initial data structure

Differential Revision: https://developer.blender.org/D14139

@@ -170,0 +161,4 @@
+ * Initializes a CustomData object with the same layers as source. The data is not copied from the
+ * source. Instead, the new layers are initialized using the given `alloctype`.
+ */
+void CustomData_copy_new(const struct CustomData *source,

@@ -2234,1 +2259,3 @@
-    if ((maxnumber != -1) && (number >= maxnumber)) {
+    if ((max_current_type_layer_count != -1) &&
+        (current_type_layer_count >= max_current_type_layer_count)) {
+      /* Don't merge this layer because the maximum amount of layers of this type is reached. */

@@ -2315,0 +2349,4 @@
+ * A #bCopyOnWrite that knows how to free the entire referenced custom data layer (including
+ * potentially separately allocated chunks like for vertex groups).
+ */
+class CustomDataLayerCOW : public bCopyOnWrite {

@@ -638,3 +638,3 @@
   CustomData_free(&pointcloud->pdata, pointcloud->totpoint);
   pointcloud->totpoint = me->totvert;
-  CustomData_merge(&me->vdata, &pointcloud->pdata, CD_MASK_PROP_ALL, CD_DUPLICATE, me->totvert);
+

@@ -0,0 +14,4 @@
+
+/**
+ * #bCopyOnWrite allows implementing copy-on-write behavior, i.e. it allows sharing read-only data
+ * between multiple independend systems (e.g. meshes). The data is only copied when it is shared

@@ -0,0 +22,4 @@
+ * data only has a single owner and is mutable. If it is larger than 1, it is shared and must be
+ * logically const.
+ *
+ * On top of containing a reference count, #bCopyOnWrite also knows how to destruct the referenced

@@ -0,0 +32,4 @@
+ *   arrays (e.g. for mesh attributes).
+ * - It can be embedded into another struct. For that it's best to use #bCopyOnWriteMixin.
+ */
+struct bCopyOnWrite : blender::NonCopyable, blender::NonMovable {

@@ -0,0 +10,4 @@
+
+namespace blender {
+
+template<typename T> class COWUser {

@@ -1689,0 +1694,4 @@
+    return;
+  }
+  MemFile &memfile = *writer->wd->mem.written_memfile;
+  if (memfile.cow_storage == nullptr) {

@@ -0,0 +10,4 @@
+
+namespace blender {
+
+template<typename T> class COWUser {

@@ -0,0 +63,4 @@
+
+  void remove_user_and_delete_if_last() const
+  {
+    const int old_user_count = users_.fetch_sub(1, std::memory_order_relaxed);

@@ -0,0 +63,4 @@
+
+  void remove_user_and_delete_if_last() const
+  {
+    const int old_user_count = users_.fetch_sub(1, std::memory_order_relaxed);