From: Robert Klemme Date: 2009-12-22T17:24:10+09:00 Subject: Re: Ruby's implementation of Fixnum-assignment 2009/12/21 RichardOnRails : > On Dec 21, 12:26 pm, Robert Klemme wrote: >> 2009/12/21 RichardOnRails : >> No variable ... has an object id. > > Robert,  I know, by virtue of reading a number of your responses in > this NG, that you’re a Ruby expert.  But the statement referenced > above is contradicted by at least two authorities on Ruby: > 1. “... every object has a unique object identifier (abbreviated as > object ID)” [“Programming Ruby”,  Second Edition, Thomas, et al, p. > 12] , where you praised the book’s usefulness :-).   Of course, the > “id” method has since be deprecated to “object_id” when we want to > reference an object ID. > 2.  “... an object in Ruby has an identity: “abc”object_id > #53744407 > This object ID is of limited utility." [“The Ruby Way”, Second > Edition, Fulton, p.26] As Marnen pointed out already, there is no contradiction between what I said and what the authorities said. A variable is just a place to store an object reference. Other than that it does not have properties, definitively no id. What is returned by invocation of #object_id is attached to the object at hand and not to the variable. Whether the object is an immediate value such as a Fixnum or a "regular" object (such as String) does not really matter with regard to the variable. >> step 3 actually mixes two separate things: evaluation of the expression of the right side of "=" and assignment > > Granted, I stuck to what I viewed as the essential issues.  Perhaps a > more complete estimate might read. > > 3.1     Ruby interprets “a = 1” to mean “a.=(1)”, i.e.  invokes the “=” > method on the object.  Normally, that reference will cascade up to > Object.=, I suppose. Now that is completely wrong. "a=1" is an assignment meaning, it takes whatever object reference the evaluation of the expression on the right side yields and puts it into the storage location denoted by "a". There is no method invoked, certainly no method on "a" which - as I said - is no object but just a place in storage. > 3.1a  If “a” is undefined in the current scope, “a” will be added to > the scope with an ID of (2a+1) = 3. > 3.1b Otherwise: > 3.1.b.1 If a’s ID is in the range of Fixnums, (or a’s ID indicates > it’s true, false, etc .), a’s ID will be set to 3 > 3.1b.2 Otherwise, if a’s ID indicates any other type, a’s ID will be > pushed to the garbage stack and a’s ID will be set to 3 > Is that better? I'm sorry, no. It's rather: 1. Make sure local scope has a place for variable "a". 2. Evaluate the expression to the right side of "=". 3. Take the resulting object reference of step 2 and store it in the location for variable "a". Steps 1 and 2 can actually be exchanged, it does not really matter much. >> For optimization purposes some object references are special in that they actually *are* the object > > That sounds like you mean in the case of Fixnum “1”,  that the number > 1 IS the ID as well as the value of the object.  But we can see “puts > 1.object_id” => 3 No, it means that in the case of Fixnum the _reference_ is the object. See the article Gary referenced for further detail: http://en.wikipedia.org/wiki/Tagged_pointer > So I think that fact (plus all the other evidence I offered) suggests > that ID’s for Fixnum’s are manufactured as 2*value+1 > QED??? The way object ids come into existence is completely unrelated to the process of variable assignment. Since in Ruby any object needs an id and Fixnums (plus a few more) are immediate values (which means there is just an object _reference_ and no object, which btw also means that there is no place to store state and hence Fixnums are immutable) it was chosen to derive object ids according to the formula you found out. There is probably also a technical reason for this formula which likely has to do with the fact that other objects need ids as well and calculation of them should be efficient but I believe it is more important to understand the object - object reference dichotomy. Actually, I believe all this reasoning about Fixnums being immediate values is totally overdone. From a Ruby programmer's perspective it is completely irrelevant (if you put performance aside for the moment). It is sufficient to know that Ruby has variables which contain object references and that evaluation of whatever expression yields an object reference. The programming model is as simple as that. Immediate values are really just an optimization under the hood to speed up math and other common operations. In Ruby land, you have no chance to distinguish an immediate value from any other immutable object - whatever methods you invoke (and there are quite a few for Fixnum) the object simply does not change its state. Granted, there are a few things that do not work, for example defining a finalizer for a Fixnum but even that raises an ordinary exception. Kind regards robert -- remember.guy do |as, often| as.you_can - without end http://blog.rubybestpractices.com/