Objects and Classes David Walker CS 320. Advanced Languages advanced programming features –ML data...

Objects and Classes

David Walker

CS 320

Advanced Languages

• advanced programming features– ML data types, exceptions, modules, objects,

concurrency, ...– fun to use, but require special techniques to

compile and optimize– today will be looking at how to compile objects

and classes similar to those found in Java• Appel chapter 14.1-14.4

Object Fun

• Add class declarations to Fun:– classes are a collection of

• field (variable) declarations• method declarations

– every class declaration gives a new name to a class

– a class may inherit methods and fields from the class it extends

– the class “object” sits at the top of the class hierarchy; it has no fields and no methods

An Object Fun Class

let class Vehicle extends Object { var position := start method move (x : int) = (position := position + x) }

in ... end

class namesuperclass thatVehicle inherits from

fielddeclaration

method declaration

Another Object Fun Class

let class Vehicle extends Object { var position := start method move (x:int) = (position := position + x) }

class Car extends Vehicle { var passengers := 0 method await(v:Vehicle) = if (v.position < position) then v.move(position – v.position) else self.move(10)

in ... end

v’s “position” field

current object’s “position” field

call to inheritedmethod

new field declaration

new method declaration

Yet Another Object Fun Class

let class Vehicle extends Object { var position := start method move (x:int) = position := position + x }

class Car extends Vehicle { ... }

class Truck extends Vehicle { method move (x:int) = if x <= 55 then position := position * x }in ... end

method override

Using the Classes

let class Vehicle extends Object { ... } class Car extends Vehicle { ... } class Truck extends Vehicle {...} val t = new Truck val c = new Car val v : Vehicle = cin c.passengers := 2; c.move(60); v.move(70); c.await(t);end

new objectcreated

subtyping allows acar to be viewed andused as a genericvehicle

subtyping allows atruck to be viewed andused as a genericvehicle

a car callsan inheritedmethod

Implementing Object Tiger

• Some key problems:– how do we access object fields?

• both inherited fields and fields for the current object?

– how do we access method code?• if the current class does not define a particular

method, where do we go to get the inherited method code?

• how do we handle method override?

Class Hierarchy

• The class hierarchy is the graph of inheritence relationships in a program:

– In a single-inheritence (SI) language, the graph is a tree– In a multiple-inheritence (MI) language, the graph is a dag– Multiple-inheritence languages are much trickier to implement

than single-inheritence languages

Object

Vehicle

Car Truck

Object Layout (SI)

• Objects are laid out somewhat like records– each variable has a slot in the record– in order to implement field lookup we need to have a

systematic way to find a given field• eg: v.position• the hard part is that v may be a generic Vehicle or it may be

a Car or a Truck• the same lookup code must work when we know the static

type (say Vehicle) but we do not know the dynamic type of the object (say Vehicle, Car or Truck)

• we need to put “position” in the same place in the record that implements vehicles, cars and trucks

Using the Classeslet class Vehicle extends Object { ... } class Car extends Vehicle { var passengers := 0 method await(v:Vehicle) = if (v.position < position) then ... }

class Truck extends Vehicle {...}

val t = new Truck val c = new Car val v : Vehicle = cin c.await(t); c.await(c); c.await(v)end

assume v is a pointer in rv

v.position (for SI) gets compiled into:

load rtemp const(rv)

what const do we use?must be the same constantregardless of whether v isa car, truck or vehicle

the layout of fields inrecords that implement cars, trucks and vehicles must be similar.

Object Layout (SI)

• Solution: extension on the right– lay out the inherited fields first in the same

order as in the parent (SI => only 1 parent)– lay out the newly declared to the right

Object Layout (SI)

class A extends Object { var a := 0 }class B extends A { var b := 0 var c := 0 }class C extends A { var d := 0 }class D extends B { var e := 0 }

Object Layout (SI)


A

a

class identifier;used to implementdowncasts;first field in all classesmust be the class identifier

first field named a;all objects extending Amust have first field a

Object Layout (SI)


B

a

b

c

A

a

Object Layout (SI)


C

a

d

B

a

b

c

A

a

Object Layout (SI)


D

a

b

c

e

C

a

d

B

a

b

c

A

a

Static & Dynamic Methods

• The result of compiling a method is some machine code located at a particular address– at a method invocation point, we need to

figure out what code location to jump to

• Java has static & dynamic methods– to resolve static method calls, we look at the

static type of the calling object– to resolve dynamic method calls, we need the

dynamic type of the calling object

Static Methods

– during semantic analysis, the compiler knows:

