Reopening classes

class Fixnum
  def divisible_by? n
    self % n == 0
  end
end

4.divisible_by? 2 #=> true
4.divisible_by? 3 #=> false

Duck Typing

• If it looks like a duck...
• and it quacks like a duck...
• then it is a duck!
• In other words
  • we don't care what an object is;
  • we care what it does

Modules and Mixins

• Any class (or object) can include a Module
  • also known as mixing in
  • so Modules are also called mixins
• Modules provide behavior but no state
  • more precisely, they affect the extending object's state
• Provides most of the power of multiple inheritance, but less confusing

Array Assignment

@width, @height = width, height
@width, @height = [width, height]

def dimensions
  [10, 20]
end
@width, @height = dimensions

Metaprogramming

• macros (class methods)
  • attr_accessor
  • has_many

Classes are objects

• class methods are really just methods on the class object
• code evaluated in the scope of a class definition acts on the class object

Domain-Specific Languages (DSLs)

• Rails
• Rake
• Cucumber
• Rspec
• etc.

No Function Overloading

• Some OO languages allow several methods with the same name but different parameters...
• ...but not Ruby!
• If you redefine a method, the new one replaces the old one
• There are tricks to save the old method if you need to call it
  • alias, alias_method, alias_method_chain

Operators are Methods

Equivalent:

1 + 2
1.+(2)
1.send "+", 2

send the object 1 the message + with the parameter 2

Operator Overriding

• operators like +, *, <<, etc. are defined as methods
• so they can be overridden like any method
• String has some great ones

"abc" * 3           #=> "abcabcabc"
"abc" << "def"      #=> "abcdef"
"%d live crew" % 2  #=> "2 live crew"