# Meta-programming **URL:** \ **Category:** Ruby\ **Created:** [December 15, 2009, 9:43am UTC](https://www.ruby-forum.com/t/meta-programming/180230 "2009-12-15T09:43:26Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 9:43am UTC](https://www.ruby-forum.com/t/meta-programming/180230/1 "2009-12-15T09:43:26Z")
I am just starting to expand my Ruby knowledge into the area of meta-programming. I want to be able to create a class dynamically. Lets call it, class Person, and then I want to add methods to it dynamically. From a static point I have managed to compile the following example: class Person def self.extend\_me class\_eval “def greet; puts ‘hello’; end” instance\_eval “def name; puts ‘Person’; end” end end Person.extend\_me Person.respond\_to? :greet Person.respond\_to? :name puts Person.name p = Person.new puts p.greet How would I declared a, ‘class Person’ dynamically and then add methods and attributes to it? Can someone point me to good documentation on this or show me some simple code example? – Kind Regards, Rajinder Y. [http://DevMentor.org](http://DevMentor.org) Do Good! - Share Freely ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 11:06am UTC](https://www.ruby-forum.com/t/meta-programming/180230/2 "2009-12-15T11:06:53Z")
Rajinder Y. wrote: > How would I declared a, ‘class Person’ dynamically Option 1: since you’re using string eval already, you could just do the same. eval “class Person; end” Option 2: Person = Class.new # superclass is Object Person = Class.new(Mammal) # superclass is Mammal I find this isn’t often done in practice though. A program which doesn’t know in advance which classes it has can be a bit too dynamic 🙂 You would probably register your classes somewhere to be able to find them. In a Hash is one option; under a Module is another, so you can use MyModule.constants to find them all. You can make your classes anonymous if you don’t bind them to a constant: my\_klass = Class.new > and then add methods and attributes to it? As you’ve done before, using string eval, is one way. Another way: ``` Person.class_eval { define_method(:greet) { puts "Hello" } } ``` This means that the method is a closure, and can access variables outside (unlike ‘def’ which starts a fresh scope), and this can be useful sometimes. Another way: put the method(s) of interest in module(s), then include the relevant ones. ``` Person.class_eval { include Greeter } ``` ActiveRecord is a plentiful source of examples. ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 11:50am UTC](https://www.ruby-forum.com/t/meta-programming/180230/3 "2009-12-15T11:50:28Z")
On Tue, Dec 15, 2009 at 9:32 AM, Rajinder Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > class\_eval “def greet; puts ‘hello’; end” > p = Person.new > puts p.greet > > How would I declared a, ‘class Person’ dynamically and then add methods and > attributes to it? Can someone point me to good documentation on this or show > me some simple code example? If you want to define the class dynamically, take a look at Class.new. This creates an anonymous class that you can assign to a constant directly or with const\_set. To define methods I would use define\_method. Adding attributes is done adding methods that set and get the attributes: irb(main):042:0\> C = Class.new do irb(main):043:1\* def self.add\_method(name, &blk) irb(main):044:2\> define\_method(name, &blk) irb(main):045:2\> end irb(main):046:1\> end =\> C irb(main):051:0\> C.add\_method(:name) {@name} =\> #Proc:0xb7cb9de8@:51(irb) irb(main):052:0\> C.add\_method(:name=) {|value| @name = value} =\> #Proc:0xb7cb3bf0@:52(irb) irb(main):053:0\> c = C.new =\> #\ irb(main):054:0\> c.name=3 =\> 3 irb(main):055:0\> c.name =\> 3 Hope this helps, Jesus. ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 12:23pm UTC](https://www.ruby-forum.com/t/meta-programming/180230/4 "2009-12-15T12:23:14Z")
Brian C. wrote: > Person = Class.new # superclass is Object > > ActiveRecord is a plentiful source of examples. Hi Brian, thanks for the excellent examples and explanation =), I can’t stop smiling about how beautiful and elegantly simple Ruby make things. – Kind Regards, Rajinder Y. [http://DevMentor.org](http://DevMentor.org) Do Good! - Share Freely ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 12:35pm UTC](https://www.ruby-forum.com/t/meta-programming/180230/5 "2009-12-15T12:35:24Z")
Jesús Gabriel y Galán wrote: > irb(main):051:0\> C.add\_method(:name) {@name} > =\> #Proc:0xb7cb9de8@:51(irb) > irb(main):052:0\> C.add\_method(:name=) {|value| @name = value} > =\> #Proc:0xb7cb3bf0@:52(irb) > irb(main):053:0\> c = C.new > =\> #\ > irb(main):054:0\> c.name=3 > =\> 3 > irb(main):055:0\> c.name > =\> 3 Of course, since ‘name’ is static here, you could just include a module. It gets more fun when the names of the accessors themselves are dynamic. At this point it really does make more sense to use string eval, rather than the alternative (instance\_variable\_get and instance\_variable\_set). Have a look at define\_method\_attribute= in the following: > Also see define\_read\_method in attribute\_methods/read.rb, and method\_missing in base.rb (this defines finder methods dynamically) ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 12:56pm UTC](https://www.ruby-forum.com/t/meta-programming/180230/6 "2009-12-15T12:56:35Z")
Brian C. wrote: > > =\> 3 > > Of course, since ‘name’ is static here, you could just include a module. > > It gets more fun when the names of the accessors themselves are dynamic. > At this point it really does make more sense to use string eval, rather > than the alternative (instance\_variable\_get and instance\_variable\_set). > > Have a look at define\_method\_attribute= in the following: > [rails/activerecord/lib/active\_record/attribute\_methods/write.rb at main · rails/rails · GitHub](http://github.com/rails/rails/blob/master/activerecord/lib/active_record/attribute_methods/write.rb) Thanks for pointing out the path to ActiveRecord, I haven’t gotten use to browsing source code, this is really helpful! > Also see define\_read\_method in attribute\_methods/read.rb, and > method\_missing in base.rb (this defines finder methods dynamically) cool I will do that =) – Kind Regards, Rajinder Y. [http://DevMentor.org](http://DevMentor.org) Do Good! - Share Freely ---
**Author:** ![Rajinder\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/91b2a8/32.png) [@Rajinder\_Y](https://www.ruby-forum.com/u/Rajinder_Y)\ **Post date:** [December 15, 2009, 12:16pm UTC](https://www.ruby-forum.com/t/meta-programming/180230/7 "2009-12-15T12:16:48Z")
Jesús Gabriel y Galán wrote: > > def self.extend\_me > > puts Person.name > > directly or with const\_set. > > irb(main):051:0\> C.add\_method(:name) {@name} > > Hope this helps, > > Jesus. Hi Jesus, yes it does help and you answered the next question I was about to ask about adding variables after reading Brian’s reply. – Kind Regards, Rajinder Y. [http://DevMentor.org](http://DevMentor.org) Do Good! - Share Freely