Software Design Lecture Notes
Fall 2004

For Friday:

1) read Chapter 13 and prepare for a quiz on Chapters 12 and 13


Classes
-------

Big dose of new vocabulary:

class, instance, instantiate, object, constructor, attribute

LEARN IT, SPEAK IT, MAKE IT A PART OF YOU.


A class is a user-defined type.

class Point:
    pass             # later there will be method definitions here

class Rectangle:
    pass


Creating a class defines a function with the same name:

blank = Point()
schmectangle = Rectangle()

These functions are called constructors.

Invoking a constuctor creates an instance of the class.

An instance is a member of a class.

An object is something that can be referred to.

A reference to an object can be assigned to a variable,
passed as an argument or returned as a return value.

In Python:

All instances are objects (can be referred to...)

AND

all objects are instances (belong to some class).


In casual speech, object and instance are used interchangeably.



Attributes
----------

Objects have attributes.

An attribute is like a variable that belongs to an object.

(recall that a variable is a name that refers to a value)

Try this:

from World import *

world = TurtleWorld()
bob = Turtle(world)
print bob.x, bob.y
print bob.color

bob.color = 'blue'
print bob.color

What color is bob?


You can add a new attribute to an object at any time!

bob.new_feature = 'whatever you want'
print bob.new_feature


Stack diagrams with objects
---------------------------

Draw a stack diagram for the following program:

def find_center(box):
    p = Point()
    p.x = blah blah
    p.y = blah blah
    return p

box = Rectangle()
box.width = 100
box.height = 200
box.corner = Point()
box.corner.x = 0
box.corner.y = 0

center = find_center(box)


Secret implementation detail
----------------------------

All user-defined objects are really, secretly, internally...

    DICTIONARIES!!!

Try this:

blank = Point()
blank.x = 100
blank.y = 200
blank.pointless = 'hello'

print blank.__dict__
{'y': 200, 'x': 100, 'pointless': 'hello'}


Now you have to eat this piece of paper.


Embedded objects
----------------

Attributes can refer to objects, sometimes called embedded
objects.

In the book, Rectangles contain an embedded Point object.


Objects can share embedded objects, which is yet another
form of...

Aliasing:

1) two variables refer to the same object

2) a calling function and a called function share references to
   an object

3) two objects refer to the same embedded object


Aliasing is sometimes a good idea, especially if the objects
are immutable, but it can spawn DEEP, EVIL BUGS!!!