Dynamically Defining Methods with define_method

If you have been hacking ruby for a while you should have heard that “Everything is an object”. To understand this concept knowing what a singleton class is important. Yehuda Katz’s post is a good source for it.

My ruby version 1.9.2.

	$ ruby -v
	ruby 1.9.2p136 (2010-12-25 revision 30365) [i686-linux]

view raw ruby_version.bash hosted with ❤ by GitHub

First I’m going to show you how to add methods to instances by using define_method.

	class Message
	[:hello, :goodbye].each do \|method_name\|
	define_method method_name do \|arg\|
	"#{method_name} #{arg}"
	end
	end
	end

	#irb
	Message.instance_methods false #=> [:hello, :goodbye]
	Message.new.hello 'emre' #=> "hello emre"
	Message.new.goodbye 'emre' #=> "goodbye emre"

view raw add_instance_methods.rb hosted with ❤ by GitHub

You can also write an instance method to create instance methods. send must be used because define_method is private.

	class Message
	def create_instance_methods *methods
	methods.each do \|method_name\|
	self.class.send :define_method, method_name do \|arg\|
	"#{method_name} #{arg}"
	end
	end
	end
	end

	#irb
	Message.instance_methods false #=> [:create_instance_methods]
	Message.new.create_instance_methods 'fork' #=> ["fork"]
	Message.instance_methods false #=> [:create_instance_methods, :fork]
	Message.new.fork 'emre' #=> "fork emre"

view raw add_instance_methods_by_instance_method.rb hosted with ❤ by GitHub

Write a singleton method to create instance methods.

	class Message
	def self.create_instance_methods *methods
	methods.each do \|method_name\|
	define_method method_name do \|arg\|
	"#{method_name} #{arg}"
	end
	end
	end
	end

	#irb
	Message.singleton_methods #=> [:create_instance_methods]
	Message.create_instance_methods 'follow' #=> ["follow"]
	Message.instance_methods false #=> [:follow]
	Message.new.follow 'emre' #=> "follow emre"

view raw add_instance_methods_by_singleton_method.rb hosted with ❤ by GitHub

Now it is time to add singleton methods. When we call define_method, it creates an instance method. What we have to do is call define_method on class’ singleton class.

Getting singleton class of a class:

	#ruby 1.9.2
	class Message
	end

	#irb
	Message.singleton_class #=> #<Class:Message>

	#ruby 1.8.x
	class Message
	#define this method to get singleton class
	def self.singleton_class
	class << self
	self
	end
	end
	end

	#irb
	Message.singleton_class #=> #<Class:Message>

view raw get_singleton_class.rb hosted with ❤ by GitHub

First defining singleton method to create singleton methods.

	class Message
	def self.create_singleton_methods *methods
	methods.each do \|method_name\|
	singleton_class.send :define_method, method_name do \|arg\|
	"#{method_name} #{arg}"
	end
	end
	end
	end

	#irb
	Message.singleton_methods false #=> [:create_singleton_methods]
	Message.create_singleton_methods 'like' #=> ["like"]
	Message.singleton_methods false #=> [:create_singleton_methods, :like]
	Message.like 'emre' #=> "like emre"

view raw add_singleton_methods_by_singleton_method.rb hosted with ❤ by GitHub

Then defining instance method to create singleton methods.

	class Message
	def create_singleton_methods *methods
	methods.each do \|method_name\|
	self.class.singleton_class.send :define_method, method_name do \|arg\|
	"#{method_name} #{arg}"
	end
	end
	end
	end

	#irb
	Message.instance_methods false #=> [:create_singleton_methods]
	Message.new.create_singleton_methods 'poke' #=> ["poke"]
	Message.singleton_methods false #=> [:poke]
	Message.poke 'emre' #=> "poke emre"

view raw add_singleton_methods_by_instance_method.rb hosted with ❤ by GitHub

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Dynamically Defining Methods with define_method

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