Implement C++ methods for DNA structures

This change makes it possible to add implementation of common
C++ methods for DNA structures which helps ensuring unsafe
operations like shallow copy are done explicitly.

For example, creating a shallow copy used to be:

  Object temp_object = *input_object;

In the C++ context it was seen like the temp_object is
properly decoupled from the input object, while in the
reality is it not. Now this code becomes:

  Object temp_object = blender:🧬:shallow_copy(*input_object);

The copy and move constructor and assignment operators are
now explicitly disabled.

Other than a more explicit resource management this change
also solves a lot of warnings generated by the implicitly
defined copy constructors w.r.t dealing with deprecated fields.
These warnings were generated by Apple Clang when a shallow
object copy was created via implicitly defined copy constructor.

In order to enable C++ methods for DNA structures a newly added
macro `DNA_DEFINE_CXX_METHODS()` is to be used:

  tpyedef struct Object {
    DNA_DEFINE_CXX_METHODS(Object)
    ...
  } Object;

For the shallow copy use `blender:🧬:shallow_copy()`.

The implementation of the memcpy is hidden via an internal DNA
function to avoid pulling `string.h` into every DNA header.
This means that the solution does not affect on the headers
dependencies.

---

Ideally `DNA_shallow_copy` would be defined in a more explicit
header, but don;t think we have a suitable one already. Maybe
we can introduce `DNA_access.h` ?

Differential Revision: https://developer.blender.org/D14427

source/blender/blenkernel/intern/mesh_convert.cc

@@ -910,7 +910,8 @@ static void curve_to_mesh_eval_ensure(Object &object)
    *
    * So we create temporary copy of the object which will use same data as the original bevel, but
    * will have no modifiers. */
-  Object bevel_object = {{nullptr}};
+  Object bevel_object;
+  memset(&bevel_object, 0, sizeof(bevel_object));
   if (curve.bevobj != nullptr) {
     memcpy(&bevel_object, curve.bevobj, sizeof(bevel_object));
     BLI_listbase_clear(&bevel_object.modifiers);
@@ -919,7 +920,8 @@ static void curve_to_mesh_eval_ensure(Object &object)
   }
 
   /* Same thing for taper. */
-  Object taper_object = {{nullptr}};
+  Object taper_object;
+  memset(&taper_object, 0, sizeof(taper_object));
   if (curve.taperobj != nullptr) {
     memcpy(&taper_object, curve.taperobj, sizeof(taper_object));
     BLI_listbase_clear(&taper_object.modifiers);

source/blender/blenkernel/intern/object.cc

@@ -3946,7 +3946,7 @@ bool BKE_object_minmax_dupli(Depsgraph *depsgraph,
       /* pass */
     }
     else {
-      Object temp_ob = *dob->ob;
+      Object temp_ob = blender::dna::shallow_copy(*dob->ob);
       /* Do not modify the original boundbox. */
       temp_ob.runtime.bb = nullptr;
       BKE_object_replace_data_on_shallow_copy(&temp_ob, dob->ob_data);

source/blender/depsgraph/intern/depsgraph_query_iter.cc

@@ -167,7 +167,7 @@ bool deg_iterator_duplis_step(DEGObjectIterData *data)
     /* Temporary object to evaluate. */
     Object *dupli_parent = data->dupli_parent;
     Object *temp_dupli_object = &data->temp_dupli_object;
-    *temp_dupli_object = *dob->ob;
+    *temp_dupli_object = blender::dna::shallow_copy(*dob->ob);
     temp_dupli_object->base_flag = dupli_parent->base_flag | BASE_FROM_DUPLI;
     temp_dupli_object->base_local_view_bits = dupli_parent->base_local_view_bits;
     temp_dupli_object->runtime.local_collections_bits =

source/blender/editors/object/object_transform.cc

@@ -1695,13 +1695,13 @@ static void object_apply_rotation(Object *ob, const float rmat[3][3])
 static void object_apply_location(Object *ob, const float loc[3])
 {
   /* quick but weak */
-  Object ob_prev = *ob;
+  Object ob_prev = blender::dna::shallow_copy(*ob);
   float mat[4][4];
   copy_m4_m4(mat, ob->obmat);
   copy_v3_v3(mat[3], loc);
   BKE_object_apply_mat4(ob, mat, true, true);
   copy_v3_v3(mat[3], ob->loc);
-  *ob = ob_prev;
+  *ob = blender::dna::shallow_copy(ob_prev);
   copy_v3_v3(ob->loc, mat[3]);
 }
 

source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc

@@ -33,7 +33,7 @@ OBJMesh::OBJMesh(Depsgraph *depsgraph, const OBJExportParams &export_params, Obj
 {
   /* We need to copy the object because it may be in temporary space. */
   Object *obj_eval = DEG_get_evaluated_object(depsgraph, mesh_object);
-  export_object_eval_ = *obj_eval;
+  export_object_eval_ = dna::shallow_copy(*obj_eval);
   export_mesh_eval_ = export_params.apply_modifiers ?
                           BKE_object_get_evaluated_mesh(&export_object_eval_) :
                           BKE_object_get_pre_modified_mesh(&export_object_eval_);

