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Saturday, August 21, 2010

Defining Methods in Ruby using #instance_eval

Last week I discussed the differences between class_eval and instance_eval.

I stated there were two basic rules to follow when choosing which method to use:

  1. When the object is a class use class_eval, you will typically be using the def keyword
  2. When the object is an instance use instance_eval

When we use instance_eval to define methods where are the located? The are stored in the instance’s singleton class. I will write more about Ruby’s singleton class in the future, but for now you may want to read Ola Bini’s excellent post on the subject.

Let’s look at an example:

class MyClass; end

my_instance = MyClass.new

my_instance.instance_eval do
  # add the method to the instance's singleton class
  def foo
    "bar" 
  end
end

puts my_instance.foo # => "bar" 

another_instance = MyClass.new

begin
  # since the method exists only on the my_instance singleton class, calling
  # it on another_instance will raise a NoMethodError exception

  puts another_instance.greeting
rescue NoMethodError
  puts "The foo method does not exist" 
end

What happens when we call instance_eval on Class?
MyClass.instance_eval do
  def baz
    "qux" 
  end
end

puts MyClass.baz # => "qux" 

As you can see it defines a class method. I would argue that you should never use instance_eval on a class (although my last post regarding the subject suggested you could). I feel for a beginner in Ruby it may confuse. Stick to class_eval for class objects, and instance_eval for instances. If you need to dynamically define a class method then use this technique:

MyClass.class_eval do

  class << self

    # define .quux as a class method
    #    
    def quux
      "corge" 
    end
  end

  def grault
    "garply" 
  end
end

puts MyClass.quux # => "corge" 
puts my_instance.grault # => "garply" 

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