def vs. define_method

Ever wondered about the difference between def and define_method? Turns out there are three implicit contexts Show archive.org snapshot in Ruby. def and define_method differ in which one they use.

def

• Ruby keyword, starts a method definition
• Opens a new, isolated scope. Variables defined outside are not accessible inside and vice versa.
• Defines an instance method on the receiver (specified before the method name, e.g. def object.foo); implicit receiver is the default definee

The default definee is not self and cannot be changed or passed around, but is determined syntactically.

Copy$string = "hello world"
class Foo  
  def $string.baz # define a singleton method on $string / an instance method on $string's singleton class
    def bar; end
  end  
end  
Foo.instance_methods(false) # => ["bar"]
$string.methods(false) # => ["baz"]

define_method

• A method defined in Module
• Defines an instance method on the receiver; implicit receiver is self
• Takes a block as method body, which is evaluated using instance_eval. Since blocks carry with them the binding in which they were created, variables from outside are accessible in define_method.

self is the "current object" and implicit receiver of method calls:

• in a method body, self is the receiver of the method
• in a class or module definition, self is the class or module object being defined
• in blocks, self is bound to the class when class_eval'd and bound to the object when instance_eval'd