source/blender/makesdna/DNA_defs.h

@@ -46,3 +46,84 @@
 
 /* non-id name variables should use this length */
 #define MAX_NAME 64
+
+/* #DNA_DEFINE_CXX_METHODS is used to define C++ methods which are needed for proper/safe resource
+ * management, making unsafe (from an ownership perspective: i.e. pointers which sometimes needs to
+ * be set to nullptr on copy, sometimes needs to be dupalloc-ed) operations explicit, and taking
+ * care of compiler specific warnings when dealing with members marked with DNA_DEPRECATED.
+ *
+ * The `class_name` argument is to match the structure name the macro is used from.
+ *
+ * Typical usage example:
+ *
+ *   typedef struct Object {
+ *     DNA_DEFINE_CXX_METHODS(Object)
+ *   } Object;
+ */
+#ifndef __cplusplus
+#  define DNA_DEFINE_CXX_METHODS(class_name)
+#else
+
+/* Forward-declared here since there is no simple header file to be pulled for this functionality.
+ * Avoids pulling `string.h` from this header to get access to #memcpy. */
+extern "C" void _DNA_internal_memcpy(void *dst, const void *src, size_t size);
+
+namespace blender::dna::internal {
+
+template<class T> class ShallowDataConstRef {
+ public:
+  constexpr explicit ShallowDataConstRef(const T &ref) : ref_(ref)
+  {
+  }
+
+  inline const T *get_pointer() const
+  {
+    return &ref_;
+  }
+
+ private:
+  const T &ref_;
+};
+
+}  // namespace blender::dna::internal
+
+#  define DNA_DEFINE_CXX_METHODS(class_name) \
+    class_name() = default; \
+    ~class_name() = default; \
+    /* Delete copy and assignment, which are not safe for resource ownership. */ \
+    class_name(const class_name &other) = delete; \
+    class_name(class_name &&other) noexcept = delete; \
+    class_name &operator=(const class_name &other) = delete; \
+    class_name &operator=(class_name &&other) = delete; \
+    /* Support for shallow copy. */ \
+    class_name(const blender::dna::internal::ShallowDataConstRef<class_name> ref) \
+    { \
+      _DNA_internal_memcpy(this, ref.get_pointer(), sizeof(class_name)); \
+    } \
+    class_name &operator=(const blender::dna::internal::ShallowDataConstRef<class_name> ref) \
+    { \
+      if (this != ref.get_pointer()) { \
+        _DNA_internal_memcpy(this, ref.get_pointer(), sizeof(class_name)); \
+      } \
+      return *this; \
+    }
+
+namespace blender::dna {
+
+/* Creates shallow copy of the given object.
+ * The entire object is copied as-is using memory copy.
+ *
+ * Typical usage:
+ *   Object temp_object = blender::dna::shallow_copy(*input_object);
+ *
+ * From the implementation detail go via copy constructor/assign operator defined in the structure.
+ */
+template<class T>
+[[nodiscard]] inline internal::ShallowDataConstRef<T> shallow_copy(const T &other)
+{
+  return internal::ShallowDataConstRef(other);
+}
+
+}  // namespace blender::dna
+
+#endif

source/blender/makesdna/DNA_object_types.h

@@ -233,6 +233,8 @@ enum eObjectLineArt_Flags {
 };
 
 typedef struct Object {
+  DNA_DEFINE_CXX_METHODS(Object)
+
   ID id;
   /** Animation data (must be immediately after id for utilities to use it). */
   struct AnimData *adt;

source/blender/makesdna/intern/dna_utils.c

@@ -308,3 +308,15 @@ const char *DNA_struct_rename_legacy_hack_alias_from_static(const char *name)
 }
 
 /** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Internal helpers for C++
+ * \{ */
+
+void _DNA_internal_memcpy(void *dst, const void *src, size_t size);
+void _DNA_internal_memcpy(void *dst, const void *src, const size_t size)
+{
+  memcpy(dst, src, size);
+}
+
+/** \} */

source/blender/makesdna/intern/makesdna.c

@@ -620,6 +620,7 @@ static int preprocess_include(char *maindata, const int maindata_len)
   int newlen = 0;
   comment = 0;
   a = maindata_len;
+  bool skip_until_closing_brace = false;
   while (a--) {
 
     if (cp[0] == '/' && cp[1] == '*') {
@@ -646,6 +647,17 @@ static int preprocess_include(char *maindata, const int maindata_len)
       a -= 13;
       cp += 13;
     }
+    else if (match_identifier(cp, "DNA_DEFINE_CXX_METHODS")) {
+      /* single values are skipped already, so decrement 1 less */
+      a -= 21;
+      cp += 21;
+      skip_until_closing_brace = true;
+    }
+    else if (skip_until_closing_brace) {
+      if (cp[0] == ')') {
+        skip_until_closing_brace = false;
+      }
+    }
     else {
       md[0] = cp[0];
       md++;