliner_animdata_test(mb->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, mb, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, mb, &te, TSE_ANIM_DATA, 0);
       }
 
       for (int a = 0; a < mb->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, mb->mat[a], te, TSE_SOME_ID, a);
+        outliner_add_element(space_outliner, mb->mat[a], &te, TSE_SOME_ID, a);
       }
       break;
     }
     case ID_MA: {
       Material *ma = (Material *)id;
       if (outliner_animdata_test(ma->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, ma, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, ma, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_TE: {
       Tex *tex = (Tex *)id;
       if (outliner_animdata_test(tex->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, tex, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, tex, &te, TSE_ANIM_DATA, 0);
       }
-      outliner_add_element(space_outliner, &te->subtree, tex->ima, te, TSE_SOME_ID, 0);
+      outliner_add_element(space_outliner, tex->ima, &te, TSE_SOME_ID, 0);
       break;
     }
     case ID_CA: {
       Camera *ca = (Camera *)id;
       if (outliner_animdata_test(ca->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, ca, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, ca, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_CF: {
       CacheFile *cache_file = (CacheFile *)id;
       if (outliner_animdata_test(cache_file->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, cache_file, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, cache_file, &te, TSE_ANIM_DATA, 0);
       }
 
       break;
@@ -631,35 +568,35 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
     case ID_LA: {
       Light *la = (Light *)id;
       if (outliner_animdata_test(la->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, la, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, la, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_SPK: {
       Speaker *spk = (Speaker *)id;
       if (outliner_animdata_test(spk->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, spk, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, spk, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_LP: {
       LightProbe *prb = (LightProbe *)id;
       if (outliner_animdata_test(prb->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, prb, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, prb, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_WO: {
       World *wrld = (World *)id;
       if (outliner_animdata_test(wrld->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, wrld, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, wrld, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_KE: {
       Key *key = (Key *)id;
       if (outliner_animdata_test(key->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, key, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, key, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
@@ -672,14 +609,13 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       bArmature *arm = (bArmature *)id;
 
       if (outliner_animdata_test(arm->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, arm, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, arm, &te, TSE_ANIM_DATA, 0);
       }
 
       if (arm->edbo) {
         int a = 0;
         LISTBASE_FOREACH_INDEX (EditBone *, ebone, arm->edbo, a) {
-          TreeElement *ten = outliner_add_element(
-              space_outliner, &te->subtree, id, te, TSE_EBONE, a);
+          TreeElement *ten = outliner_add_element(space_outliner, id, &te, TSE_EBONE, a);
           ten->directdata = ebone;
           ten->name = ebone->name;
           ebone->temp.p = ten;
@@ -689,20 +625,21 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
                                static_cast<TreeElement *>(((EditBone *)arm->edbo->first)->temp.p) :
                                nullptr;
         while (ten) {
-          TreeElement *nten = ten->next, *par;
+          TreeElement *nten = ten->next;
+
           EditBone *ebone = (EditBone *)ten->directdata;
           if (ebone->parent) {
-            BLI_remlink(&te->subtree, ten);
-            par = static_cast<TreeElement *>(ebone->parent->temp.p);
-            BLI_addtail(&par->subtree, ten);
-            ten->parent = par;
+            std::unique_ptr ten_uptr = te.child_elements.remove(*ten);
+
+            TreeElement *new_parent = static_cast<TreeElement *>(ebone->parent->temp.p);
+            new_parent->child_elements.add_back(std::move(ten_uptr));
           }
           ten = nten;
         }
       }
       else {
         /* do not extend Armature when we have posemode */
-        tselem = TREESTORE(te->parent);
+        tselem = TREESTORE(te.parent);
         if (TSE_IS_REAL_ID(tselem) && GS(tselem->id->name) == ID_OB &&
             ((Object *)tselem->id)->mode & OB_MODE_POSE) {
           /* pass */
@@ -710,7 +647,7 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
         else {
           int a = 0;
           LISTBASE_FOREACH (Bone *, bone, &arm->bonebase) {
-            outliner_add_bone(space_outliner, &te->subtree, id, bone, te, &a);
+            outliner_add_bone(space_outliner, id, bone, &te, &a);
           }
         }
       }
@@ -720,12 +657,12 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       FreestyleLineStyle *linestyle = (FreestyleLineStyle *)id;
 
       if (outliner_animdata_test(linestyle->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, linestyle, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, linestyle, &te, TSE_ANIM_DATA, 0);
       }
 
       for (int a = 0; a < MAX_MTEX; a++) {
         if (linestyle->mtex[a]) {
-          outliner_add_element(space_outliner, &te->subtree, linestyle->mtex[a]->tex, te, 0, a);
+          outliner_add_element(space_outliner, linestyle->mtex[a]->tex, &te, 0, a);
         }
       }
       break;
@@ -734,50 +671,50 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       bGPdata *gpd = (bGPdata *)id;
 
       if (outliner_animdata_test(gpd->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, gpd, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, gpd, &te, TSE_ANIM_DATA, 0);
       }
 
       /* TODO: base element for layers? */
       int index = 0;
       LISTBASE_FOREACH_BACKWARD (bGPDlayer *, gpl, &gpd->layers) {
-        outliner_add_element(space_outliner, &te->subtree, gpl, te, TSE_GP_LAYER, index);
+        outliner_add_element(space_outliner, gpl, &te, TSE_GP_LAYER, index);
         index++;
       }
       break;
     }
     case ID_GR: {
       /* Don't expand for instances, creates too many elements. */
-      if (!(te->parent && te->parent->idcode == ID_OB)) {
+      if (!(te.parent && te.parent->idcode == ID_OB)) {
         Collection *collection = (Collection *)id;
-        outliner_add_collection_recursive(space_outliner, collection, te);
+        outliner_add_collection_recursive(space_outliner, collection, &te);
       }
       break;
     }
     case ID_CV: {
       Curves *curves = (Curves *)id;
       if (outliner_animdata_test(curves->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, curves, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, curves, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_PT: {
       PointCloud *pointcloud = (PointCloud *)id;
       if (outliner_animdata_test(pointcloud->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, pointcloud, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, pointcloud, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_VO: {
       Volume *volume = (Volume *)id;
       if (outliner_animdata_test(volume->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, volume, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, volume, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
     case ID_SIM: {
       Simulation *simulation = (Simulation *)id;
       if (outliner_animdata_test(simulation->adt)) {
-        outliner_add_element(space_outliner, &te->subtree, simulation, te, TSE_ANIM_DATA, 0);
+        outliner_add_element(space_outliner, simulation, &te, TSE_ANIM_DATA, 0);
       }
       break;
     }
@@ -787,7 +724,6 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
 }
 
 TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
-                                  ListBase *lb,
                                   void *idv,
                                   TreeElement *parent,
                                   short type,
@@ -823,27 +759,38 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
     BLI_assert(TREESTORE_ID_TYPE(id));
   }
 
-  TreeElement *te = MEM_new<TreeElement>(__func__);
-  /* add to the visual tree */
-  BLI_addtail(lb, te);
+  std::unique_ptr<TreeElement> te_uptr = std::make_unique<TreeElement>();
+  TreeElement &te = *te_uptr;
+
+  /* Add to the visual tree. Moves ownership the the proper element storage. */
+  if (parent) {
+    parent->child_elements.add_back(std::move(te_uptr));
+  }
+  else {
+    space_outliner->runtime->root_elements.add_back(std::move(te_uptr));
+  }
+
   /* add to the storage */
   check_persistent(space_outliner, te, id, type, index);
-  TreeStoreElem *tselem = TREESTORE(te);
+  TreeStoreElem *tselem = TREESTORE(&te);
 
   /* if we are searching for something expand to see child elements */
   if (SEARCHING_OUTLINER(space_outliner)) {
     tselem->flag |= TSE_CHILDSEARCH;
   }
 
-  te->parent = parent;
-  te->index = index; /* For data arrays. */
+  if (te.parent != parent) {
+    printf("error\n");
+  }
+  BLI_assert(te.parent == parent);
+  te.index = index; /* For data arrays. */
 
   /* New inheritance based element representation. Not all element types support this yet,
    * eventually it should replace #TreeElement entirely. */
-  te->abstract_element = AbstractTreeElement::createFromType(type, *te, idv);
-  if (te->abstract_element) {
+  te.abstract_element = AbstractTreeElement::createFromType(type, te, idv);
+  if (te.abstract_element) {
     /* Element types ported to the new design are expected to have their name set at this point! */
-    BLI_assert(te->name != nullptr);
+    BLI_assert(te.name != nullptr);
   }
 
   if (ELEM(type, TSE_SEQUENCE, TSE_SEQ_STRIP, TSE_SEQUENCE_DUP)) {
@@ -865,7 +812,7 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
     /* pass */
   }
   else if (type == TSE_SOME_ID) {
-    if (!te->abstract_element) {
+    if (!te.abstract_element) {
       BLI_assert_msg(0, "Expected this ID type to be ported to new Outliner tree-element design");
     }
   }
@@ -873,7 +820,7 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
                 TSE_LIBRARY_OVERRIDE_BASE,
                 TSE_LIBRARY_OVERRIDE,
                 TSE_LIBRARY_OVERRIDE_OPERATION)) {
-    if (!te->abstract_element) {
+    if (!te.abstract_element) {
       BLI_assert_msg(0,
                      "Expected override types to be ported to new Outliner tree-element design");
     }
@@ -884,21 +831,21 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
 
