classes-and-modules.md

Classes and Modules

Classes

Class Definition

• singular nouns

• capital camel case ShoppingCart

• each class defined in separate file

• group classes in dir

• can be redefined or modified

  • add new changes without whenever

• Attributes = State

• Methods = behavior

add attributes by defining them as custom methods in def

class ShoppingCart
  
  # methods part of class template
  def quantity
    # ...
  end
  
  def total
    # ...
  end
  
end

Instances

string = "pasta"  # "pasta"
string = String.new  # ""
array = Array.new  # []
hash = Hash.new  # {}

1. first create method in class
2. create new instance
3. then call method

class Person
  
  def say_hello
    'Hello!'
  end
end

person = Person.new  # new instance
puts person.say_hello  # call method in new instance

Attributes

values that persists inside an instance

• attributes are blanks filled in
• properties of an object
• i.e. the car is green
• stored in instance variables

@variable

@= scope indicator

class Animal
  
  def make_noise
    @noise
  end
  
  def set_noise
    @noise = 'Oink!'
  end
end

pig = Animal.new
pig.set_noise  # sets string to inst variable
puts pig.make_noise
# Oink!

• the value persists even tho it has changed to a different method
• local variable can’t do that
• the variable persists throughout the instance
• an instance of a class always has access to its instance variables
• ↑ can pull up a value of an instance variable from anywhere inside the class

Reader/writer methods

• aka getter/setter methods
• allow reading/writing to an attribute

Instance Variables

• instantiation always has access to its instance variables
• instance variable inaccessible outside an instance
• methods accessible from outside instance
• use methods to access instance variables

class Animal
  
  def get_noise  # getting back the value of noise
    @noise
  end
  
  def set_noise(value)
    @noise = value
  end

end

# ruby convention:

class Animal

  def noise  # getting back the value of noise
    @noise
  end

  def noise=(value)  # syntactic sugar, just assign a value to noise and have it work
    @noise = value
  end

end

# calling noise the method returns the instance variable @noise

pig = Animal.new
pig.noise = "Oink!"

• feels like asking for the value of the variable, but we are actually calling a method

Attribute Methods

attr_* methods

• attr_reader
• attr_writer
• attr_accessor

attr_reader :name

# same as adding the definition to the class
def name
  @name
end


attr_writer :name

# same as setting a value
def name=(value)
  @name = value
end


# reader + writer both in one line
attr_accessor :name

# same as both combined above
def name
  @name
end

def name=(value)
  @name = value
end

class Animal
  attr_accessor :noise
end

pig = Animal.new
pig.noise = 'Oink!' # write to the instance variable
puts pig.noise  # read back from instance variable
# Oink!

Instances calling their own methods

Helps ruby understand to call the method

• use self to reference the current instance from the code inside the instance
• add self when calling writer methods self.first_name=
• omit self when calling other methods first_name
• safest to always include self

Method Access Control

• methods = primary interfaces to class
  • used to access instance variables
• only expose when necessary so we have control over them
• Access control restricts access to methods from outside an instance
  • decides which methods are callable and which aren’t

3 levels of control:

• public anyone can access (default)
• protected can only be called by instances of the class and its subclasses
• private can only be called by instances of a class (only from within the code definition from the class itself, not from dot method)
  • cannot be called by subclasses

class Newsletter
  # public methods by default
  def add_subscriptions # can still access private methods below
    subscribe_to_product_updates
    subscribe_to_press_releases
  end
  
private
  # use keyword to define private or protected methods
  def subscribe_to_product_updates
  end
  
  def subscribe_to_press_releases
  end
  
end

class SomeClass
  
  def method1
  end
  
protected
  def method2
  end
  
private
  def method3
  end
  
end

Initialize Method

• called automatically once a new instance of a class is created
  • ShoppingCart.new
• default or specific state
  • specific state accepts arguments to the method that gets passed on to initialize method

class Animal
  
  attr_accessor :noise
  
  def initialize # this is the code to run whenever a new instance is created
    @noise = 'Oink!'
    puts "New animal instantiated"
  end

end

pig = Animal.new
# New animal instantiated
puts pig.noise # default value is Oink!
# Oink!

class Animal
  
  attr_accessor :noise

  def initialize(options={})
    @noise = options[:noise] || 'Grrr!'
  end

end

pig = Animal.new({noise: 'Oink!'}) # curly brackets optional
puts pig.noise
# Oink!