Ruby Method Search (Comparison with JavaScript)

Question

I want to better understand how objects in Ruby have access methods defined in classes and modules. In particular, I want to compare and contrast it with JavaScript (with which I am more familiar).

In JavaScript, objects look for methods for the object itself, and if it cannot find it there, it will look for a method on the prototype object. This process will continue until achieved Object.prototype.

// JavaScript Example
var parent = {
  someMethod: function () {
    console.log( 'Inside Parent' );
  }
};

var child = Object.create( parent );
child.someMethod = function () {
  console.log( 'Inside Child' );
};

var obj1 = Object.create( child );
var obj2 = Object.create( child );

obj1.someMethod(); // 'Inside Child'
obj2.someMethod(); // 'Inside Child'

In the JavaScript example obj1, both obj2do not have functions someMethodfor the object itself. The key is to note that:

• There's a copy of the function someMethodin the object childand the two obj1, and obj2is delegated object child.
• , obj, obj2 someMethod .
• child someMethod, parent.

Ruby:

# Ruby Example
class Parent
  def some_method
    put 'Inside Parent'
  end
end

class Child < Parent
  def some_method
    puts 'Inside Child'
  end
end

obj1 = Child.new
obj2 = Child.new

obj1.some_method  # 'Inside Child'
obj2.some_method  # 'Inside Child'

:

• obj1 obj2 Ruby some_method? JavaScript, some_method ( Child)?
• , Ruby, Ruby ?

, Ruby , , . , Ruby JavaScript, , , , , , BasicObject.

Answer 1

• obj1 obj2 Ruby some_method? JavaScript, some_method ( Child)?

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ruby -e 'p ObjectSpace.each_object(String).count'
# => 10013

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, , Ruby, . , ( -Ruby):

struct Object
  object_id: Id
  ivars: Dictionary<Symbol, *Object>
  class: *Class
end

, :

struct Module
  methods: Dictionary<Symbol, *Method>
  constants: Dictionary<Symbol, *Object>
  cvars: Dictionary<Symbol, *Object>
end

, :

struct Class
  methods: Dictionary<Symbol, *Method>
  constants: Dictionary<Symbol, *Object>
  cvars: Dictionary<Symbol, *Object>
  superclass: *Class
end

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, : ! . , , singleton :

struct Object
  object_id: Id
  ivars: Dictionary<Symbol, *Object>
  class: *Class
  singleton_class: Class
end

, .

? , , :

struct Module
  methods: Dictionary<Symbol, *Method>
  constants: Dictionary<Symbol, *Object>
  cvars: Dictionary<Symbol, *Object>
  mixins: List<*Module>
end

struct Class
  methods: Dictionary<Symbol, *Method>
  constants: Dictionary<Symbol, *Object>
  cvars: Dictionary<Symbol, *Object>
  superclass: *Class
  mixins: List<*Module>
end

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struct Object
  object_id: Id
  ivars: Dictionary<Symbol, *Object>
  class: *Class
  singleton_class: Class
end

, . . , , , :

struct Class
  methods: *Dictionary<Symbol, *Method>
  constants: *Dictionary<Symbol, *Object>
  cvars: *Dictionary<Symbol, *Object>
  superclass: *Class
  visible?: Bool
end

, singleton . mixin M C, Ruby M′, , cvar- mixin. mixin C, C mixin:

M′ = Class.new(
  methods = M->methods
  constants = M->constants
  cvars = M->cvars
  superclass = C->superclass
  visible? = false
)

C->superclass = *M'

, , , M ( ), , .

:

def lookup(meth, obj)
  c = obj->class

  until res = c->methods[meth]
    c = c->superclass
    raise MethodNotFound, meth if c.nil?
  end

  res
end

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Answer 2

IRB , :

> obj1.method(:some_method)
=> #<Method: Child#some_method>
> obj1.method(:some_method).source_location
=> ["(irb)", 8]
> obj2.method(:some_method)
=> #<Method: Child#some_method>
> obj2.method(:some_method).source_location
=> ["(irb)", 8]

Ah ok, . , ...

> obj1.instance_eval do
>   def some_method
>     puts 'what is going on here?'
>   end
> end
=> nil
> obj1.some_method
what is going on here?
=> nil
> obj2.some_method
Inside Child
=> nil
> obj1.method(:some_method)
=> #<Method: #<Child:0x2b9c128>.some_method>
> obj1.method(:some_method).source_location
=> ["(irb)", 19]

, .

, , https://blog.jcoglan.com/2013/05/08/how-ruby-method-dispatch-works/

, . , - MRI, JRuby Rubinius, , Ruby .

Answer 3

More food for thought

> obj1.instance_eval do
>  def some_method
>    puts "Inside Instance"
>    super
>  end
> end 
=> :some_method
Inside Instance
Inside Child