     /* The new type design sets the name already, don't override that here. We need to figure out
      * how to deal with the idcode for non-TSE_SOME_ID types still. Some rely on it... */
-    if (!te->abstract_element) {
-      te->name = id->name + 2; /* Default, can be overridden by Library or non-ID data. */
+    if (!te.abstract_element) {
+      te.name = id->name + 2; /* Default, can be overridden by Library or non-ID data. */
     }
-    te->idcode = GS(id->name);
+    te.idcode = GS(id->name);
   }
 
   if (!expand) {
     /* Pass */
   }
-  else if (te->abstract_element && te->abstract_element->isExpandValid()) {
-    tree_element_expand(*te->abstract_element, *space_outliner);
+  else if (te.abstract_element && te.abstract_element->isExpandValid()) {
+    tree_element_expand(*te.abstract_element, *space_outliner);
   }
   else if (type == TSE_SOME_ID) {
     /* ID types not (fully) ported to new design yet. */
-    if (te->abstract_element->expandPoll(*space_outliner)) {
+    if (te.abstract_element->expandPoll(*space_outliner)) {
       outliner_add_id_contents(space_outliner, te, tselem, id);
     }
   }
@@ -919,7 +866,17 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
     BLI_assert_msg(false, "Element type should already use new AbstractTreeElement design");
   }
 
-  return te;
+  return &te;
+}
+
+TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
+                                  void *idv,
+                                  SubTree &subtree,
+                                  short type,
+                                  short index,
+                                  const bool expand)
+{
+  return outliner_add_element(space_outliner, idv, subtree.parent(), type, index, expand);
 }
 
 /* ======================================================= */
@@ -931,12 +888,11 @@ BLI_INLINE void outliner_add_collection_init(TreeElement *te, Collection *collec
 }
 
 BLI_INLINE void outliner_add_collection_objects(SpaceOutliner *space_outliner,
-                                                ListBase *tree,
                                                 Collection *collection,
                                                 TreeElement *parent)
 {
   LISTBASE_FOREACH (CollectionObject *, cob, &collection->gobject) {
-    outliner_add_element(space_outliner, tree, cob->ob, parent, TSE_SOME_ID, 0);
+    outliner_add_element(space_outliner, cob->ob, parent, TSE_SOME_ID, 0);
   }
 }
 
@@ -947,12 +903,11 @@ TreeElement *outliner_add_collection_recursive(SpaceOutliner *space_outliner,
   outliner_add_collection_init(ten, collection);
 
   LISTBASE_FOREACH (CollectionChild *, child, &collection->children) {
-    outliner_add_element(
-        space_outliner, &ten->subtree, &child->collection->id, ten, TSE_SOME_ID, 0);
+    outliner_add_element(space_outliner, &child->collection->id, ten, TSE_SOME_ID, 0);
   }
 
   if (space_outliner->outlinevis != SO_SCENES) {
-    outliner_add_collection_objects(space_outliner, &ten->subtree, collection, ten);
+    outliner_add_collection_objects(space_outliner, collection, ten);
   }
 
   return ten;
@@ -1504,7 +1459,7 @@ static bool outliner_element_visible_get(ViewLayer *view_layer,
   return true;
 }
 
-static bool outliner_filter_has_name(TreeElement *te, const char *name, int flags)
+static bool outliner_filter_has_name(const TreeElement &te, const char *name, int flags)
 {
   int fn_flag = 0;
 
@@ -1512,7 +1467,7 @@ static bool outliner_filter_has_name(TreeElement *te, const char *name, int flag
     fn_flag |= FNM_CASEFOLD;
   }
 
-  return fnmatch(name, te->name, fn_flag) == 0;
+  return fnmatch(name, te.name, fn_flag) == 0;
 }
 
 static bool outliner_element_is_collection_or_object(TreeElement *te)
@@ -1531,54 +1486,57 @@ static bool outliner_element_is_collection_or_object(TreeElement *te)
   return false;
 }
 
-static TreeElement *outliner_extract_children_from_subtree(TreeElement *element,
-                                                           ListBase *parent_subtree)
+static SubTree::iterator outliner_extract_children_from_subtree(
+    SubTree::iterator te_iter,
+    /* The tree the given elemet is in. */
+    SubTree &parent_tree)
 {
-  TreeElement *te_next = element->next;
+  TreeElement &te = *te_iter;
+  SubTree::iterator te_next = std::next(te_iter);
+
+  BLI_assert(parent_tree.has_child(te));
 
-  if (outliner_element_is_collection_or_object(element)) {
-    TreeElement *te_prev = nullptr;
-    for (TreeElement *te = static_cast<TreeElement *>(element->subtree.last); te; te = te_prev) {
-      te_prev = te->prev;
+  if (outliner_element_is_collection_or_object(&te)) {
+    for (std::reverse_iterator child_iter = te.child_elements.rbegin();
+         child_iter != te.child_elements.rend();
+         ++child_iter) {
+      TreeElement &child = *child_iter;
 
-      if (!outliner_element_is_collection_or_object(te)) {
+      if (!outliner_element_is_collection_or_object(&child)) {
         continue;
       }
 
-      te_next = te;
-      BLI_remlink(&element->subtree, te);
-      BLI_insertlinkafter(parent_subtree, element->prev, te);
-      te->parent = element->parent;
+      std::unique_ptr floating_child = te.child_elements.remove(child);
+      parent_tree.insert_before(te_iter, std::move(floating_child));
+      te_next = child_iter.base();
     }
   }
 
-  outliner_free_tree_element(element, parent_subtree);
+  parent_tree.remove(te);
   return te_next;
 }
 
 static int outliner_filter_subtree(SpaceOutliner *space_outliner,
                                    ViewLayer *view_layer,
-                                   ListBase *lb,
+                                   SubTree &subtree,
                                    const char *search_string,
                                    const int exclude_filter)
 {
-  TreeElement *te, *te_next;
-  TreeStoreElem *tselem;
+  for (SubTree::iterator te_iter = subtree.begin(); te_iter != subtree.end(); ++te_iter) {
+    TreeElement &te = *te_iter;
 
-  for (te = static_cast<TreeElement *>(lb->first); te; te = te_next) {
-    te_next = te->next;
-    if ((outliner_element_visible_get(view_layer, te, exclude_filter) == false)) {
+    if ((outliner_element_visible_get(view_layer, &te, exclude_filter) == false)) {
       /* Don't free the tree, but extract the children from the parent and add to this tree. */
       /* This also needs filtering the subtree prior (see T69246). */
       outliner_filter_subtree(
-          space_outliner, view_layer, &te->subtree, search_string, exclude_filter);
-      te_next = outliner_extract_children_from_subtree(te, lb);
+          space_outliner, view_layer, te.child_elements, search_string, exclude_filter);
+      te_iter = outliner_extract_children_from_subtree(te_iter, subtree);
       continue;
     }
     if ((exclude_filter & SO_FILTER_SEARCH) == 0) {
       /* Filter subtree too. */
       outliner_filter_subtree(
-          space_outliner, view_layer, &te->subtree, search_string, exclude_filter);
+          space_outliner, view_layer, te.child_elements, search_string, exclude_filter);
       continue;
     }
 
@@ -1589,31 +1547,31 @@ static int outliner_filter_subtree(SpaceOutliner *space_outliner,
        * - otherwise, we can't see within the subtree and the item doesn't match,
        *     so these can be safely ignored (i.e. the subtree can get freed)
        */
-      tselem = TREESTORE(te);
+      TreeStoreElem *tselem = TREESTORE(&te);
 
       /* flag as not a found item */
       tselem->flag &= ~TSE_SEARCHMATCH;
 
       if ((!TSELEM_OPEN(tselem, space_outliner)) ||
           outliner_filter_subtree(
-              space_outliner, view_layer, &te->subtree, search_string, exclude_filter) == 0) {
-        outliner_free_tree_element(te, lb);
+              space_outliner, view_layer, te.child_elements, search_string, exclude_filter) == 0) {
+        subtree.remove(te);
       }
     }
     else {
-      tselem = TREESTORE(te);
+      TreeStoreElem *tselem = TREESTORE(&te);
 
       /* flag as a found item - we can then highlight it */
       tselem->flag |= TSE_SEARCHMATCH;
 
       /* filter subtree too */
       outliner_filter_subtree(
-          space_outliner, view_layer, &te->subtree, search_string, exclude_filter);
+          space_outliner, view_layer, te.child_elements, search_string, exclude_filter);
     }
   }
 
   /* if there are still items in the list, that means that there were still some matches */
-  return (BLI_listbase_is_empty(lb) == false);
+  return !subtree.is_empty();
 }
 
 static void outliner_filter_tree(SpaceOutliner *space_outliner, ViewLayer *view_layer)
@@ -1636,8 +1594,11 @@ static void outliner_filter_tree(SpaceOutliner *space_outliner, ViewLayer *view_
     search_string = search_buff;
   }
 
