# C extension: can rb\_ivar\_set be used to set class variable? **URL:** \ **Category:** Ruby\ **Created:** [December 14, 2007, 6:21am UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118 "2007-12-14T06:21:39Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 6:21am UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/1 "2007-12-14T06:21:39Z")
Hi all, This is a question on Ruby C extension. I have been hacking some existed ruby c extension, and found following code: VALUE klass = rb\_define\_class(name, pClass); rb\_ivar\_set(klass, id\_some\_var, someValue); From the book “Programming Ruby, Second Edition”, I know the prototype for rb\_ivar\_set is like: VALUE rb\_ivar\_set( VALUE obj, ID name, VALUE value ) And it says the first parameter should be an obj, while as we can see from above, the code I am hacking is passing “klass” returned by rb\_define\_class to rb\_ivar\_set. So I wonder if this is a way to set class variable for a class. I am really confused because the document says it should be used to set value of instance variable to a given obj. Am I missing some knowledge? Any help is appreciated and thanks in advance! ---
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 6:29am UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/2 "2007-12-14T06:29:15Z")
Hi, At Fri, 14 Dec 2007 14:20:01 +0900, lianliming wrote in [ruby-talk:283492]: > So I wonder if this is a way to set class variable for a class. I am > really confused because the document says it should be used to set > value of instance variable to a given obj. Am I missing some > knowledge? No, use rb\_cvar\_set to set class variables. rb\_ivar\_set sets an instance variable of a class instance. I.E., class Foo @foo = “ivar” @@foo = “cvar” end ---
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 7:16am UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/3 "2007-12-14T07:16:39Z")
Thanks for your reply! > No, use rb\_cvar\_set to set class variables. rb\_ivar\_set sets > an instance variable of a class instance. I.E., > > class Foo > @foo = “ivar” > @@foo = “cvar” > end Will the instance variable “@foo”'s value declared as the above example can be referenced in an object of Class Foo? Please look at following example: class Foo @foo = “ivar” def printFoo puts @foo end end f=Foo.new f.printFoo ========\> nil So now, I am curious to know, why the function rb\_ivar\_set allows passing a “klass” as the first parameter to set value of a instance variable which can’t be referenced by its objects. ---
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 2:06pm UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/4 "2007-12-14T14:06:59Z")
On 12/14/07, lianliming [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Will the instance variable “@foo”'s value declared as the above > end > > f=Foo.new > f.printFoo > ========\> nil > > So now, I am curious to know, why the function rb\_ivar\_set allows > passing a “klass” as the first parameter to set value of a instance > variable which can’t be referenced by its objects. rb\_ivar\_set takes an object as the first parameter. Classes are objects, and as such can have their own instance variables which are visible in class methods. These are called class instance variables, and are quite different from class variables. Being instance variables, class instance variables are only visible to methods of the object to which they are attached, which in the case of class instance variables would be class methods. There’s also a subtlety to the Ruby object model which can be surprising to folks familiar with other OO languages. In many languages including Java, C++ and Smalltalk, one of the functions of a class is to serve as a template which maps the layout of the object in memory, in a manner analogous to a C struct, so that method code can reference instance variables by offset from the address of the instance. Ruby classes don’t really “know” about the instance variables of their instances, since instance variables( actually all variables) aren’t declared in Ruby. Instance variables get attached to an instance dynamically when a method is executed which mentions that instance variable. In fact different instances of the same class can have different sets of instance variables, and the set of instance variables of a particular object can change over time. Conceptually, and actually at least for Matz’s Ruby Implementation, instance variables are resolved by accessing a per-instance internal hash table which maps the instance variable name (including the @ sigil) as a symbol to the value. Class variables, marked with the @@ sigil are also looked up in a hash associated with the class object. The difference is that for class variables, if the key is not found in the hash, the search considers iteratively up the superclass chain, and for class methods, it’s the class and not the metaclass chain which is traversed. This means that class variables are visible to both instance and class methods of the class where they live and that classes subclasses. The MRI, at least the last time I looked, actually puts class variables in the same hash as class instance variables, i.e. the class objects instance variable hash. It’s the difference in the sigils which affects how they are seen and used. – Rick DeNatale My blog on Ruby [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) ---
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 7:51am UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/5 "2007-12-14T07:51:43Z")
Hi, At Fri, 14 Dec 2007 15:15:02 +0900, lianliming wrote in [ruby-talk:283498]: > Will the instance variable “@foo”'s value declared as the above > example can be referenced in an object of Class Foo? No, two different objects’s instance variables. Now your question is not related to C extension. I guess others could explain it much better than me. ---
**Author:** ![lianliming](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/b38774/32.png) [@lianliming](https://www.ruby-forum.com/u/lianliming)\ **Post date:** [December 14, 2007, 4:38pm UTC](https://www.ruby-forum.com/t/c-extension-can-rb-ivar-set-be-used-to-set-class-variable/123118/6 "2007-12-14T16:38:59Z")
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