Add method to an instanced object

Question

obj = SomeObject.new

def obj.new_method
  "do some things"
end

puts obj.new_method
> "do some things"

This works ok. However, I need to do same thing inside an existing method:

def some_random_method
  def obj.new_method
    "do some things"
  end
end

Works ok as well, but having a method inside a method looks pretty horrible. The question is, is there any alternate way of adding such a method?

Are you sure the second snippet of code works? Because I suppose it should be accepting `obj` as a method argument first to define a singleton method over it. — Chirantan, Dec 11 '09 at 12:57
Just scroll down a bit. SO's algorithm sometimes puts newer answers at the top. — vise, Oct 24 '19 at 09:54

score 94 · Accepted Answer · edited Oct 31 '21 at 11:40

94

In ruby 1.9+, there's a better way of doing this using define_singleton_method, as follows:

obj = SomeObject.new

obj.define_singleton_method(:new_method) do
  "do some things"
end

edited Oct 31 '21 at 11:40

Jim U

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answered Oct 11 '12 at 14:25

vise

12,713
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2

What will happen if I use `@field` inside of this `do` block? Will I access a field from current class, or will it use field of `obj`? – ciembor Jan 31 '13 at 10:30
1

Haven't really tested this properly, but as far as I can tell the mixin and this approach are not equivalent. This has access to outside values, whereas a mixin does not. – vise Jan 31 '13 at 13:10

score 57 · Answer 2 · answered Dec 11 '09 at 12:48

57

Use a Mixin.

module AdditionalMethods
  def new_method
    "do some things"
  end
end

obj = SomeObject.new
obj.extend(AdditionalMethods)

puts obj.new_method
> "do some things"

answered Dec 11 '09 at 12:48

Rinzi

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David Rissato Cruz · Answer 3 · 2021-03-17T06:19:56.637

There are several ways to achieve this, and they are all related to the singleton class:

You can use class << idiom to open the singleton class definition:

 obj = Object.new
 class << obj
   def my_new_method
      ...
   end
 end

Or you can use define_singleton_method on the obj:

 obj = Object.new
 obj.define_singleton_method(:my_new_method) do
      ...
 end

You can also use define_method from the singleton class:

 obj = Object.new
 obj.singleton_class.define_method(:my_new_method) do
      ...
 end

Or you can use def directly:

 obj = Object.new
 def obj.my_new_method
      ...
 end

Pay attention to example 3, I think the concept of a singleton class becomes clearer on that one. There is a difference between these two examples:

    a = Object.new
    b = Object.new
    
    # -- defining a new method in the object's "class" --
    a.class.define_method(:abc) do
      puts "hello abc"
    end
    
    a.abc # prints "hello abc"
    b.abc # also prints "hello abc"

    # -- defining a new method in the object's "singleton class" --
    a.singleton_class.define_method(:bcd) do
      puts "hello bcd"
    end
    
    a.bcd # prints "hello bcd"
    b.bcd # error undefined method

This is because every object has its own singleton class:

    a = Object.new
    b = Object.new

    p a.class # prints "Object"
    p a.singleton_class # prints "#<Class:#<Object:0x000055ebc0b84438>>"

    p b.class # also prints "Object"
    p b.singleton_class # prints "#<Class:#<Object:0x000055ebc0b84410>>" (a different reference address)

Hi, there is spelling error. The define_sigleton_method which in 2's code should be define_singleton_method. — fanjieqi, Mar 17 '21 at 02:36

score 8 · Answer 4 · answered Dec 11 '09 at 15:27

Just an interesting point to note:

if you had instead gone:

def my_method
    def my_other_method; end
end

Then my_other_method would actually be defined on the CLASS of the object not withstanding that the receiver ofmy_method is an instance.

However if you go (as you did):

def my_method
    def self.my_other_method; end
end

Then my_other_method is defined on the eigenclass of the instance.

Not directly relevant to your question but kind of interesting nonetheless ;)

score 6 · Answer 5 · answered Dec 11 '09 at 12:50

You can use modules.

module ObjSingletonMethods
  def new_method
    "do some things"
  end
end


obj.extend ObjSingletonMethods

puts obj.new_method # => do some things

Now if you need to add more methods to that object, you just need to implement the methods in the module and you are done.

score 2 · Answer 6 · answered Jul 14 '18 at 04:37

2

Use instance_eval:

obj = SomeObject.new

obj.instance_eval do
  def new_method
    puts 'do something new'
  end
end

obj.new_method 
> "do something new"

answered Jul 14 '18 at 04:37

EJAg

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kajamite · Answer 7 · 2018-12-03T06:37:51.467

class Some
end 

obj = Some.new

class << obj
  def hello 
    puts 'hello'
  end 
end 

obj.hello

obj2 = Some.new
obj2.hello # error

Syntax class << obj means that we are opening definition of the class for an object. As you probably know we can define Ruby class methods using syntax like this:

class Math

    class << self 

        def cos(x)
            ...
        end 

        def sin(x)
            ...
        end 
    end 
end

Then we can use those methods like this:

Math.cos(1)

In Ruby, everything is an object - even classes. self here is an object of Math class itself (you can access that object with Math.class). So syntax class << self means we are opening class for Math class object. Yes, it means that Math class has class too (Math.class.class).

score 0 · Answer 8 · answered Oct 24 '19 at 22:23

0

Another way to use a Mixin

obj = SomeObject.new
class << obj
  include AnotherModule
end

This includes all of the methods from AnotherModule into the current object.

answered Oct 24 '19 at 22:23

HippoMan

2,119
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48

Add method to an instanced object

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