-  outliner_filter_subtree(
-      space_outliner, view_layer, &space_outliner->tree, search_string, exclude_filter);
+  outliner_filter_subtree(space_outliner,
+                          view_layer,
+                          space_outliner->runtime->root_elements,
+                          search_string,
+                          exclude_filter);
 }
 
 static void outliner_clear_newid_from_main(Main *bmain)
@@ -1691,8 +1652,7 @@ void outliner_build_tree(Main *mainvar,
   OutlinerTreeElementFocus focus;
   outliner_store_scrolling_position(space_outliner, region, &focus);
 
-  outliner_free_tree(&space_outliner->tree);
-  outliner_storage_cleanup(space_outliner);
+  outliner_cleanup_tree(space_outliner);
 
   space_outliner->runtime->tree_display = AbstractTreeDisplay::createFromDisplayMode(
       space_outliner->outlinevis, *space_outliner);
@@ -1701,7 +1661,8 @@ void outliner_build_tree(Main *mainvar,
   BLI_assert(space_outliner->runtime->tree_display != nullptr);
 
   TreeSourceData source_data{*mainvar, *scene, *view_layer};
-  space_outliner->tree = space_outliner->runtime->tree_display->buildTree(source_data);
+  space_outliner->runtime->root_elements = space_outliner->runtime->tree_display->buildTree(
+      source_data);
 
   if ((space_outliner->flag & SO_SKIP_SORT_ALPHA) == 0) {
     outliner_sort(&space_outliner->tree);
diff --git a/source/blender/editors/space_outliner/outliner_utils.cc b/source/blender/editors/space_outliner/outliner_utils.cc
index ff5292e2883..dd29fedaef2 100644
--- a/source/blender/editors/space_outliner/outliner_utils.cc
+++ b/source/blender/editors/space_outliner/outliner_utils.cc
@@ -70,8 +70,7 @@ TreeElement *outliner_find_item_at_y(const SpaceOutliner *space_outliner,
         return te_iter;
       }
 
-      if (BLI_listbase_is_empty(&te_iter->subtree) ||
-          !TSELEM_OPEN(TREESTORE(te_iter), space_outliner)) {
+      if (te_iter->child_elements.is_empty() || !TSELEM_OPEN(TREESTORE(te_iter), space_outliner)) {
         /* No need for recursion. */
         continue;
       }
@@ -95,35 +94,31 @@ TreeElement *outliner_find_item_at_y(const SpaceOutliner *space_outliner,
   return nullptr;
 }
 
-static TreeElement *outliner_find_item_at_x_in_row_recursive(const TreeElement *parent_te,
+static TreeElement *outliner_find_item_at_x_in_row_recursive(TreeElement *parent_te,
                                                              float view_co_x,
                                                              bool *r_is_merged_icon)
 {
-  TreeElement *child_te = static_cast<TreeElement *>(parent_te->subtree.first);
-
-  while (child_te) {
-    const bool over_element = (view_co_x > child_te->xs) && (view_co_x < child_te->xend);
-    if ((child_te->flag & TE_ICONROW) && over_element) {
-      return child_te;
+  for (TreeElement &child_te : parent_te->child_elements) {
+    const bool over_element = (view_co_x > child_te.xs) && (view_co_x < child_te.xend);
+    if ((child_te.flag & TE_ICONROW) && over_element) {
+      return &child_te;
     }
-    if ((child_te->flag & TE_ICONROW_MERGED) && over_element) {
+    if ((child_te.flag & TE_ICONROW_MERGED) && over_element) {
       if (r_is_merged_icon) {
         *r_is_merged_icon = true;
       }
-      return child_te;
+      return &child_te;
     }
 
     TreeElement *te = outliner_find_item_at_x_in_row_recursive(
-        child_te, view_co_x, r_is_merged_icon);
-    if (te != child_te) {
+        &child_te, view_co_x, r_is_merged_icon);
+    if (te != &child_te) {
       return te;
     }
-
-    child_te = child_te->next;
   }
 
   /* return parent if no child is hovered */
-  return (TreeElement *)parent_te;
+  return parent_te;
 }
 
 TreeElement *outliner_find_item_at_x_in_row(const SpaceOutliner *space_outliner,
diff --git a/source/blender/editors/space_outliner/space_outliner.cc b/source/blender/editors/space_outliner/space_outliner.cc
index 76b7197b86a..d6cdf80779f 100644
--- a/source/blender/editors/space_outliner/space_outliner.cc
+++ b/source/blender/editors/space_outliner/space_outliner.cc
@@ -39,6 +39,10 @@
 
 namespace blender::ed::outliner {
 
+SpaceOutliner_Runtime::SpaceOutliner_Runtime() : root_elements()
+{
+}
+
 SpaceOutliner_Runtime::SpaceOutliner_Runtime(const SpaceOutliner_Runtime & /*other*/)
     : tree_display(nullptr), tree_hash(nullptr)
 {
@@ -345,7 +349,6 @@ static void outliner_free(SpaceLink *sl)
 {
   SpaceOutliner *space_outliner = (SpaceOutliner *)sl;
 
-  outliner_free_tree(&space_outliner->tree);
   if (space_outliner->treestore) {
     BLI_mempool_destroy(space_outliner->treestore);
   }
diff --git a/source/blender/editors/space_outliner/tree/common.cc b/source/blender/editors/space_outliner/tree/common.cc
index 199c80f021a..448df9c1e7d 100644
--- a/source/blender/editors/space_outliner/tree/common.cc
+++ b/source/blender/editors/space_outliner/tree/common.cc
@@ -36,25 +36,26 @@ const char *outliner_idcode_to_plural(short idcode)
 
 /** \} */
 
-void outliner_make_object_parent_hierarchy(ListBase *lb)
+void outliner_make_object_parent_hierarchy(SubTree &subtree)
 {
   /* build hierarchy */
   /* XXX also, set extents here... */
-  TreeElement *te = static_cast<TreeElement *>(lb->first);
-  while (te) {
-    TreeElement *ten = te->next;
-    TreeStoreElem *tselem = TREESTORE(te);
-
-    if ((tselem->type == TSE_SOME_ID) && te->idcode == ID_OB) {
-      Object *ob = (Object *)tselem->id;
-      if (ob->parent && ob->parent->id.newid) {
-        BLI_remlink(lb, te);
-        TreeElement *tep = (TreeElement *)ob->parent->id.newid;
-        BLI_addtail(&tep->subtree, te);
-        te->parent = tep;
-      }
+  for (TreeElement &te : subtree) {
+    TreeStoreElem *tselem = TREESTORE(&te);
+
+    if ((tselem->type != TSE_SOME_ID) || te.idcode != ID_OB) {
+      continue;
+    }
+
+    Object *ob = (Object *)tselem->id;
+    if (!ob->parent || !ob->parent->id.newid) {
+      continue;
     }
-    te = ten;
+
+    TreeElement *new_parent = reinterpret_cast<TreeElement *>(ob->parent->id.newid);
+
+    std::unique_ptr floating_te = subtree.remove(te);
+    new_parent->child_elements.add_back(std::move(floating_te));
   }
 }
 
diff --git a/source/blender/editors/space_outliner/tree/common.hh b/source/blender/editors/space_outliner/tree/common.hh
index ba2d1c3fab6..286e81c4a0c 100644
--- a/source/blender/editors/space_outliner/tree/common.hh
+++ b/source/blender/editors/space_outliner/tree/common.hh
@@ -10,9 +10,11 @@ struct ListBase;
 
 namespace blender::ed::outliner {
 
+class SubTree;
+
 const char *outliner_idcode_to_plural(short idcode);
 
-void outliner_make_object_parent_hierarchy(ListBase *lb);
+void outliner_make_object_parent_hierarchy(SubTree &subtree);
 bool outliner_animdata_test(const struct AnimData *adt);
 
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_display.hh b/source/blender/editors/space_outliner/tree/tree_display.hh
index 295eeb59eaa..31e35ad0ed8 100644
--- a/source/blender/editors/space_outliner/tree/tree_display.hh
+++ b/source/blender/editors/space_outliner/tree/tree_display.hh
@@ -34,6 +34,7 @@ struct ViewLayer;
 
 namespace blender::ed::outliner {
 
+class SubTree;
 struct TreeElement;
 class TreeElementID;
 
@@ -73,7 +74,7 @@ class AbstractTreeDisplay {
    * Build a tree for this display mode with the Blender context data given in \a source_data and
    * the view settings in \a space_outliner.
    */
-  virtual ListBase buildTree(const TreeSourceData &source_data) = 0;
+  virtual SubTree buildTree(const TreeSourceData &source_data) = 0;
 
   /**
    * Define if the display mode should be allowed to show a mode column on the left. This column
@@ -111,14 +112,14 @@ class TreeDisplayViewLayer final : public AbstractTreeDisplay {
  public:
   TreeDisplayViewLayer(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
   bool supportsModeColumn() const override;
 
  private:
-  void add_view_layer(Scene &, ListBase &, TreeElement *);
-  void add_layer_collections_recursive(ListBase &, ListBase &, TreeElement &);
-  void add_layer_collection_objects(ListBase &, LayerCollection &, TreeElement &);
+  void add_view_layer(Scene &, SubTree &);
+  void add_layer_collections_recursive(ListBase &, TreeElement &);
+  void add_layer_collection_objects(LayerCollection &, TreeElement &);
   void add_layer_collection_objects_children(TreeElement &);
 };
 