• static type (class) of the object calling the method

• the list of methods in each class

– and determines• the closest method (up the

class hierarchy) with the given name

– and inserts• instructions to pass object

as self parameter• a direct call to the known

method

let class A extends Object { static method foo (x:int) = ... static method bar (x:int) = ... } class B extends A { static method foo (x:int) = ... } var a : A = new A var b : A = new B var c : B = new Bin a.foo(3); (* calls foo in class A *) b.foo(3); (* calls foo in class A *) c.bar(3); (* calls bar in class A *) c.foo(3); (* calls foo in class B *)

Dynamic Methods

– Method called depends on object’s dynamic type

• During semantic analysis, may be unknown

– At run-time, we determine which code to jump to

• object stores a pointer to its method table (v-table) as well as its object vars

– At compile-time, we generate code to

• look up v-table in object• extract method from table• jump to method body

let class A extends Object { method foo (x:int) = ... method bar (x:int) = ... } class B extends A { method foo (x:int) = ... } var a : A = new A var b : A = new B var c : A = if long-and-tricky-computation then a else bin c.foo(3)

Object Layout II (SI)

class A extends Object { var a := 0; method f () }class B extends A { method g () }class C extends B { method g () }class D extends C { var b := 0 ; method f () }



A

a

A_f



B

a

A

a

A_f A_f

B_g



C

a

B

a

A

a

A_f A_f

B_g

A_f

C_g



D

a

b

C

a

B

a

A

a

A_f A_f

B_g

A_f

C_g

D_f

C_g



D

a

b

C

a

B

a

A

a

A_f A_f

B_g

A_f

C_g

D_f

C_g

D

a

b

Multiple Inheritence

• Multiple inheritence is trickier to implement than single inheritence because creating objects of a subclass from their subclass by “extension on the right” doesn’t work– if C inherits from both A and B, we can’t put

A’s variables at the front and put B’s variables at the front of the object in the same place!

– we need to do a global analysis to determine object layout

Object Layout (MI)

class A extends Object { var a := 0 }class B extends Object { var b := 0 var c := 0 }class C extends A { var d := 0 }class D extends A,B,C { var e := 0 }

D

a

b

c

e

C

a

d

B

b

c

A

a

d

Object Layout (MI)


D

a

b

c

e

C

a

d

B

b

c

A

a

d

Determine object layout by:• global graph coloring!• a node for each field name• an interference edge betweennames that coexist in the sameclass (via inheritence or otherwise)

Object Layout (MI)


D

a

b

c

e

C

a

d

B

b

c

A

a

dwasted space inevery object

Object Layout II (MI)


D

a

b

c

e

C

a

d

B

b

c

A

a

d1 1

2

1

2

1

2

3

5

4wasted spaceper class

a: a:

b:

a:

d:

a:

b:

c:

d:

e:

Object Layout II (MI)


B

b

c

A

a

1 1

2

a: a:

b:

To fetch a field using this representation, we:

• load the first field of the objectto get the class descriptor c• load M [c + fieldOffset]

Note: fieldOffset can be precomputed using global graph coloring as before

A Problem

• Global analyses are the bane of modern computing– many applications use dynamically linked libraries,

mobile code, get patches to fix bugs, etc.– when we don’t have the whole program at compile-

time, we can’t do global analyses!– solution (most of the time):

• a smart custom linker• still tricky when new code is linked dynamically to code that

is already running

– hence, Java has single inheritence

Other OO Features

• Down-casts and type tests– Java has casting mechanism “(C) x” to cast

variable x to class C• at run time we look up x’s dynamic type and

determine whether it is a subtype of C• these type casts are currently pervasive and a

source of both inefficiency and errors• soon, Java and C# will be adding parametric

polymorphism, a la ML, to make many of these unnecessary casts go away

Other OO Features

• Protection mechanisms– to encapsulate local state within an object,

Java has “private” “protected” and “public” qualifiers

• private methods/fields can’t be called/used outside of the class in which they are defined

– during semantic analysis (type checking), the compiler maintains this information in the symbol table for each class

Summary

• Object-oriented languages provide new challenges for compiler writers– how to find fields and methods– how to make field and method access just as

efficient as ordinary function call and variable lookup

– lots of ongoing research in OO language implementation tackles these and other interesting questions

Objects and Classes David Walker CS 320. Advanced Languages advanced programming features –ML data...

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