Class_eval vs (class << object)

ruby-talk
1

What is the difference between class_eval and "class << object"?

It seems like they are the same. I am thinking that class_eval affects
the greater class and "class_object" is used on a singleton class. Are
they mutually exclusive?

Thanks

2

Ruby Freak wrote:

What is the difference between class_eval and "class << object"?

It seems like they are the same. I am thinking that class_eval affects
the greater class and "class_object" is used on a singleton class. Are
they mutually exclusive?

Thanks

class Dog
  attr_accessor :age

  def initialize(a)
    @age = a
  end
end

Dog.class_eval{
  def show
    puts @age
  end
}

d1 = Dog.new(5)
d2 = Dog.new(2)

d1.show #5
d2.show #2

class Dog
  attr_accessor :age

  def initialize(a)
    @age = a
  end
end

d1 = Dog.new(5)

class << d1
  def show
    puts age
  end
end

d1.show #5

d2 = Dog.new(2)
d2.show

--output:--
undefined method `show' for #<Dog:0x25260 @age=2> (NoMethodError)

···

--
Posted via http://www.ruby-forum.com/\.

3

Ruby Freak wrote:

What is the difference between class_eval and "class << object"?

It seems like they are the same. I am thinking that class_eval affects
the greater class and "class_object" is used on a singleton class.

Are
they mutually exclusive?

class Dog
  attr_accessor :age

  def initialize(a)
    @age = a
  end
end

class << Dog
  def greet
    puts "hello"
  end
end

Dog.class_eval{
  def show
    puts @age
  end
}

Dog.greet #hello
d = Dog.new(5)
d.show #5

···

--
Posted via http://www.ruby-forum.com/\.

4

Hi --

What is the difference between class_eval and "class << object"?

It seems like they are the same. I am thinking that class_eval affects
the greater class and "class_object" is used on a singleton class. Are
they mutually exclusive?

class << object is the class keyword, which starts a class definition
block (in this case, on the singleton class of object) and therefore a
new local scope.

class_eval is a method, not a keyword, so it requires a receiver --
namely, a Module or Class object. It takes a block, and the block does
shares the variables of the existing local scope.

David

···

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