zend_register_class() overwrites ce->create_object with parent_ce->create_object?

If I define a descendant class, I obviously know what create_object should
do, as this should only effect internal classes. So could anybody please
explain to me why the parent's create_object is enforced?

It causes to write hacks like:

---8<---
zend_class_entry ce;
zend_object_value (__create_object)(zend_class_entry *ce TSRMLS_DC);

INIT_CLASS_ENTRY(ce, "classname", php_my_method_entry);
ce.create_object = php_my_object_new;
/* hack starts here */
__create_object = parent_ce->create_object;
parent_ce->create_object = php_my_object_new;
php_my_class_entry = zend_register_internal_class_ex(&ce, parent_ce, NULL TSRMLS_CC);
parent->create_object = __create_object;
--->8---

Why can't you simply overwrite it afterwards? Just like it's done in
SPL:

http://php-og.mgdm.net/opengrok/xref/PHP_5_3/ext/spl/spl_functions.c#41

Best,

If this limitation is there without a really good reason, I'd like to get rid of it.

Thanks,
Mike

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Etienne Kneuss