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CSE 413 Autumn 2008

Objects & DynamicDispatch

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an

Weve learned a great deal about functionaland object-oriented programming

Now,

Look at semantics and rinci les more carefull

Look at O-O and functional programming whatare the essential differences and similarities

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oo - p u es

Key idea in any language: how ares mbols names identifiers resolved

Functional programming first-class, , . .,

dont use set!)

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oo - p u es n u y

In Ruby, use syntactic distinctions instance fields x , class fields x vs

method/block variables and method names (x)

No shadowing of fields, unlike Java Can shadow method names with variables

So: is m+2 a variable looku or a method call?

We wont worry about this for the most part

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rs - ass

If something can be computed, stored in fields/variables, used as arguments, returned as results,we sa it is first-class

All objects in Ruby are first-class

& most things are objects ngs a are no :

Message names cant write x.(if b then m else n end)

Blocks (but procs are)Argument lists

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ar a e oo up n u y

To resolve a variable (e.g., x) Inside a code block x e , x resolves to local

variable (the argument)

Not strictly true in Ruby 1.8 & earlier if x already exists

Else x resolves to x defined in enclosing method

Lexical sco e as in Scheme

Implies Ruby implementation needs to build closures atleast some of the time

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essage oo up n u y

To resolve a message (e.g., m)All messages are sent to an object (e.g., e.m), so

Get class of obj (e.g., A) Every object has a class and carries a reference to the

If m defined in A (instance methods first, thenclass methods), call it, otherwise recursively look

Mixins complicate this somewhat (later) If no match up the chain, method not found error

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a s se

Evaluation always takes place in anenvironment

self is always bound to some object in any

Determines resolution for self and super

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r ncpes

Inheritance and override Private fields (just abstraction) The semantics of message send

To send m to obj means evaluate body of methodmap to arguments and self is bound to obj

This is exactly late binding, dynamic dispatch,

And why superclass code can call code defined insubclasses

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n xamp e c eme

Suppose this is defined at top-level(define (even x) (if (= x 0) #t (odd (- x 1))))

(define (odd x) (if (= x 0) #f (even (- x 1))))

Suppose we evaluate odd 42 in an innerscope where even is defined to be

(define (even x) (= 0 (modulo x 2)))

Nothing changes odd calls original even(static scope)

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xamp e u y u c asses

class Adef even x

==

class B < Adef even x

odd(x-1) end

end

x % 2 == 0

ende o x

if x == 0 then false else

even(x-1) end

en

Now odd, as well as

endend

,instances of B

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erspec ves on a e n ng

More complicated semanticsRub without self is easier to define and

reason about

Seems natural onl because ou have hadmonths of this in previous courses

Hard to reason about code which method is

really called here?

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erspec ves on a e n ng

But often an elegant pattern for reuseOO without self is not OO

Fits well with object analogy

code even when other code wasnt written tobe specialized

More reuse/abuse

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ower- eve ew

A definition in one language is often a patternin another an s mu a e a e- n ng n c eme eas y

enough

objects and late binding are implementedNave, but accurate view can give a way to

,implementations contain more sophisticatedengineering

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a e n ng n c eme

Key idea: extend all methods to take anextra ar ument i.e. self

An object is a record (closure) holding

Self is passed as an explicit argument

Message resolution always uses self

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s s ea

Its a fine pattern, but It doesnt model Rub where methods can be

added/removed dynamically and an objects

class determines behavior In the example we model classless objects

Space inefficient duplicate methods

Time inefficient method lookup needs to beconstant time in real systems

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etter ng neer ng, etter ea ty

To model classes, add a level ofindirection: all instances created b thesame constructor share a list of methods

And for Rub we can chan e the list

Use better data structures (array or hash)

-And add tricks so subclassing works