Did You Really Understand Super?

What You Will Learn

Revisit

• super
• Message lookup
• Class methods

A Little Puzzle

Dice class >> new

  | inst |
  inst := super new. 
  inst initialize.
  ^ inst

We execute the following expression: Dice new

Questions

Dice class >> new

  | inst |
  inst := super new. 
  inst initialize.
  ^ inst

• What is inst?
• What is super?
• What is super new?

Hint: super is Not...

Dice class >> new

  | inst |
  inst := super new. 
  inst initialize.
  ^ inst

• No super is not the superclass
• No inst is not an instance of the superclass

Hint 2: super is the Message Receiver

Dice class >> new

  | inst |
  inst := super new. 
  inst initialize.
  ^ inst

• The message is Dice new
• So the receiver is the class Dice

Sending a Message: Lookup + Apply on Receiver

Remember: Method Lookup

Solution

Dice class >> new

  | inst |
  inst := super new. 
  inst initialize.
  ^ inst

• super is the receiver: the class Dice
• Look for new in the superclass of the class Dice class (Pay attention not Dice)
• Once found we apply to the receiver: Dice
• We get an instance of the class Dice and send it initialize and return it

Solution

Summary

• Sending a message is looking up for the method and applying it on the receiver
• Now you should really understand super :)
• super is the receiver of the message and the method lookup starts in the superclass of the class containing the expression

Challenge?

Challenge!

Imagine we have

A >> foo
   ^ super class == self class 

What is the result of A new foo and why?