@@ -132,10 +133,10 @@ class TreeDisplayLibraries final : public AbstractTreeDisplay {
  public:
   TreeDisplayLibraries(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
  private:
-  TreeElement *add_library_contents(Main &, ListBase &, Library *);
+  TreeElement *add_library_contents(Main &, SubTree &, Library *);
   bool library_id_filter_poll(const Library *lib, ID *id) const;
   short id_filter_get() const;
 };
@@ -150,10 +151,10 @@ class TreeDisplayOverrideLibraryProperties final : public AbstractTreeDisplay {
  public:
   TreeDisplayOverrideLibraryProperties(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
  private:
-  ListBase add_library_contents(Main &);
+  SubTree add_library_contents(Main &);
   short id_filter_get() const;
 };
 
@@ -164,14 +165,14 @@ class TreeDisplayOverrideLibraryHierarchies final : public AbstractTreeDisplay {
  public:
   TreeDisplayOverrideLibraryHierarchies(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
   bool is_lazy_built() const override;
 
  private:
-  ListBase build_hierarchy_for_lib_or_main(Main *bmain,
-                                           TreeElement &parent_te,
-                                           Library *lib = nullptr);
+  void build_hierarchy_for_lib_or_main(Main *bmain,
+                                       TreeElement &parent_te,
+                                       Library *lib = nullptr);
 };
 
 /* -------------------------------------------------------------------- */
@@ -190,7 +191,7 @@ class TreeDisplaySequencer final : public AbstractTreeDisplay {
  public:
   TreeDisplaySequencer(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
  private:
   TreeElement *add_sequencer_contents() const;
@@ -211,7 +212,7 @@ class TreeDisplayIDOrphans final : public AbstractTreeDisplay {
  public:
   TreeDisplayIDOrphans(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
  private:
   bool datablock_has_orphans(ListBase &) const;
@@ -227,7 +228,7 @@ class TreeDisplayScenes final : public AbstractTreeDisplay {
  public:
   TreeDisplayScenes(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
   bool supportsModeColumn() const override;
 };
@@ -242,7 +243,7 @@ class TreeDisplayDataAPI final : public AbstractTreeDisplay {
  public:
   TreeDisplayDataAPI(SpaceOutliner &space_outliner);
 
-  ListBase buildTree(const TreeSourceData &source_data) override;
+  SubTree buildTree(const TreeSourceData &source_data) override;
 
   bool is_lazy_built() const override;
 };
diff --git a/source/blender/editors/space_outliner/tree/tree_display_data.cc b/source/blender/editors/space_outliner/tree/tree_display_data.cc
index 3d9b927fbf1..2206eda2c23 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_data.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_data.cc
@@ -22,15 +22,15 @@ TreeDisplayDataAPI::TreeDisplayDataAPI(SpaceOutliner &space_outliner)
 {
 }
 
-ListBase TreeDisplayDataAPI::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayDataAPI::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   PointerRNA mainptr;
   RNA_main_pointer_create(source_data.bmain, &mainptr);
 
   TreeElement *te = outliner_add_element(
-      &space_outliner_, &tree, (void *)&mainptr, nullptr, TSE_RNA_STRUCT, -1);
+      &space_outliner_, (void *)&mainptr, tree, TSE_RNA_STRUCT, -1);
 
   /* On first view open parent data elements */
   const int show_opened = !space_outliner_.treestore ||
diff --git a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
index 405f1dd73f4..d8268080974 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
@@ -29,9 +29,9 @@ TreeDisplayLibraries::TreeDisplayLibraries(SpaceOutliner &space_outliner)
 {
 }
 
-ListBase TreeDisplayLibraries::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayLibraries::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   {
     /* current file first - mainvar provides tselem with unique pointer - not used */
@@ -56,13 +56,13 @@ ListBase TreeDisplayLibraries::buildTree(const TreeSourceData &source_data)
   }
 
   /* make hierarchy */
-  for (TreeElement *ten : List<TreeElement>(tree)) {
-    if (ten == tree.first) {
+  for (TreeElement &ten : tree) {
+    if (&ten == &*tree.begin()) {
       /* First item is main, skip. */
       continue;
     }
 
-    TreeStoreElem *tselem = TREESTORE(ten);
+    TreeStoreElem *tselem = TREESTORE(&ten);
     Library *lib = (Library *)tselem->id;
     BLI_assert(!lib || (GS(lib->id.name) == ID_LI));
     if (!lib || !lib->parent) {
@@ -73,16 +73,12 @@ ListBase TreeDisplayLibraries::buildTree(const TreeSourceData &source_data)
 
     if (tselem->id->tag & LIB_TAG_INDIRECT) {
       /* Only remove from 'first level' if lib is not also directly used. */
-      BLI_remlink(&tree, ten);
-      BLI_addtail(&parent->subtree, ten);
-      ten->parent = parent;
+      std::unique_ptr<TreeElement> ten_uptr = tree.remove(ten);
+      parent->child_elements.add_back(std::move(ten_uptr));
     }
     else {
       /* Else, make a new copy of the libtree for our parent. */
-      TreeElement *dupten = add_library_contents(*source_data.bmain, parent->subtree, lib);
-      if (dupten) {
-        dupten->parent = parent;
-      }
+      add_library_contents(*source_data.bmain, parent->child_elements, lib);
     }
   }
   /* restore newid pointers */
@@ -93,7 +89,9 @@ ListBase TreeDisplayLibraries::buildTree(const TreeSourceData &source_data)
   return tree;
 }
 
-TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar, ListBase &lb, Library *lib)
+TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar,
+                                                        SubTree &subtree,
+                                                        Library *lib)
 {
   const short filter_id_type = id_filter_get();
 
@@ -135,10 +133,10 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar, ListBase
       if (!tenlib) {
         /* Create library tree element on demand, depending if there are any data-blocks. */
         if (lib) {
-          tenlib = outliner_add_element(&space_outliner_, &lb, lib, nullptr, TSE_SOME_ID, 0);
+          tenlib = outliner_add_element(&space_outliner_, lib, subtree, TSE_SOME_ID, 0);
         }
         else {
-          tenlib = outliner_add_element(&space_outliner_, &lb, &mainvar, nullptr, TSE_ID_BASE, 0);
+          tenlib = outliner_add_element(&space_outliner_, &mainvar, subtree, TSE_ID_BASE, 0);
           tenlib->name = IFACE_("Current File");
         }
       }
@@ -151,15 +149,14 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar, ListBase
           ten = tenlib;
         }
         else {
-          ten = outliner_add_element(
-              &space_outliner_, &tenlib->subtree, lib, nullptr, TSE_ID_BASE, a);
+          ten = outliner_add_element(&space_outliner_, lib, subtree, TSE_ID_BASE, a);
           ten->directdata = lbarray[a];
           ten->name = outliner_idcode_to_plural(GS(id->name));
         }
 
         for (ID *id : List<ID>(lbarray[a])) {
           if (library_id_filter_poll(lib, id)) {
-            outliner_add_element(&space_outliner_, &ten->subtree, id, ten, TSE_SOME_ID, 0);
+            outliner_add_element(&space_outliner_, id, ten, TSE_SOME_ID, 0);
           }
         }
       }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
index d5ecfd4edf3..d7885bb1aa6 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
@@ -27,9 +27,9 @@ TreeDisplayIDOrphans::TreeDisplayIDOrphans(SpaceOutliner &space_outliner)
 {
 }
 
-ListBase TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
   ListBase *lbarray[INDEX_ID_MAX];
   short filter_id_type = (space_outliner_.filter & SO_FILTER_ID_TYPE) ?
                              space_outliner_.filter_id_type :
@@ -56,7 +56,7 @@ ListBase TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
     TreeElement *te = nullptr;
     if (!filter_id_type) {
       ID *id = (ID *)lbarray[a]->first;
-      te = outliner_add_element(&space_outliner_, &tree, lbarray[a], nullptr, TSE_ID_BASE, 0);
+      te = outliner_add_element(&space_outliner_, lbarray[a], tree, TSE_ID_BASE, 0);
       te->directdata = lbarray[a];
       te->name = outliner_idcode_to_plural(GS(id->name));
     }
@@ -64,8 +64,7 @@ ListBase TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
     /* Add the orphaned data-blocks - these will not be added with any subtrees attached. */
     for (ID *id : List<ID>(lbarray[a])) {
       if (ID_REAL_USERS(id) <= 0) {
-        outliner_add_element(
-            &space_outliner_, (te) ? &te->subtree : &tree, id, te, TSE_SOME_ID, 0);
+        outliner_add_element(&space_outliner_, id, te ? te->child_elements : tree, TSE_SOME_ID, 0);
       }
     }
   }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_override_library_hierarchies.cc b/source/blender/editors/space_outliner/tree/tree_display_override_library_hierarchies.cc
index fa4479d0d9d..679627d6518 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_override_library_hierarchies.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_override_library_hierarchies.cc
@@ -33,13 +33,13 @@ TreeDisplayOverrideLibraryHierarchies::TreeDisplayOverrideLibraryHierarchies(
 {
 }
 
