Hi Bert,
My answers inline below.

On 2009.08.22 17:26, Bert Freudenberg wrote:
> On 22.08.2009, at 12:39, Trygve Reenskaug wrote:
>
>> /Object>>asOop
>>     "Primitive. Answer a SmallInteger whose value is half of the 
>> receiver's 
>>     object pointer (interpreting object pointers as 16-bit signed 
>> quantities)..."/
>>
>> /ProtoObject>>identityHash
>>     "Answer a SmallInteger whose value is related to the receiver's 
>> identity..."
>> /This is pretty vague, but the term 'Hash' indicates that many 
>> objects can share the same value. So it is clearly cannot be used as 
>> the objectID.
>>
>> The Morph>>printOn: implementation is less than ideal because it 
>> should apply to all objects, not only Morphs. Also, it should use 
>> 'asOop' rather than 'identityHash' to let us hope for uniqueness.
>
> You might have overlooked that both these methods are identical.
I do know it and interpret this to mean that the current Squeak VM 
happens to have unique hash values. There is no guarantee that another 
version of the VM will have this invariant.
>
>> In short:
>>
>>     * Smalltalk claims to be object oriented and I regard it as a
>>       serious flaw that the object ID is not explicitly visible.
>>
>
> What if there simply *is* no object ID?
In Smalltalk, a value is a pointer to an object. Bluebook p. 564: "Each 
object is associated with a unique identifier called its object pointer. 
..."
Smalltalk MUST have such unique identifiers to work. I see this pointer 
as the object's ID. I have seen from the discussion in this track that 
the VM can actually change this "unique" pointer.

IMO, a Smalltalk user/programmer should not need to understand the VM. 
Alan Kay' once said something like "everything in computing is about 
illusions". One such illusion is that the unique object identifier is 
immutable. When debugging, I also find it useful that it shall be 
possible to make it visible, but this is not essential for Smalltalk to 
work.
>
>>     * We probably need a 64-bit VM to represent it properly, e.g.,
>>       using the standardized definition of OID, but a value that is
>>       guaranteed to be unique within the current set of objects would
>>       be acceptable..
>>
>
> What is the "standardized definition of OID"?
See the Wikipedia article http://en.wikipedia.org/wiki/Object_identifier


>
>>     * I regard it as a bug that Object>>printOn: does not present
>>       some kind of object ID.
>>
>
> Has never bothered me.
I tend to think and work in terms of objects and object structures. Then 
I need it.

>
> - Bert -
>
>
--Trygve
-- 

Trygve Reenskaug       mailto: trygver at ifi.uio.no

Morgedalsvn. 5A         http://heim.ifi.uio.no/~trygver

N-0378 Oslo               Tel: (+47) 22 49 57 27

Norway

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