Re: How does Pharo store instance variables?

On 04/10/16 03:37, CodeDmitry wrote:
> "static" variables in Pharo outlive their Playground call!

There is no such thing as a "static" method or "static" variable in
Smalltalk. At least not in the Java sense.

In most languages a static method is simply a function attached to a
class (because there is no other place to put it). But it does not imply
any meaning to this/self. It's similar for variables ... because Global
Variables are frowned upon most programmers simply attach them to a
class - making them "static".

That's different in Smalltalk. In Smalltalk /every method is an instance
method/!
"Regular methods" are defined by the instance's class and executed
within an instance context.
"Class methods" are defined by the instances's *meta*class and also
executed in an instance context - only that the instance at that point
is the class itself!
Because there is a 1:1 relationship between the class (defining the
instance behavior) and it's meta-class (defining the class behavior) the
Smalltalk Browser provides a unified view. E.g. if you create a class in
the class browser you'll also create it's metaclass. The code you see
when browsing "the instance side" is the method dictionary of the class.
If you switch to "the class side" in the class browser you are
essentially browsing the method dictionary of it's metaclass!

It's a bit more complex for variables.
Ignoring regular instance variables a "class variable" is defined by a
class and is shared with all it's subclasses. E.g. a class hierarchy,
where only one of the subclasses is used for a Singleton at a time could
use a class variable "Current".
A "class instance variable" on the other hand does not share it's value
with subclasses. E.g. the class name and it's method dictionary could be
candidates for class inctance variables - i.e. they are unique per class
- not per hierarchy!

IMHO this implementation of instances/classes/metaclasses is very
different from the "static" stuff used today. Take a look here
(especially Chapter 5 in the Blue Book) for more details:
http://esug.org/data/Articles/Columns/EwingPapers/cvars&cinst_vars.pdf
http://stephane.ducasse.free.fr/FreeBooks/BlueBook/Bluebook.pdf


CU,

Udo

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