Only allocate dict pointer when needed for dynamic attributes

include/pybind11/common.h

@@ -297,7 +297,6 @@ inline std::string error_string();
 template <typename type> struct instance_essentials {
     PyObject_HEAD
     type *value;
-    PyObject *dict;
     PyObject *weakrefs;
     bool owned : 1;
     bool constructed : 1;

include/pybind11/pybind11.h

@@ -574,24 +574,24 @@ public:
 
 NAMESPACE_BEGIN(detail)
 extern "C" inline PyObject *get_dict(PyObject *op, void *) {
-    auto *self = (instance<void> *) op;
-    if (!self->dict) {
-        self->dict = PyDict_New();
+    PyObject *&dict = *_PyObject_GetDictPtr(op);
+    if (!dict) {
+        dict = PyDict_New();
     }
-    Py_XINCREF(self->dict);
-    return self->dict;
+    Py_XINCREF(dict);
+    return dict;
 }
 
-extern "C" inline int set_dict(PyObject *op, PyObject *dict, void *) {
-    if (!PyDict_Check(dict)) {
+extern "C" inline int set_dict(PyObject *op, PyObject *new_dict, void *) {
+    if (!PyDict_Check(new_dict)) {
         PyErr_Format(PyExc_TypeError, "__dict__ must be set to a dictionary, not a '%.200s'",
-                     Py_TYPE(dict)->tp_name);
+                     Py_TYPE(new_dict)->tp_name);
         return -1;
     }
-    auto *self = (instance<void> *) op;
-    Py_INCREF(dict);
-    Py_CLEAR(self->dict);
-    self->dict = dict;
+    PyObject *&dict = *_PyObject_GetDictPtr(op);
+    Py_INCREF(new_dict);
+    Py_CLEAR(dict);
+    dict = new_dict;
     return 0;
 }
 
@@ -714,7 +714,8 @@ protected:
         /* Support dynamic attributes */
         if (rec->dynamic_attr) {
             type->ht_type.tp_flags |= Py_TPFLAGS_HAVE_GC;
-            type->ht_type.tp_dictoffset = offsetof(instance_essentials<void>, dict);
+            type->ht_type.tp_dictoffset = type->ht_type.tp_basicsize; // place the dict at the end
+            type->ht_type.tp_basicsize += sizeof(PyObject *); // and allocate enough space for it
             type->ht_type.tp_getset = generic_getset;
             type->ht_type.tp_traverse = traverse;
             type->ht_type.tp_clear = clear;
@@ -822,20 +823,23 @@ protected:
             if (self->weakrefs)
                 PyObject_ClearWeakRefs((PyObject *) self);
 
-            Py_CLEAR(self->dict);
+            PyObject **dict_ptr = _PyObject_GetDictPtr((PyObject *) self);
+            if (dict_ptr) {
+                Py_CLEAR(*dict_ptr);
+            }
         }
         Py_TYPE(self)->tp_free((PyObject*) self);
     }
 
     static int traverse(PyObject *op, visitproc visit, void *arg) {
-        auto *self = (instance<void> *) op;
-        Py_VISIT(self->dict);
+        PyObject *&dict = *_PyObject_GetDictPtr(op);
+        Py_VISIT(dict);
         return 0;
     }
 
     static int clear(PyObject *op) {
-        auto *self = (instance<void> *) op;
-        Py_CLEAR(self->dict);
+        PyObject *&dict = *_PyObject_GetDictPtr(op);
+        Py_CLEAR(dict);
         return 0;
     }