tag variables

Whose Variable Is It Anyway?

Posted by Rick DeNatale Fri, 08 Feb 2008 21:14:00 GMT

The more I think about Ruby in relation to other object programming languages I’ve worked with, the more I realize that there’s a continuum of static vs. dynamic typing.

Ruby fits close to one end of that continuum. Understanding this can help understand how to best use the language. I recently had a quick look at Russ Olsen’s new book Design Patterns in Ruby and looked at his section on the observer pattern. I’d just posted to ruby-talk about this pattern, how it was implemented in Smalltalk, and a more Rubyish implementation. I’ll get to that at the end of this article, but first I really feel the urge to talk about instance variables.

If we view a type as a particular interpretation of a memory layout, I see something like this

Language	Outside	inside
Java	static	static
Smalltalk	encapsulated	static
Ruby	encapsulated	dynamic

On Variables, Values, and Objects

Posted by Rick DeNatale Wed, 13 Sep 2006 19:47:00 GMT

I’ve recently observed some posts to ruby-talk which evidence some confusion on the part of the posters about the relationship between variables and objects in Ruby. One currently active thread concerns several participants who are upset that instances of the Matrix class in the Ruby standard library can’t be changed once created.

In Ruby, like in other uniformly object-oriented languages, the relationship between variables and their values is subtly different than in other languages, and this is a crucial paradigm shift, which must be crossed in order to understand Ruby.

In many languages a variable names an area in memory which holds a “bag-of-bits” representing the value of the variable. The size of that area depends on the type of the variable. A variable holding an integer might be 4-bytes long, while one holding a particular structure might be 325 (or whatever) bytes.

In a uniformly object-oriented language, all variables reference objects, and any variable can reference any object, or different objects over time. My good friend at IBM, the late David N. Smith, used to say that in such a language, “all variables are the same size,” when he wrote about or taught Smalltalk.

This distinction can trip up the unwary. Let’s try to clear some of the stumbling blocks out of the way.