Class_eval vs (class << object)

Ruby
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 F. 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)

3

Ruby F. 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

4

Hi –

On Wed, 26 Mar 2008, Ruby F. 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 << 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