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::dna::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::dna::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

2 files changed, 24 insertions, 0 deletions

diff --git a/source/blender/makesdna/intern/dna_utils.c b/source/blender/makesdna/intern/dna_utils.c
index 8c86ef69ebd..560b91b925e 100644
--- a/source/blender/makesdna/intern/dna_utils.c
+++ b/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);
+}
+
+/** \} */
diff --git a/source/blender/makesdna/intern/makesdna.c b/source/blender/makesdna/intern/makesdna.c
index 0d2f265a9b5..12ec7262906 100644
--- a/source/blender/makesdna/intern/makesdna.c
+++ b/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++;