BLI: add kdtree range search method that accepts c++ lambda

This is easier to use in C++ code compared to passing a function
and user-data separately.

source/blender/blenlib/BLI_kdtree.h

@@ -2,10 +2,6 @@
 
 #pragma once
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /** \file
  * \ingroup bli
  * \brief A KD-tree for nearest neighbor search.
@@ -54,7 +50,3 @@ extern "C" {
 #undef KDTree
 #undef KDTreeNearest
 #undef KDTREE_PREFIX_ID
-
-#ifdef __cplusplus
-}
-#endif

source/blender/blenlib/BLI_kdtree_impl.h

@@ -12,6 +12,10 @@
 #define _BLI_CONCAT(MACRO_ARG1, MACRO_ARG2) _BLI_CONCAT_AUX(MACRO_ARG1, MACRO_ARG2)
 #define BLI_kdtree_nd_(id) _BLI_CONCAT(KDTREE_PREFIX_ID, _##id)
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct KDTree;
 typedef struct KDTree KDTree;
 
@@ -80,6 +84,29 @@ int BLI_kdtree_nd_(range_search_with_len_squared_cb)(
                        const void *user_data),
     const void *user_data) ATTR_NONNULL(1, 2) ATTR_WARN_UNUSED_RESULT;
 
+#ifdef __cplusplus
+}
+#endif
+
+#ifdef __cplusplus
+template<typename Fn>
+inline void BLI_kdtree_nd_(range_search_cb_cpp)(const KDTree *tree,
+                                                const float co[KD_DIMS],
+                                                float distance,
+                                                const Fn &fn)
+{
+  BLI_kdtree_nd_(range_search_cb)(
+      tree,
+      co,
+      distance,
+      [](void *user_data, const int index, const float *co, const float dist_sq) {
+        const Fn &fn = *static_cast<const Fn *>(user_data);
+        return fn(index, co, dist_sq);
+      },
+      const_cast<Fn *>(&fn));
+}
+#endif
+
 #undef _BLI_CONCAT_AUX
 #undef _BLI_CONCAT
 #undef BLI_kdtree_nd_