gh-105059: In C++, Py_INCREF() uses a function call

In C++, Py_INCREF() and Py_DECREF() are now implemented as function
calls. Moreover, in C++, PyObject.ob_refcnt type is just Py_ssize_t:
don't use an anonymous union to avoid compiler warnings (anonymous
unions were added to C11).

Author: vstinner

Include/object.h

@@ -134,13 +134,21 @@ check by comparing the reference count field to the immortality reference count.
         { _Py_IMMORTAL_REFCNT },    \
         (type)                      \
     },
-#else
+#elif !defined(__cplusplus)
 #define PyObject_HEAD_INIT(type) \
     {                            \
         _PyObject_EXTRA_INIT     \
         { 1 },                   \
         (type)                   \
     },
+#else
+    // gh-105059: In C++, ob_refcnt type is Py_ssize_t
+#define PyObject_HEAD_INIT(type) \
+    {                            \
+        _PyObject_EXTRA_INIT     \
+        1,                       \
+        (type)                   \
+    },
 #endif /* Py_BUILD_CORE */
 
 #define PyVarObject_HEAD_INIT(type, size) \
@@ -165,12 +173,18 @@ check by comparing the reference count field to the immortality reference count.
  */
 struct _object {
     _PyObject_HEAD_EXTRA
+#ifndef __cplusplus
     union {
        Py_ssize_t ob_refcnt;
 #if SIZEOF_VOID_P > 4
        PY_UINT32_T ob_refcnt_split[2];
 #endif
     };
+#else
+    // gh-105059: In C++, avoid anonymous union for ob_refcnt.
+    // C++ uses opaque function calls for Py_INCREF() and Py_DECREF().
+    Py_ssize_t ob_refcnt;
+#endif
     PyTypeObject *ob_type;
 };
 
@@ -611,15 +625,18 @@ PyAPI_FUNC(void) _Py_DecRef(PyObject *);
 static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 {
 #if defined(Py_LIMITED_API) && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-    // Stable ABI implements Py_INCREF() as a function call on limited C API
-    // version 3.12 and newer, and on Python built in debug mode. _Py_IncRef()
-    // was added to Python 3.10.0a7, use Py_IncRef() on older Python versions.
-    // Py_IncRef() accepts NULL whereas _Py_IncRef() doesn't.
+    // In limited C API version 3.12 and newer and on Python built in debug
+    // mode, Py_INCREF() is implemented as a function call. _Py_IncRef() was
+    // added to Python 3.10.0a7, use Py_IncRef() on older Python versions.
+    // Py_IncRef() accepts NULL, whereas _Py_IncRef() doesn't.
 #  if Py_LIMITED_API+0 >= 0x030a00A7
     _Py_IncRef(op);
 #  else
     Py_IncRef(op);
 #  endif
+#elif defined(__cplusplus)
+    // gh-105059: In C++, Py_INCREF() is implemented as a function call
+    _Py_IncRef(op);
 #else
     // Non-limited C API and limited C API for Python 3.9 and older access
     // directly PyObject.ob_refcnt.
@@ -649,10 +666,10 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 #endif
 
 #if defined(Py_LIMITED_API) && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-// Stable ABI implements Py_DECREF() as a function call on limited C API
-// version 3.12 and newer, and on Python built in debug mode. _Py_DecRef() was
-// added to Python 3.10.0a7, use Py_DecRef() on older Python versions.
-// Py_DecRef() accepts NULL whereas _Py_IncRef() doesn't.
+// In limited C API version 3.12 and newer and on Python built in debug mode,
+// Py_DECREF() is implemented as a function call. _Py_DecRef() was added to
+// Python 3.10.0a7, use Py_DecRef() on older Python versions. Py_DecRef()
+// accepts NULL, whereas _Py_DecRef() doesn't.
 static inline void Py_DECREF(PyObject *op) {
 #  if Py_LIMITED_API+0 >= 0x030a00A7
     _Py_DecRef(op);
@@ -662,6 +679,13 @@ static inline void Py_DECREF(PyObject *op) {
 }
 #define Py_DECREF(op) Py_DECREF(_PyObject_CAST(op))
 
+#elif defined(__cplusplus)
+// gh-105059: In C++, Py_DECREF() is implemented as a function call
+static inline void Py_DECREF(PyObject *op) {
+    _Py_DecRef(op);
+}
+#define Py_DECREF(op) Py_DECREF(_PyObject_CAST(op))
+
 #elif defined(Py_REF_DEBUG)
 static inline void Py_DECREF(const char *filename, int lineno, PyObject *op)
 {