Object Model

Question 1

Where is the method table stored?

Answer 1

The class representation, namely the class object.

Question 2

Why do we need a class tun-time representation?

Answer 2

1. dynamic binding
2. Run-time type info
3. Reflection

Question 3

What is reflection?

Answer 3

The ability of the program to examine and possibly modify itself

Question 4

what is serialization?

Answer 4

Serialization is the process of turning an object in memory into a stream of bytes so you can do stuff like store it on disk or send it over the network.

Question 5

What is a meta-class?

Answer 5

a class whose instances are classes

Question 6

What’s a terminal instance?

Answer 6

cannot be instantiated further

Question 7

What is a 1-level system?

Answer 7

Object only.
object describe themselves, no classes: objects can be instantiated from other objects and inherited from them.
an object is essentially a map from names (strings) to values or methods.

Example: Javascript, Self

Question 8

What is prototype inheritance?

Answer 8

קישור בזמן-ריצה במקום
subclassing
בזמן הידור

Question 9

1-level pros and cons

Answer 9

Pros:
- Extremely flexible
- As Object Oriented as it gets… (literally)
Cons:
- Performance penalty on member lookup (need to go to all the “parents” and look there)
- No compile time validations and optimizations (everything is done dynamically)
- Not very common

Question 10

What is the 2-level system?

Answer 10

• Objects are instances of classes
• Classes are compile time creatures
  (No runtime representation)
• Minimal runtime overhead
  Zero overhead (“supermarket”) principle
  But object types cannot always be determined
  On some cases, run-time type information is available
• No class (static) members

Question 11

What is the 3-level system?

Answer 11

• Objects are instances of classes
• Classes are also objects
• All classes are instances of one metaclass

Question 12

Can a user-defined class affect the Class object or the meta-class in a 3-level system?

Answer 12

A user-defined class can only affect class objects state (for example, diffrent method dictionary for every user-class).

Question 13

object members are defined in the object's class.
in a 3-level system where are class members defined?

Answer 13

They should be defined in the meta-class, but there is only one so it's not possible.
we can emulate class members with static members since the are stored in some global space.

Question 14

What is the 4-level system?

Answer 14

Each object is an instance of a class
Each class is an instance of a metaclass
Each metaclass is an instance of a meta-metaclass
One meta-metaclass (instance of itself and inherits from Object or Class)

Question 15

What is the 5-level system?

Answer 15

Same as 4-level, except meta-metaclass (we'll call Metaclass) is and instance of metaclass class
and metaclass class is instance of Metaclass.

Question 16

Draw the squeak object model. (and check yourself!)