-ListBase TreeDisplayOverrideLibraryHierarchies::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayOverrideLibraryHierarchies::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   /* First step: Build "Current File" hierarchy. */
   TreeElement *current_file_te = outliner_add_element(
-      &space_outliner_, &tree, source_data.bmain, nullptr, TSE_ID_BASE, -1);
+      &space_outliner_, source_data.bmain, tree, TSE_ID_BASE, -1);
   current_file_te->name = IFACE_("Current File");
   AbstractTreeElement::uncollapse_by_default(current_file_te);
   {
@@ -48,7 +48,7 @@ ListBase TreeDisplayOverrideLibraryHierarchies::buildTree(const TreeSourceData &
     /* Add dummy child if there's nothing to display. */
     if (BLI_listbase_is_empty(&current_file_te->subtree)) {
       TreeElement *dummy_te = outliner_add_element(
-          &space_outliner_, &current_file_te->subtree, nullptr, current_file_te, TSE_ID_BASE, 0);
+          &space_outliner_, nullptr, current_file_te, TSE_ID_BASE, 0);
       dummy_te->name = IFACE_("No Library Overrides");
     }
   }
@@ -56,19 +56,18 @@ ListBase TreeDisplayOverrideLibraryHierarchies::buildTree(const TreeSourceData &
   /* Second step: Build hierarchies for external libraries. */
   for (Library *lib = (Library *)source_data.bmain->libraries.first; lib;
        lib = (Library *)lib->id.next) {
-    TreeElement *tenlib = outliner_add_element(
-        &space_outliner_, &tree, lib, nullptr, TSE_SOME_ID, 0);
+    TreeElement *tenlib = outliner_add_element(&space_outliner_, lib, tree, TSE_SOME_ID, 0);
     build_hierarchy_for_lib_or_main(source_data.bmain, *tenlib, lib);
   }
 
   /* Remove top level library elements again that don't contain any overrides. */
-  LISTBASE_FOREACH_MUTABLE (TreeElement *, top_level_te, &tree) {
-    if (top_level_te == current_file_te) {
+  for (TreeElement &top_level_te : tree) {
+    if (&top_level_te == current_file_te) {
       continue;
     }
 
-    if (BLI_listbase_is_empty(&top_level_te->subtree)) {
-      outliner_free_tree_element(top_level_te, &tree);
+    if (top_level_te.child_elements.is_empty()) {
+      tree.remove(top_level_te);
     }
   }
 
@@ -115,11 +114,10 @@ class OverrideIDHierarchyBuilder {
                                         TreeElement &te_to_expand);
 };
 
-ListBase TreeDisplayOverrideLibraryHierarchies::build_hierarchy_for_lib_or_main(
-    Main *bmain, TreeElement &parent_te, Library *lib)
+void TreeDisplayOverrideLibraryHierarchies::build_hierarchy_for_lib_or_main(Main *bmain,
+                                                                            TreeElement &parent_te,
+                                                                            Library *lib)
 {
-  ListBase tree = {nullptr};
-
   /* Ensure #Main.relations contains the latest mapping of relations. Must be freed before
    * returning. */
   BKE_main_relations_create(bmain, 0);
@@ -141,26 +139,20 @@ ListBase TreeDisplayOverrideLibraryHierarchies::build_hierarchy_for_lib_or_main(
     }
 
     TreeElement *new_base_te = id_base_te_map.lookup_or_add_cb(GS(iter_id->name), [&]() {
-      TreeElement *new_te = outliner_add_element(&space_outliner_,
-                                                 &parent_te.subtree,
-                                                 lib ? (void *)lib : bmain,
-                                                 &parent_te,
-                                                 TSE_ID_BASE,
-                                                 base_index++);
+      TreeElement *new_te = outliner_add_element(
+          &space_outliner_, lib ? (void *)lib : bmain, &parent_te, TSE_ID_BASE, base_index++);
       new_te->name = outliner_idcode_to_plural(GS(iter_id->name));
       return new_te;
     });
 
     TreeElement *new_id_te = outliner_add_element(
-        &space_outliner_, &new_base_te->subtree, iter_id, new_base_te, TSE_SOME_ID, 0, false);
+        &space_outliner_, iter_id, new_base_te, TSE_SOME_ID, 0, false);
 
     builder.build_hierarchy_for_ID(*iter_id, *new_id_te);
   }
   FOREACH_MAIN_ID_END;
 
   BKE_main_relations_free(bmain);
-
-  return tree;
 }
 
 void OverrideIDHierarchyBuilder::build_hierarchy_for_ID(ID &override_root_id,
@@ -220,7 +212,7 @@ void OverrideIDHierarchyBuilder::build_hierarchy_for_ID_recursive(const ID &pare
     }
 
     TreeElement *new_te = outliner_add_element(
-        &space_outliner_, &te_to_expand.subtree, &id, &te_to_expand, TSE_SOME_ID, 0, false);
+        &space_outliner_, &id, &te_to_expand, TSE_SOME_ID, 0, false);
 
     build_data.sibling_ids.add(&id);
 
diff --git a/source/blender/editors/space_outliner/tree/tree_display_override_library_properties.cc b/source/blender/editors/space_outliner/tree/tree_display_override_library_properties.cc
index 5713a4a6a1f..1cd946f152e 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_override_library_properties.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_override_library_properties.cc
@@ -31,12 +31,12 @@ TreeDisplayOverrideLibraryProperties::TreeDisplayOverrideLibraryProperties(
 {
 }
 
-ListBase TreeDisplayOverrideLibraryProperties::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayOverrideLibraryProperties::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = add_library_contents(*source_data.bmain);
+  SubTree tree = add_library_contents(*source_data.bmain);
 
-  for (TreeElement *top_level_te : List<TreeElement>(tree)) {
-    TreeStoreElem *tselem = TREESTORE(top_level_te);
+  for (TreeElement &top_level_te : tree) {
+    TreeStoreElem *tselem = TREESTORE(&top_level_te);
     if (!tselem->used) {
       tselem->flag &= ~TSE_CLOSED;
     }
@@ -45,9 +45,9 @@ ListBase TreeDisplayOverrideLibraryProperties::buildTree(const TreeSourceData &s
   return tree;
 }
 
-ListBase TreeDisplayOverrideLibraryProperties::add_library_contents(Main &mainvar)
+SubTree TreeDisplayOverrideLibraryProperties::add_library_contents(Main &mainvar)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   const short filter_id_type = id_filter_get();
 
@@ -82,33 +82,32 @@ ListBase TreeDisplayOverrideLibraryProperties::add_library_contents(Main &mainva
 
     /* Create data-block list parent element on demand. */
     TreeElement *id_base_te = nullptr;
-    ListBase *lb_to_expand = &tree;
+    SubTree *subtree_to_expand = &tree;
 
     if (!filter_id_type) {
-      id_base_te = outliner_add_element(
-          &space_outliner_, &tree, lbarray[a], nullptr, TSE_ID_BASE, 0);
+      id_base_te = outliner_add_element(&space_outliner_, lbarray[a], tree, TSE_ID_BASE, 0);
       id_base_te->directdata = lbarray[a];
       id_base_te->name = outliner_idcode_to_plural(GS(id->name));
 
-      lb_to_expand = &id_base_te->subtree;
+      subtree_to_expand = &id_base_te->child_elements;
     }
 
     for (ID *id : List<ID>(lbarray[a])) {
       if (ID_IS_OVERRIDE_LIBRARY_REAL(id) && !ID_IS_LINKED(id)) {
         TreeElement *override_tree_element = outliner_add_element(
-            &space_outliner_, lb_to_expand, id, id_base_te, TSE_LIBRARY_OVERRIDE_BASE, 0);
+            &space_outliner_, id, id_base_te, TSE_LIBRARY_OVERRIDE_BASE, 0);
 
-        if (BLI_listbase_is_empty(&override_tree_element->subtree)) {
-          outliner_free_tree_element(override_tree_element, lb_to_expand);
+        if (override_tree_element->child_elements.is_empty()) {
+          subtree_to_expand->remove(*override_tree_element);
         }
       }
     }
   }
 
   /* Remove ID base elements that turn out to be empty. */
-  LISTBASE_FOREACH_MUTABLE (TreeElement *, te, &tree) {
-    if (BLI_listbase_is_empty(&te->subtree)) {
-      outliner_free_tree_element(te, &tree);
+  for (TreeElement &te : tree) {
+    if (te.child_elements.is_empty()) {
+      tree.remove(te);
     }
   }
 
diff --git a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
index 6b1de7f8b95..671a2b8c113 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
@@ -31,17 +31,16 @@ bool TreeDisplayScenes::supportsModeColumn() const
   return true;
 }
 
-ListBase TreeDisplayScenes::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayScenes::buildTree(const TreeSourceData &source_data)
 {
   /* On first view we open scenes. */
   const int show_opened = !space_outliner_.treestore ||
                           !BLI_mempool_len(space_outliner_.treestore);
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   for (ID *id : List<ID>(source_data.bmain->scenes)) {
     Scene *scene = reinterpret_cast<Scene *>(id);
-    TreeElement *te = outliner_add_element(
-        &space_outliner_, &tree, scene, nullptr, TSE_SOME_ID, 0);
+    TreeElement *te = outliner_add_element(&space_outliner_, scene, tree, TSE_SOME_ID, 0);
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* New scene elements open by default */
@@ -49,7 +48,7 @@ ListBase TreeDisplayScenes::buildTree(const TreeSourceData &source_data)
       tselem->flag &= ~TSE_CLOSED;
     }
 
