Also, I would: <code>
-#if !defined(ALLOC_IMPL)
+#if 0 // !defined(ALLOC_IMPL) // explain why disabled
</code>
this leaves the reader with less to think through in deciding whether the disabled condition on ALLOC_IMPL is *nested* ouside the #if 0 or replaced by it.  You can I can see that, in this diff, but it won't be so obvious to someone reading the final result.