# In Depth: attr\_reader/writer/accessor **URL:** \ **Category:** Ruby\ **Created:** [November 23, 2008, 10:50pm UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958 "2008-11-23T22:50:10Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [November 23, 2008, 10:50pm UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/1 "2008-11-23T22:50:10Z")
Can anyone explain exactly what happens when the interpreter interprets attr\_reader and its kin? E.G. where is the method located (in which module, class) and what does it do with the symbols in order to expand the method call into an instance method of the calling class object? ---
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [November 23, 2008, 11:32pm UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/2 "2008-11-23T23:32:38Z")
On Sun, Nov 23, 2008 at 4:45 PM, Matthew M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Can anyone explain exactly what happens when the interpreter interprets > attr\_reader and its kin? > > E.G. where is the method located (in which module, class) and what does > it do with the symbols in order to expand the method call into an > instance method of the calling class object? [http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/241709](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/241709) HTH Michael G. ---
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [November 24, 2008, 9:44am UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/3 "2008-11-24T09:44:17Z")
Hi – On Mon, 24 Nov 2008, Matthew M. wrote: > Can anyone explain exactly what happens when the interpreter interprets > attr\_reader and its kin? > > E.G. where is the method located (in which module, class) and what does > it do with the symbols in order to expand the method call into an > instance method of the calling class object? attr\_reader and friends are private instance methods of the class Module: > > Module.private\_instance\_methods(false).grep(/attr/) > > =\> [“attr\_accessor”, “attr\_writer”, “attr\_reader”, “attr”] which means that as long as self is an instance of Module or one of its subclasses, those methods can be called. The Class class is a subclass of Module, so instances of Class can execute those methods too. That’s what’s happening when you do: class C attr\_accessor :x end As for what it does: see Greg’s Ruby implementation, and also the original C code which is in eval.c (look for the definition of rb\_attr). It basically calls rb\_define\_method, which is a general method for adding methods to classes. I used implementing attr\_\* as a metaprogramming exercise in a Ruby course I taught last week, and it was very instructive. Amazing what you can do in Ruby, without even getting terribly convoluted or opaque. David ---
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [November 24, 2008, 10:03am UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/4 "2008-11-24T10:03:31Z")
David A. Black wrote: > Hi – > > On Mon, 24 Nov 2008, Matthew M. wrote: > > > Can anyone explain exactly what happens when the interpreter interprets > > attr\_reader and its kin? > > > > E.G. where is the method located (in which module, class) and what does > > it do with the symbols in order to expand the method call into an > > instance method of the calling class object? > > attr\_reader and friends are private instance methods of the class > Module: > > > > Module.private\_instance\_methods(false).grep(/attr/) > > > =\> [“attr\_accessor”, “attr\_writer”, “attr\_reader”, “attr”] > > which means that as long as self is an instance of Module or one of > its subclasses, those methods can be called. The Class class is a > subclass of Module, so instances of Class can execute those methods > too. That’s what’s happening when you do: > > class C > attr\_accessor :x > end > > As for what it does: see Greg’s Ruby implementation, and also the > original C code which is in eval.c (look for the definition of > rb\_attr). It basically calls rb\_define\_method, which is a general > method for adding methods to classes. > > I used implementing attr\_\* as a metaprogramming exercise in a Ruby > course I taught last week, and it was very instructive. Amazing what > you can do in Ruby, without even getting terribly convoluted or > opaque. > > David Not that I don’t appreciate the other responses but this is exactly what I was looking for. Thank you for your help everyone. Also, David, can you recommend any reference texts for Ruby that delve into this much detail for other similar concepts. I’m nearly finished with Programming Ruby: Pragmatic Programmers and I feel like I want a bit more. Thanks! ---
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [November 24, 2008, 12:04am UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/5 "2008-11-24T00:04:14Z")
Not an answer to the OP’s question, but… On Sun, Nov 23, 2008 at 5:27 PM, Michael G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > On Sun, Nov 23, 2008 at 4:45 PM, Matthew M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > [http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/241709](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/241709) This implementation: > def new\_attr\_reader cl, sym > str = “def #{sym.to\_s}; @#{sym.to\_s}; end” > cl.class\_eval str > end is more dangerous than it needs to be. Instead, you can write: def new\_attr\_reader(\*args) args.each { |e| define\_method(e) { instance\_variable\_get(“@#{e}”) } } end Which allows for: class A new\_attr\_reader :a, :b, :c def initialize @a = 1 @b = 2 @c = 3 end end a = A.new p [a.a, a.b, a.c] #=\> [1, 2, 3] Of course, substitute instance\_variable\_set for writers. ---
**Author:** ![mansfiem](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@mansfiem](https://www.ruby-forum.com/u/mansfiem)\ **Post date:** [January 19, 2009, 6:12pm UTC](https://www.ruby-forum.com/t/in-depth-attr-reader-writer-accessor/153958/6 "2009-01-19T18:12:41Z")
Hi – On Mon, 24 Nov 2008, Matthew M. wrote: > > > instance method of the calling class object? > > > too. That’s what’s happening when you do: > > > I used implementing attr\_\* as a metaprogramming exercise in a Ruby > > > detail for other similar concepts. I’m nearly finished with Programming > > > Ruby: Pragmatic Programmers and I feel like I want a bit more. Sorry – this got lost in the inbox pipeline. I have an answer for you, though: The Well-Grounded Rubyist (by me), due out in March. See [The Well-Grounded Rubyist](http://www.manning.com/black2). There’s no one-stop-shopping source for all such stuff but I think you’ll find a good bit in there. David – David A. Black / Ruby Power and Light, LLC Ruby/Rails consulting & training: [http://www.rubypal.com](http://www.rubypal.com) Coming in 2009: The Well-Grounded Rubyist ([The Well-Grounded Rubyist](http://manning.com/black2)) [http://www.wishsight.com](http://www.wishsight.com) =\> Independent, social wishlist management!