Outliner: Refactor outliner tree-hash interfaces with C++

- Turn storage into an object with "automatic" memory management (RAII)
  so freeing is implicit and reliable.
- Turn functions into member functions, to have the data and its
  functions close together with controlled access that increases
  encapsulation and hiding implementation details.
- Use references to indicate null is not an expected value.
- Related minor cleanup (comments, use const etc.)

Couldn't spot any changes in performance.

1 files changed, 34 insertions, 27 deletions

diff --git a/source/blender/blenkernel/BKE_outliner_treehash.hh b/source/blender/blenkernel/BKE_outliner_treehash.hh
index fc0ab35cf38..f72cec21dbc 100644
--- a/source/blender/blenkernel/BKE_outliner_treehash.hh
+++ b/source/blender/blenkernel/BKE_outliner_treehash.hh
@@ -3,45 +3,52 @@
 
 /** \file
  * \ingroup bke
+ *
+ * Hash table of tree-store elements (#TreeStoreElem) for fast lookups via a (id, type, index)
+ * tuple as key.
+ *
+ * The Outliner may have to perform many lookups for rebuilding complex trees, so this should be
+ * treated as performance sensitive.
  */
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+#include <memory>
 
 struct BLI_mempool;
 struct ID;
 struct GHash;
 struct TreeStoreElem;
 
-/* create and fill hashtable with treestore elements */
-GHash *BKE_outliner_treehash_create_from_treestore(BLI_mempool *treestore);
+namespace blender::bke::outliner::treehash {
 
-/* full rebuild for already allocated hashtable */
-GHash *BKE_outliner_treehash_rebuild_from_treestore(GHash *treehash, BLI_mempool *treestore);
+class TreeHash {
+  GHash *treehash_ = nullptr;
 
-/* clear element usage flags */
-void BKE_outliner_treehash_clear_used(GHash *treehash);
+ public:
+  ~TreeHash();
 
-/* Add/remove hashtable elements */
-void BKE_outliner_treehash_add_element(GHash *treehash, TreeStoreElem *elem);
-void BKE_outliner_treehash_remove_element(GHash *treehash, TreeStoreElem *elem);
+  /* create and fill hashtable with treestore elements */
+  static std::unique_ptr<TreeHash> create_from_treestore(BLI_mempool &treestore);
 
-/* find first unused element with specific type, nr and id */
-struct TreeStoreElem *BKE_outliner_treehash_lookup_unused(GHash *treehash,
-                                                          short type,
-                                                          short nr,
-                                                          ID *id);
+  /* full rebuild for already allocated hashtable */
+  void rebuild_from_treestore(BLI_mempool &treestore);
 
-/* find user or unused element with specific type, nr and id */
-struct TreeStoreElem *BKE_outliner_treehash_lookup_any(GHash *treehash,
-                                                       short type,
-                                                       short nr,
-                                                       ID *id);
+  /* clear element usage flags */
+  void clear_used();
 
-/* free treehash structure */
-void BKE_outliner_treehash_free(GHash *treehash);
+  /* Add/remove hashtable elements */
+  void add_element(TreeStoreElem &elem);
+  void remove_element(TreeStoreElem &elem);
 
-#ifdef __cplusplus
-}
-#endif
+  /* find first unused element with specific type, nr and id */
+  TreeStoreElem *lookup_unused(short type, short nr, ID *id) const;
+
+  /* find user or unused element with specific type, nr and id */
+  TreeStoreElem *lookup_any(short type, short nr, ID *id) const;
+
+ private:
+  TreeHash() = default;
+
+  void fill_treehash(BLI_mempool &treestore);
+};
+
+}  // namespace blender::bke::outliner::treehash