-    outliner_make_object_parent_hierarchy(&te->subtree);
+    outliner_make_object_parent_hierarchy(te->child_elements);
   }
 
   return tree;
diff --git a/source/blender/editors/space_outliner/tree/tree_display_sequencer.cc b/source/blender/editors/space_outliner/tree/tree_display_sequencer.cc
index c2739d876f4..530ce40bae7 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_sequencer.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_sequencer.cc
@@ -28,9 +28,9 @@ TreeDisplaySequencer::TreeDisplaySequencer(SpaceOutliner &space_outliner)
 {
 }
 
-ListBase TreeDisplaySequencer::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplaySequencer::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree tree;
 
   Editing *ed = SEQ_editing_get(source_data.scene);
   if (ed == nullptr) {
@@ -40,11 +40,10 @@ ListBase TreeDisplaySequencer::buildTree(const TreeSourceData &source_data)
   for (Sequence *seq : List<Sequence>(ed->seqbasep)) {
     SequenceAddOp op = need_add_seq_dup(seq);
     if (op == SEQUENCE_DUPLICATE_NONE) {
-      outliner_add_element(&space_outliner_, &tree, seq, nullptr, TSE_SEQUENCE, 0);
+      outliner_add_element(&space_outliner_, seq, tree, TSE_SEQUENCE, 0);
     }
     else if (op == SEQUENCE_DUPLICATE_ADD) {
-      TreeElement *te = outliner_add_element(
-          &space_outliner_, &tree, seq, nullptr, TSE_SEQUENCE_DUP, 0);
+      TreeElement *te = outliner_add_element(&space_outliner_, seq, tree, TSE_SEQUENCE_DUP, 0);
       add_seq_dup(seq, te, 0);
     }
   }
@@ -101,7 +100,7 @@ void TreeDisplaySequencer::add_seq_dup(Sequence *seq, TreeElement *te, short ind
     }
 
     if (STREQ(p->strip->stripdata->name, seq->strip->stripdata->name)) {
-      outliner_add_element(&space_outliner_, &te->subtree, (void *)p, te, TSE_SEQUENCE, index);
+      outliner_add_element(&space_outliner_, (void *)p, te, TSE_SEQUENCE, index);
     }
     p = p->next;
   }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
index c8869d90eca..cbaba863950 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
@@ -60,9 +60,9 @@ bool TreeDisplayViewLayer::supportsModeColumn() const
   return true;
 }
 
-ListBase TreeDisplayViewLayer::buildTree(const TreeSourceData &source_data)
+SubTree TreeDisplayViewLayer::buildTree(const TreeSourceData &source_data)
 {
-  ListBase tree = {nullptr};
+  SubTree subtree;
   Scene *scene = source_data.scene;
   show_objects_ = !(space_outliner_.filter & SO_FILTER_NO_OBJECT);
 
@@ -74,23 +74,23 @@ ListBase TreeDisplayViewLayer::buildTree(const TreeSourceData &source_data)
         continue;
       }
 
-      add_view_layer(*scene, tree, (TreeElement *)nullptr);
+      add_view_layer(*scene, subtree);
     }
     else {
       TreeElement &te_view_layer = *outliner_add_element(
-          &space_outliner_, &tree, scene, nullptr, TSE_R_LAYER, 0);
+          &space_outliner_, scene, subtree, TSE_R_LAYER, 0);
       TREESTORE(&te_view_layer)->flag &= ~TSE_CLOSED;
       te_view_layer.name = view_layer->name;
       te_view_layer.directdata = view_layer;
 
-      add_view_layer(*scene, te_view_layer.subtree, &te_view_layer);
+      add_view_layer(*scene, te_view_layer.child_elements);
     }
   }
 
-  return tree;
+  return subtree;
 }
 
-void TreeDisplayViewLayer::add_view_layer(Scene &scene, ListBase &tree, TreeElement *parent)
+void TreeDisplayViewLayer::add_view_layer(Scene &scene, SubTree &subtree)
 {
   const bool show_children = (space_outliner_.filter & SO_FILTER_NO_CHILDREN) == 0;
 
@@ -98,18 +98,18 @@ void TreeDisplayViewLayer::add_view_layer(Scene &scene, ListBase &tree, TreeElem
     /* Show objects in the view layer. */
     for (Base *base : List<Base>(view_layer_->object_bases)) {
       TreeElement *te_object = outliner_add_element(
-          &space_outliner_, &tree, base->object, parent, TSE_SOME_ID, 0);
+          &space_outliner_, base->object, subtree, TSE_SOME_ID, 0);
       te_object->directdata = base;
     }
 
     if (show_children) {
-      outliner_make_object_parent_hierarchy(&tree);
+      outliner_make_object_parent_hierarchy(subtree);
     }
   }
   else {
     /* Show collections in the view layer. */
     TreeElement &ten = *outliner_add_element(
-        &space_outliner_, &tree, &scene, parent, TSE_VIEW_COLLECTION_BASE, 0);
+        &space_outliner_, &scene, subtree, TSE_VIEW_COLLECTION_BASE, 0);
     ten.name = IFACE_("Scene Collection");
     TREESTORE(&ten)->flag &= ~TSE_CLOSED;
 
@@ -119,9 +119,9 @@ void TreeDisplayViewLayer::add_view_layer(Scene &scene, ListBase &tree, TreeElem
       return;
     }
 
-    add_layer_collections_recursive(ten.subtree, lc->layer_collections, ten);
+    add_layer_collections_recursive(lc->layer_collections, ten);
     if (show_objects_) {
-      add_layer_collection_objects(ten.subtree, *lc, ten);
+      add_layer_collection_objects(*lc, ten);
     }
     if (show_children) {
       add_layer_collection_objects_children(ten);
@@ -129,8 +129,7 @@ void TreeDisplayViewLayer::add_view_layer(Scene &scene, ListBase &tree, TreeElem
   }
 }
 
-void TreeDisplayViewLayer::add_layer_collections_recursive(ListBase &tree,
-                                                           ListBase &layer_collections,
+void TreeDisplayViewLayer::add_layer_collections_recursive(ListBase &layer_collections,
                                                            TreeElement &parent_ten)
 {
   for (LayerCollection *lc : List<LayerCollection>(layer_collections)) {
@@ -142,8 +141,7 @@ void TreeDisplayViewLayer::add_layer_collections_recursive(ListBase &tree,
     }
     else {
       ID *id = &lc->collection->id;
-      ten = outliner_add_element(
-          &space_outliner_, &tree, id, &parent_ten, TSE_LAYER_COLLECTION, 0);
+      ten = outliner_add_element(&space_outliner_, id, &parent_ten, TSE_LAYER_COLLECTION, 0);
 
       ten->name = id->name + 2;
       ten->directdata = lc;
@@ -155,21 +153,19 @@ void TreeDisplayViewLayer::add_layer_collections_recursive(ListBase &tree,
       }
     }
 
-    add_layer_collections_recursive(ten->subtree, lc->layer_collections, *ten);
+    add_layer_collections_recursive(lc->layer_collections, *ten);
     if (!exclude && show_objects_) {
-      add_layer_collection_objects(ten->subtree, *lc, *ten);
+      add_layer_collection_objects(*lc, *ten);
     }
   }
 }
 
-void TreeDisplayViewLayer::add_layer_collection_objects(ListBase &tree,
-                                                        LayerCollection &lc,
-                                                        TreeElement &ten)
+void TreeDisplayViewLayer::add_layer_collection_objects(LayerCollection &lc, TreeElement &ten)
 {
   for (CollectionObject *cob : List<CollectionObject>(lc.collection->gobject)) {
     Base *base = BKE_view_layer_base_find(view_layer_, cob->ob);
     TreeElement *te_object = outliner_add_element(
-        &space_outliner_, &tree, base->object, &ten, TSE_SOME_ID, 0);
+        &space_outliner_, base->object, &ten, TSE_SOME_ID, 0);
     te_object->directdata = base;
   }
 }
@@ -261,9 +257,10 @@ void ObjectsChildrenBuilder::make_object_parent_hierarchy_collections()
       for (TreeElement *child_ob_tree_element : child_ob_tree_elements) {
         if (child_ob_tree_element->parent == parent_ob_collection_tree_element) {
           /* Move from the collection subtree into the parent object subtree. */
-          BLI_remlink(&parent_ob_collection_tree_element->subtree, child_ob_tree_element);
-          BLI_addtail(&parent_ob_tree_element->subtree, child_ob_tree_element);
-          child_ob_tree_element->parent = parent_ob_tree_element;
+          std::unique_ptr<TreeElement> removed_te =
+              parent_ob_collection_tree_element->child_elements.remove(*child_ob_tree_element);
+          parent_ob_tree_element->child_elements.add_back(std::move(removed_te));
+
           found = true;
           break;
         }
@@ -272,13 +269,8 @@ void ObjectsChildrenBuilder::make_object_parent_hierarchy_collections()
       if (!found) {
         /* We add the child in the tree even if it is not in the collection.
          * We don't expand its sub-tree though, to make it less prominent. */
-        TreeElement *child_ob_tree_element = outliner_add_element(&outliner_,
-                                                                  &parent_ob_tree_element->subtree,
-                                                                  child,
-                                                                  parent_ob_tree_element,
-                                                                  TSE_SOME_ID,
-                                                                  0,
-                                                                  false);
+        TreeElement *child_ob_tree_element = outliner_add_element(
+            &outliner_, child, parent_ob_tree_element, TSE_SOME_ID, 0, false);
         child_ob_tree_element->flag |= TE_CHILD_NOT_IN_COLLECTION;
         child_ob_tree_elements.append(child_ob_tree_element);
       }
diff --git a/source/blender/editors/space_outliner/tree/tree_element.cc b/source/blender/editors/space_outliner/tree/tree_element.cc
index 4a540c3ce87..4b85784d2ae 100644
--- a/source/blender/editors/space_outliner/tree/tree_element.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element.cc
@@ -146,4 +146,75 @@ void tree_element_expand(const AbstractTreeElement &tree_element, SpaceOutliner
   tree_element.expand(space_outliner);
 }
 
