diff --git a/Include/cpython/objimpl.h b/Include/cpython/objimpl.h
index 2f802e9..832622c 100644
--- a/Include/cpython/objimpl.h
+++ b/Include/cpython/objimpl.h
@@ -6,6 +6,56 @@
 extern "C" {
 #endif
 
+#define _PyObject_SIZE(typeobj) ( (typeobj)->tp_basicsize )
+
+/* _PyObject_VAR_SIZE returns the number of bytes (as size_t) allocated for a
+   vrbl-size object with nitems items, exclusive of gc overhead (if any).  The
+   value is rounded up to the closest multiple of sizeof(void *), in order to
+   ensure that pointer fields at the end of the object are correctly aligned
+   for the platform (this is of special importance for subclasses of, e.g.,
+   str or int, so that pointers can be stored after the embedded data).
+
+   Note that there's no memory wastage in doing this, as malloc has to
+   return (at worst) pointer-aligned memory anyway.
+*/
+#if ((SIZEOF_VOID_P - 1) & SIZEOF_VOID_P) != 0
+#   error "_PyObject_VAR_SIZE requires SIZEOF_VOID_P be a power of 2"
+#endif
+
+#define _PyObject_VAR_SIZE(typeobj, nitems)     \
+    _Py_SIZE_ROUND_UP((typeobj)->tp_basicsize + \
+        (nitems)*(typeobj)->tp_itemsize,        \
+        SIZEOF_VOID_P)
+
+
+/* This example code implements an object constructor with a custom
+   allocator, where PyObject_New is inlined, and shows the important
+   distinction between two steps (at least):
+       1) the actual allocation of the object storage;
+       2) the initialization of the Python specific fields
+      in this storage with PyObject_{Init, InitVar}.
+
+   PyObject *
+   YourObject_New(...)
+   {
+       PyObject *op;
+
+       op = (PyObject *) Your_Allocator(_PyObject_SIZE(YourTypeStruct));
+       if (op == NULL)
+       return PyErr_NoMemory();
+
+       PyObject_Init(op, &YourTypeStruct);
+
+       op->ob_field = value;
+       ...
+       return op;
+   }
+
+   Note that in C++, the use of the new operator usually implies that
+   the 1st step is performed automatically for you, so in a C++ class
+   constructor you would start directly with PyObject_Init/InitVar. */
+
+
 /* Inline functions trading binary compatibility for speed:
    PyObject_INIT() is the fast version of PyObject_Init(), and
    PyObject_INIT_VAR() is the fast version of PyObject_InitVar().