Fwd: [squeak-dev] Re: Newbie Question (about OOPs, maybe) (sorry)

> From: "David T. Lewis" <[hidden email]>
> Date: August 23, 2009 5:52:25 PM CEDT
> To: The general-purpose Squeak developers list <[hidden email]
> >
> Subject: Re: [squeak-dev] Re: Newbie Question (about OOPs, maybe)  
> (sorry)
> Reply-To: The general-purpose Squeak developers list <[hidden email]
> >
>
> On Sun, Aug 23, 2009 at 04:36:24PM +0200, Bert Freudenberg wrote:
>> On 23.08.2009, at 11:42, Trygve Reenskaug wrote:
>>> On 2009.08.22 17:26, Bert Freudenberg wrote:
>>>> On 22.08.2009, at 12:39, Trygve Reenskaug wrote:
>>>>> 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. ..."
>>
>> ... digs out Blue Book ...
>>
>> Yes. This is the VM implementation chapter. IMHO it does not mean an
>> OOP has to be accessible from the image as an integer.
>>
>>> 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.
>>
>> The VM can do anything as long as you can't tell from the image. And
>> you can't. OOPs cannot be accessed directly.
>
> To illustrate the current behavior in Squeak, I used a plugin that
> displays OOP values, addresses, etc. Thus:
>
>  (1 to: 100) do: [:e | Array with: e]. "make some garbage"
>  aString := 'Hello there'. "allocate an object"
>  aString asOop. => 1422 "identity hash for the object"
>  OopPlugin oopPrintStringHex: aString. '16r0FF3BE64' "current OOP  
> value"
>  OopPlugin machineAddressPrintStringHex: aString.  
> '16r00002B66BBE8AE54' "C pointer to object header"
>  Smalltalk garbageCollect. "cause objects to move around in the  
> object memory"
>  aString asOop. => 1422 "identity hash remains the same"
>  OopPlugin oopPrintStringHex: aString. => '16r0FF36954' "OOP value  
> has changed"
>  OopPlugin machineAddressPrintStringHex: aString. =>  
> '16r00002B66BBE85944' "object has moved"
>
> The OOP is a unique identifier for the object at a particular  
> instant in
> time, but the VM is free to change it at any time without telling you.
>
> Dave
>
>