# Attr\_reader explained **URL:** \ **Category:** ruby-talk\ **Created:** [3 March 2007 17:55 UTC](https://rubytalk.org/t/attr-reader-explained/35834 "2007-03-03T17:55:04Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![libsfan01](https://avatars.discourse-cdn.com/v4/letter/l/779978/32.png) [@libsfan01](https://rubytalk.org/u/libsfan01)\ **Post date:** [3 March 2007 17:55 UTC](https://rubytalk.org/t/attr-reader-explained/35834/1 "2007-03-03T17:55:04Z")
can someone explain how attr\_reader works? i can't find a good explanation anywhere. please help! ---
**Author:** ![Tim\_Becker](https://avatars.discourse-cdn.com/v4/letter/t/5fc32e/32.png) [@Tim\_Becker](https://rubytalk.org/u/Tim_Becker)\ **Post date:** [3 March 2007 18:14 UTC](https://rubytalk.org/t/attr-reader-explained/35834/2 "2007-03-03T18:14:48Z")
> can someone explain how attr\_reader works? > > i can't find a good explanation anywhere. > > please help! It's built in to the language, so it 'works' by magic for all practical purposes, it's implemented in `object.c` if you want to take a look at the implementation. If you wanted to implement it in Ruby, you do something like this: > def new\_attr\_reader cl, sym >   str = "def #{sym.to\_s}; @#{sym.to\_s}; end" >   cl.class\_eval str > end > class Test; end > new\_attr\_reader Test, :my\_new\_reader > Test.new().my\_new\_reader The built in `attr_reader` takes an array of sym's, but you get the idea.    -tim > **···** > > On 3/3/07, libsfan01 \ wrote: ---
**Author:** ![Raj\_Sahae](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/raj_sahae/32/1840_2.png) [@Raj\_Sahae](https://rubytalk.org/u/Raj_Sahae)\ **Post date:** [3 March 2007 18:21 UTC](https://rubytalk.org/t/attr-reader-explained/35834/3 "2007-03-03T18:21:19Z")
libsfan01 wrote: > can someone explain how attr\_reader works? > > i can't find a good explanation anywhere. > > please help! >    attr\_reader, in it's common usage, creates instance variables and defines a method by which you can read them. Class Test     def initialize(num)        @test=num     end     def test        @test     end end If you were to use attr\_reader instead, your class definition would be Class Test     attr\_reader :test     def initialize(num)        @test = num     end end ---
**Author:** ![Stefano\_Crocco](https://avatars.discourse-cdn.com/v4/letter/s/5f9b8f/32.png) [@Stefano\_Crocco](https://rubytalk.org/u/Stefano_Crocco)\ **Post date:** [3 March 2007 19:47 UTC](https://rubytalk.org/t/attr-reader-explained/35834/4 "2007-03-03T19:47:31Z")
Alle sabato 3 marzo 2007, Raj Sahae ha scritto: > attr\_reader, in it's common usage, creates instance variables and > defines a method by which you can read them. Actually, attr\_reader doesn't create the variable, just the accessor method. To see this, do the following in irb: > class C > attr\_reader :var > def var\_defined? > defined?(@var) > end > end =\> nil > c=C.new =\> #\ > c.var\_defined? =\> nil > class C > def initialize > @var=nil > end > end =\> nil > c1=C.new =\> #\ > c1.var\_defined? =\> "instance-variable" As you can see, in the first case (the variable called c) doesn't have the @var instance variable defined. Even after you call the var method defined using attr\_reader, the variable is still not defined. To define it, you need to explicitly assign it to a value (using @var= something inside an instance method of the class or using instance\_variable\_set) Stefano ---
**Author:** ![Austin\_Ziegler5](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/austin_ziegler5/32/1985_2.png) [@Austin\_Ziegler5](https://rubytalk.org/u/Austin_Ziegler5)\ **Post date:** [3 March 2007 23:25 UTC](https://rubytalk.org/t/attr-reader-explained/35834/5 "2007-03-03T23:25:52Z")
> \> can someone explain how attr\_reader works? > \> i can't find a good explanation anywhere. > It's built in to the language, so it 'works' by magic for all > practical purposes, it's implemented in `object.c` if you want to take > a look at the implementation. If you wanted to implement it in Ruby, > you do something like this: Actually, that's not true. It's written in C, but that's not the same as being built into the language. It's a method on Module, so it \*can\* be overridden. > > class Module > >   alias old\_attr\_reader attr\_reader > >   def attr\_reader(\*names) > >     puts "Making attribute readers for #{names.join(", ")}" > >     old\_attr\_reader \*names > >   end > > end =\> nil > > class Foo > >   attr\_reader :bar > >   attr\_reader :baz, :quux > > end Making attribute readers for bar Making attribute readers for baz, quux -austin > **···** > > On 3/3/07, Tim Becker \ wrote: > > > On 3/3/07, libsfan01 \ wrote: > > -- > Austin Ziegler \* halostatue@gmail.com \* [http://www.halostatue.ca/](http://www.halostatue.ca/) >                \* austin@halostatue.ca \* [You are in a maze of twisty little passages, all alike. // halo • statue](http://www.halostatue.ca/feed/) >                \* austin@zieglers.ca ---
**Author:** ![Eric\_Hodel1](https://avatars.discourse-cdn.com/v4/letter/e/a9adbd/32.png) [@Eric\_Hodel1](https://rubytalk.org/u/Eric_Hodel1)\ **Post date:** [5 March 2007 21:28 UTC](https://rubytalk.org/t/attr-reader-explained/35834/6 "2007-03-05T21:28:49Z")
attr\* also cheats, so methods defined by attr\* are slightly faster than methods defined with def: [http://blog.segment7.net/articles/2006/03/06/attr-vs-method-vs-define\_method](http://blog.segment7.net/articles/2006/03/06/attr-vs-method-vs-define_method) > **···** > > On Mar 3, 2007, at 15:25, Austin Ziegler wrote: > > > On 3/3/07, Tim Becker \ wrote: > > > > > On 3/3/07, libsfan01 \ wrote: > > > \> can someone explain how attr\_reader works? > > > \> i can't find a good explanation anywhere. > > > It's built in to the language, so it 'works' by magic for all > > > practical purposes, it's implemented in `object.c` if you want to take > > > a look at the implementation. If you wanted to implement it in Ruby, > > > you do something like this: > > > > Actually, that's not true. It's written in C, but that's not the same > > as being built into the language. It's a method on Module, so it \*can\* > > be overridden.