+/* -------------------------------------------------------------------- */
+
+SubTree::SubTree(TreeElement &parent) : parent_(&parent)
+{
+}
+
+SubTree::SubTree(SpaceOutliner_Runtime &) : parent_(nullptr)
+{
+}
+
+void SubTree::add_back(std::unique_ptr<TreeElement> element)
+{
+  element->parent = parent_;
+  /* TODO is this the right place for this? */
+  element->child_elements.parent_ = element.get();
+  elements_.push_back(std::move(element));
+}
+
+void SubTree::insert_before(Iterator pos, std::unique_ptr<TreeElement> element)
+{
+  element->parent = parent_;
+  elements_.insert(pos.iter_, std::move(element));
+}
+
+std::unique_ptr<TreeElement> SubTree::remove(TreeElement &element)
+{
+  std::unique_ptr<TreeElement> element_uptr = nullptr;
+  /* TODO doesn't free element entirely yet, #TreeElement.name may need freeing! See
+   * #outliner_free_tree_element(). */
+
+  for (decltype(elements_)::iterator iter = elements_.begin(); iter != elements_.end(); ++iter) {
+    if (iter->get() == &element) {
+      element_uptr = std::move(*iter);
+      elements_.erase(iter);
+      break;
+    }
+  }
+  BLI_assert(!has_child(element));
+
+  if (element_uptr) {
+    element_uptr->parent = nullptr;
+  }
+  return element_uptr;
+}
+
+void SubTree::clear()
+{
+  elements_.clear();
+  /* No need to clear the parent pointer, elements get destructed entirely anyway. */
+}
+
+bool SubTree::has_child(const TreeElement &needle) const
+{
+  for (const TreeElement &te : *this) {
+    if (&te == &needle) {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool SubTree::is_empty() const
+{
+  return elements_.empty();
+}
+
+TreeElement *SubTree::parent() const
+{
+  return parent_;
+}
+
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_element.hh b/source/blender/editors/space_outliner/tree/tree_element.hh
index 1b145a48daa..71f64cfe610 100644
--- a/source/blender/editors/space_outliner/tree/tree_element.hh
+++ b/source/blender/editors/space_outliner/tree/tree_element.hh
@@ -18,6 +18,7 @@ struct SpaceOutliner;
 namespace blender::ed::outliner {
 
 struct TreeElement;
+class SubTree;
 
 /* -------------------------------------------------------------------- */
 /* Tree-Display Interface */
@@ -114,21 +115,28 @@ class AbstractTreeElement {
  * TODO: this function needs to be split up! It's getting a bit too large...
  *
  * \note "ID" is not always a real ID.
- * \note If child items are only added to the tree if the item is open,
- * the `TSE_` type _must_ be added to #outliner_element_needs_rebuild_on_open_change().
+ * \note If child items are only added to the tree if the item is open, the `TSE_` type _must_ be
+ *       added to #outliner_element_needs_rebuild_on_open_change().
  *
+ * \param parent: The parent element to add the new element to. If this is null, the new element
+ *                will be added to the root level of the tree.
  * \param expand: If true, the element may add its own sub-tree. E.g. objects will list their
  *                animation data, object data, constraints, modifiers, ... This often adds visual
  *                noise, and can be expensive to add in big scenes. So prefer setting this to
  *                false.
  */
-struct TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
-                                         ListBase *lb,
-                                         void *idv,
-                                         struct TreeElement *parent,
-                                         short type,
-                                         short index,
-                                         const bool expand = true);
+TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
+                                  void *idv,
+                                  TreeElement *parent,
+                                  short type,
+                                  short index,
+                                  const bool expand = true);
+TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
+                                  void *idv,
+                                  SubTree &subtree,
+                                  short type,
+                                  short index,
+                                  const bool expand = true);
 
 void tree_element_expand(const AbstractTreeElement &tree_element, SpaceOutliner &space_outliner);
 
diff --git a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
index bb514cf6f3c..20f48e2421b 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
@@ -30,8 +30,7 @@ TreeElementAnimData::TreeElementAnimData(TreeElement &legacy_te, AnimData &anim_
 void TreeElementAnimData::expand(SpaceOutliner &space_outliner) const
 {
   /* Animation data-block itself. */
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, anim_data_.action, &legacy_te_, TSE_SOME_ID, 0);
+  outliner_add_element(&space_outliner, anim_data_.action, &legacy_te_, TSE_SOME_ID, 0);
 
   expand_drivers(space_outliner);
   expand_NLA_tracks(space_outliner);
@@ -42,8 +41,7 @@ void TreeElementAnimData::expand_drivers(SpaceOutliner &space_outliner) const
   if (BLI_listbase_is_empty(&anim_data_.drivers)) {
     return;
   }
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, &anim_data_, &legacy_te_, TSE_DRIVER_BASE, 0);
+  outliner_add_element(&space_outliner, &anim_data_, &legacy_te_, TSE_DRIVER_BASE, 0);
 }
 
 void TreeElementAnimData::expand_NLA_tracks(SpaceOutliner &space_outliner) const
@@ -51,7 +49,7 @@ void TreeElementAnimData::expand_NLA_tracks(SpaceOutliner &space_outliner) const
   if (BLI_listbase_is_empty(&anim_data_.nla_tracks)) {
     return;
   }
-  outliner_add_element(&space_outliner, &legacy_te_.subtree, &anim_data_, &legacy_te_, TSE_NLA, 0);
+  outliner_add_element(&space_outliner, &anim_data_, &legacy_te_, TSE_NLA, 0);
 }
 
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_element_driver.cc b/source/blender/editors/space_outliner/tree/tree_element_driver.cc
index f8072d2814d..acd75fefff3 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_driver.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_driver.cc
@@ -40,8 +40,7 @@ void TreeElementDriverBase::expand(SpaceOutliner &space_outliner) const
         DRIVER_TARGETS_USED_LOOPER_BEGIN (dvar) {
           if (lastadded != dtar->id) {
             /* XXX this lastadded check is rather lame, and also fails quite badly... */
-            outliner_add_element(
-                &space_outliner, &legacy_te_.subtree, dtar->id, &legacy_te_, TSE_LINKED_OB, 0);
+            outliner_add_element(&space_outliner, dtar->id, &legacy_te_, TSE_LINKED_OB, 0);
             lastadded = dtar->id;
           }
         }
diff --git a/source/blender/editors/space_outliner/tree/tree_element_id.cc b/source/blender/editors/space_outliner/tree/tree_element_id.cc
index 86f5fd4eff5..78f734a5579 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_id.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_id.cc
@@ -107,8 +107,7 @@ void TreeElementID::expand_animation_data(SpaceOutliner &space_outliner,
                                           const AnimData *anim_data) const
 {
   if (outliner_animdata_test(anim_data)) {
-    outliner_add_element(
-        &space_outliner, &legacy_te_.subtree, &id_, &legacy_te_, TSE_ANIM_DATA, 0);
+    outliner_add_element(&space_outliner, &id_, &legacy_te_, TSE_ANIM_DATA, 0);
   }
 }
 
diff --git a/source/blender/editors/space_outliner/tree/tree_element_id_scene.cc b/source/blender/editors/space_outliner/tree/tree_element_id_scene.cc
index 328c19c56fd..753de08c72a 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_id_scene.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_id_scene.cc
@@ -36,26 +36,22 @@ void TreeElementIDScene::expand(SpaceOutliner &space_outliner) const
 
 void TreeElementIDScene::expandViewLayers(SpaceOutliner &space_outliner) const
 {
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, &scene_, &legacy_te_, TSE_R_LAYER_BASE, 0);
+  outliner_add_element(&space_outliner, &scene_, &legacy_te_, TSE_R_LAYER_BASE, 0);
 }
 
 void TreeElementIDScene::expandWorld(SpaceOutliner &space_outliner) const
 {
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, scene_.world, &legacy_te_, TSE_SOME_ID, 0);
+  outliner_add_element(&space_outliner, scene_.world, &legacy_te_, TSE_SOME_ID, 0);
 }
 
 void TreeElementIDScene::expandCollections(SpaceOutliner &space_outliner) const
 {
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, &scene_, &legacy_te_, TSE_SCENE_COLLECTION_BASE, 0);
+  outliner_add_element(&space_outliner, &scene_, &legacy_te_, TSE_SCENE_COLLECTION_BASE, 0);
 }
 
 void TreeElementIDScene::expandObjects(SpaceOutliner &space_outliner) const
 {
-  outliner_add_element(
-      &space_outliner, &legacy_te_.subtree, &scene_, &legacy_te_, TSE_SCENE_OBJECTS_BASE, 0);
+  outliner_add_element(&space_outliner, &scene_, &legacy_te_, TSE_SCENE_OBJECTS_BASE, 0);
 }
 
