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# What is the difference between class_eval and instance_eval ?

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class_eval:

Context: class_eval evaluates a string or block within the context of a class.
Purpose: It allows you to reopen the class and define additional behavior on it.
Method Definition: When you create a method inside the class_eval block, it defines an instance method on the class.
Access to Class Variables: You can access class variables and methods.
Example:

class Greeting
  def greet
    puts "Hello!"
  end
end

Greeting.class_eval do
  def farewell
    puts "Goodbye!"
  end
end

greeting = Greeting.new
greeting.farewell # Calls the method defined by class_eval
Benefits: 
  1. You can call class_eval on a variable pointing to a class, allowing dynamic addition of behavior.
  2. It fails immediately if the class doesn’t exist or if there’s a misspelled class name.

instance_eval:

Context: instance_eval evaluates a string or block within the context of the receiving object.
Purpose: It allows you to run code as if you were inside a method of the object, giving access to its private methods and instance variables.
Method Definition: When you create a method inside the instance_eval block, it defines a class method associated with the class object (but not visible to instances of that class).
Access to Instance Variables: You can access instance variables directly.

Example:

class Klass
  def initialize(name)
    @name = name
  end

  private def classified
    puts "private stuff"
  end
end

kl = Klass.new("Klein")
kl.instance_eval { @name } # Accesses the @name instance variable
Note: Methods defined via instance_eval are only accessible by the specific object it’s called on.
In summary:
  • class_eval modifies the class itself, while instance_eval operates within the context of an object.
  • Use class_eval for class-level changes and instance_eval for object-specific modifications. 🚀