Inheritance and Lookup

3: super

Goal

• Sending a message
• Method lookup
• super semantics and the differences with self

What is super?

Take 5 min and write the definition of super

• your definition should have two points:
  • what does super represent?
  • how is a method looked up when a message is sent to super?

Challenge Yourself With super!

    A new bar
    > ...
    B new bar
    > ...
    C new bar
    > ...

Challenge Yourself With super!

    A new bar
    > 10
    B new bar
    > 20
    C new bar
    > 100

super Changes Where the Lookup Starts

Evaluation of aC bar

1. aC's class is C
2. no method bar in C
3. look up in B - bar is found
4. method bar is executed
5. bar is sent to super
6. super is aC but lookup starts in A
7. bar is found in A and executed
8. foo is sent to aC
9. foo is found in C

super Changes Where the Lookup Starts

• super refers to the receiver of the message (just like self)
• The method lookup starts in the superclass of the class containing the super expression

self is Dynamic

We don't know which foo method bar refers to

super is Static

• at compilation-time, we know that B>>foo refers to A>>foo
• we should look above the class containing the method using super

Even Some Books Got it Wrong

• Wrong definition: super looks for the method in the superclass of the receiver's class
• With this definition, this example would loop forever:

In reality it does not loop, the definition is wrong

What You Should Know

• self always represents the receiver
• super always represents the receiver
• super changes the lookup:
  • a super send starts the lookup in the class above it
• self sends act as a hook: code of subclasses may be invoked