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_element_nla.cc b/source/blender/editors/space_outliner/tree/tree_element_nla.cc
index df19b2e8f4b..75ae4d0744b 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_nla.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_nla.cc
@@ -30,7 +30,7 @@ void TreeElementNLA::expand(SpaceOutliner &space_outliner) const
 {
   int a = 0;
   for (NlaTrack *nlt : ListBaseWrapper<NlaTrack>(anim_data_.nla_tracks)) {
-    outliner_add_element(&space_outliner, &legacy_te_.subtree, nlt, &legacy_te_, TSE_NLA_TRACK, a);
+    outliner_add_element(&space_outliner, nlt, &legacy_te_, TSE_NLA_TRACK, a);
     a++;
   }
 }
@@ -48,8 +48,7 @@ void TreeElementNLATrack::expand(SpaceOutliner &space_outliner) const
 {
   int a = 0;
   for (NlaStrip *strip : ListBaseWrapper<NlaStrip>(track_.strips)) {
-    outliner_add_element(
-        &space_outliner, &legacy_te_.subtree, strip->act, &legacy_te_, TSE_NLA_ACTION, a);
+    outliner_add_element(&space_outliner, strip->act, &legacy_te_, TSE_NLA_ACTION, a);
     a++;
   }
 }
diff --git a/source/blender/editors/space_outliner/tree/tree_element_overrides.cc b/source/blender/editors/space_outliner/tree/tree_element_overrides.cc
index 11067d37966..f4684d89d45 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_overrides.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_overrides.cc
@@ -353,12 +353,8 @@ void OverrideRNAPathTreeBuilder::build_path(TreeElement &parent,
    * values), so the element may already be present. At this point they are displayed as a single
    * property in the tree, so don't add it multiple times here. */
   else if (!path_te_map.contains(override_data.override_property.rna_path)) {
-    outliner_add_element(&space_outliner_,
-                         &te_to_expand->subtree,
-                         &override_data,
-                         te_to_expand,
-                         TSE_LIBRARY_OVERRIDE,
-                         index++);
+    outliner_add_element(
+        &space_outliner_, &override_data, te_to_expand, TSE_LIBRARY_OVERRIDE, index++);
   }
 
   MEM_delete(elem_path);
@@ -403,7 +399,6 @@ void OverrideRNAPathTreeBuilder::ensure_entire_collection(
       override_op_data.operation = item_operation;
 
       current_te = outliner_add_element(&space_outliner_,
-                                        &te_to_expand.subtree,
                                         /* Element will store a copy. */
                                         &override_op_data,
                                         &te_to_expand,
@@ -446,7 +441,6 @@ TreeElement &OverrideRNAPathTreeBuilder::ensure_label_element_for_prop(
 {
   return *path_te_map.lookup_or_add_cb(elem_path, [&]() {
     TreeElement *new_te = outliner_add_element(&space_outliner_,
-                                               &parent.subtree,
                                                (void *)RNA_property_ui_name(&prop),
                                                &parent,
                                                TSE_GENERIC_LABEL,
@@ -469,7 +463,6 @@ TreeElement &OverrideRNAPathTreeBuilder::ensure_label_element_for_ptr(TreeElemen
 
     TreeElement *new_te = outliner_add_element(
         &space_outliner_,
-        &parent.subtree,
         (void *)(dyn_name ? dyn_name : RNA_struct_ui_name(ptr.type)),
         &parent,
         TSE_GENERIC_LABEL,
diff --git a/source/blender/editors/space_outliner/tree/tree_element_rna.cc b/source/blender/editors/space_outliner/tree/tree_element_rna.cc
index 9e1f22b49d6..9a0563f36fa 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_rna.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_rna.cc
@@ -118,8 +118,7 @@ void TreeElementRNAStruct::expand(SpaceOutliner &space_outliner) const
       PointerRNA propptr;
       RNA_property_collection_lookup_int(&ptr, iterprop, index, &propptr);
       if (!(RNA_property_flag(static_cast<PropertyRNA *>(propptr.data)) & PROP_HIDDEN)) {
-        outliner_add_element(
-            &space_outliner, &legacy_te_.subtree, &ptr, &legacy_te_, TSE_RNA_PROPERTY, index);
+        outliner_add_element(&space_outliner, &ptr, &legacy_te_, TSE_RNA_PROPERTY, index);
       }
     }
   }
@@ -168,8 +167,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
 
     if (pptr.data) {
       if (TSELEM_OPEN(&tselem, &space_outliner)) {
-        outliner_add_element(
-            &space_outliner, &legacy_te_.subtree, &pptr, &legacy_te_, TSE_RNA_STRUCT, -1);
+        outliner_add_element(&space_outliner, &pptr, &legacy_te_, TSE_RNA_STRUCT, -1);
       }
       else {
         legacy_te_.flag |= TE_PRETEND_HAS_CHILDREN;
@@ -184,8 +182,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
       for (int index = 0; index < tot; index++) {
         PointerRNA pptr;
         RNA_property_collection_lookup_int(&rna_ptr, rna_prop_, index, &pptr);
-        outliner_add_element(
-            &space_outliner, &legacy_te_.subtree, &pptr, &legacy_te_, TSE_RNA_STRUCT, index);
+        outliner_add_element(&space_outliner, &pptr, &legacy_te_, TSE_RNA_STRUCT, index);
       }
     }
     else if (tot) {
@@ -198,12 +195,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
 
     if (TSELEM_OPEN(&tselem, &space_outliner)) {
       for (int index = 0; index < tot; index++) {
-        outliner_add_element(&space_outliner,
-                             &legacy_te_.subtree,
-                             &rna_ptr,
-                             &legacy_te_,
-                             TSE_RNA_ARRAY_ELEM,
-                             index);
+        outliner_add_element(&space_outliner, &rna_ptr, &legacy_te_, TSE_RNA_ARRAY_ELEM, index);
       }
     }
     else if (tot) {
diff --git a/source/blender/editors/space_outliner/tree/tree_element_scene_objects.cc b/source/blender/editors/space_outliner/tree/tree_element_scene_objects.cc
index b47707e8891..0f6b9363bf9 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_scene_objects.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_scene_objects.cc
@@ -29,10 +29,10 @@ TreeElementSceneObjectsBase::TreeElementSceneObjectsBase(TreeElement &legacy_te,
 void TreeElementSceneObjectsBase::expand(SpaceOutliner &space_outliner) const
 {
   FOREACH_SCENE_OBJECT_BEGIN (&scene_, ob) {
-    outliner_add_element(&space_outliner, &legacy_te_.subtree, ob, &legacy_te_, TSE_SOME_ID, 0);
+    outliner_add_element(&space_outliner, ob, &legacy_te_, TSE_SOME_ID, 0);
   }
   FOREACH_SCENE_OBJECT_END;
-  outliner_make_object_parent_hierarchy(&legacy_te_.subtree);
+  outliner_make_object_parent_hierarchy(legacy_te_.child_elements);
 }
 
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_element_seq.cc b/source/blender/editors/space_outliner/tree/tree_element_seq.cc
index 550a72d246d..dbe587075f8 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_seq.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_seq.cc
@@ -45,13 +45,11 @@ void TreeElementSequence::expand(SpaceOutliner &space_outliner) const
 
   if (sequence_.type == SEQ_TYPE_META) {
     LISTBASE_FOREACH (Sequence *, child, &sequence_.seqbase) {
-      outliner_add_element(
-          &space_outliner, &legacy_te_.subtree, child, &legacy_te_, TSE_SEQUENCE, 0);
+      outliner_add_element(&space_outliner, child, &legacy_te_, TSE_SEQUENCE, 0);
     }
   }
   else {
-    outliner_add_element(
-        &space_outliner, &legacy_te_.subtree, sequence_.strip, &legacy_te_, TSE_SEQ_STRIP, 0);
+    outliner_add_element(&space_outliner, sequence_.strip, &legacy_te_, TSE_SEQ_STRIP, 0);
   }
 }
 
diff --git a/source/blender/editors/space_outliner/tree/tree_element_view_layer.cc b/source/blender/editors/space_outliner/tree/tree_element_view_layer.cc
index da82854facb..ba11af3110a 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_view_layer.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_view_layer.cc
@@ -29,7 +29,7 @@ void TreeElementViewLayerBase::expand(SpaceOutliner &space_outliner) const
 {
   for (auto *view_layer : ListBaseWrapper<ViewLayer>(scene_.view_layers)) {
     TreeElement *tenlay = outliner_add_element(
-        &space_outliner, &legacy_te_.subtree, &scene_, &legacy_te_, TSE_R_LAYER, 0);
+        &space_outliner, &scene_, &legacy_te_, TSE_R_LAYER, 0);
     tenlay->name = view_layer->name;
     tenlay->directdata = view_layer;
   }
-- 
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