# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1298162115 -3600
# Node ID 7cc5058cbfb6d2be3308c6ad98079ca22a92e874
# Parent  5692f2b87aabb20840650d4f254b793a48be6987
Fixed __class__ to always be at instance attribute position 0, thus matching its
class attribute position.
Added notes about the __class__ attribute, its definition in the object table,
and the copying of the attribute from instance templates upon instantiation.

diff -r 5692f2b87aab -r 7cc5058cbfb6 docs/concepts.txt
--- a/docs/concepts.txt	Sun Feb 20 01:10:24 2011 +0100
+++ b/docs/concepts.txt	Sun Feb 20 01:35:15 2011 +0100
@@ -192,7 +192,8 @@
 A limitation of this representation is that instance attributes may not shadow
 class attributes: if an attribute with a given name is not defined on an
 instance, an attribute with the same name cannot be provided by the class of
-the instance or any superclass of the instance's class.
+the instance or any superclass of the instance's class. The exception to this
+restriction is the __class__ attribute, as described below.
 
 The table can be compacted using a representation known as a displacement
 list (referred to as an object list in this context):
@@ -240,9 +241,15 @@
 each class and its instances. Since the table elements can only refer to a
 single absolute address, thus providing only a single value, such absolute
 references which are sufficient for most class attributes would not be
-appropriate for the __class__ attribute. However, using an object-relative
-location would require both classes and instances to retain an attribute
-location specifically to hold the value appropriate for each object type.
+appropriate for the __class__ attribute. Using a common object-relative
+location of 0 permits the first attribute to be accessed via an object address
+regardless of whether a class or instance is involved.
+
+Obviously, this requires both classes and instances to retain an attribute
+location specifically to hold the value appropriate for each object type,
+whereas a scheme which omits the __class__ attribute on classes would be able
+to employ an absolute address in the table and maintain only a single address
+to refer to the class for all instances.
 
 Comparing Tables as Matrices with Displacement Lists
 ----------------------------------------------------
@@ -532,6 +539,8 @@
 resulting instance, along with locations for the attributes of the instance.
 Since the instance header contains data common to all instances of a class, a
 template header is copied to the start of the newly reserved memory region.
+The __class__ attribute is also an essential part of instances, and this is
+also copied to the new memory region.
 
 Register Usage
 ==============
diff -r 5692f2b87aab -r 7cc5058cbfb6 micropython/data.py
--- a/micropython/data.py	Sun Feb 20 01:10:24 2011 +0100
+++ b/micropython/data.py	Sun Feb 20 01:35:15 2011 +0100
@@ -1395,7 +1395,13 @@
         # instance.
 
         for name in self.instattr:
-            instattr[name] = set() # position not yet defined
+
+            # Special case: __class__ has to be at position 0.
+
+            if name == "__class__":
+                instattr[name] = set([0])
+            else:
+                instattr[name] = set() # position not yet defined
 
         reversed_bases = self.bases[:]
         reversed_bases.reverse()