Implications of Substitution

Implications ofSubstitution

Muhammad AdilRaja

Introduction

The is-aRelationship

Summary

References

IMPLICATIONS OF SUBSTITUTION

Muhammad Adil Raja

Roaming Researchers, Inc.

cbna

April 19, 2015


Muhammad AdilRaja

Introduction


Summary

References

OUTLINE I

INTRODUCTION

THE IS-A RELATIONSHIP

SUMMARY

REFERENCES


Muhammad AdilRaja

Introduction


Summary

References

INTRODUCTION

I We will investigate some of the implications of theprinciple of substitution in statically typedobject-oriented programming languages.

I In particular, we will consider:I The impact on memory managementI The meaning of assignmentI The distinction between testing for identity and

testing for equality


Muhammad AdilRaja

Introduction


Summary

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THE IDEALIZATION OF IS-A RELATIONSHIP

MESSAGE SYNTAX

I A TextWindow is-a Window.I Because TextWindow is subclassed from Window, all

behavior associated with Windows is also manifestby instances of TextWindow.

I Therefore, a variable declared as maintaining aninstance of Window should be able to hold a value oftype TextWindow.

I Unfortunately, practical programming languageimplementation issues complicate this idealizedpicture.


Muhammad AdilRaja

Introduction


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MEMORY ALLOCATION – STACK AND HEAP

BASED

I Generally, programming languages use two differenttechniques for allocation of memory.

I Stack-based allocation.I Amount of space required is determined at compile

time, based on static types of variables.I Memory allocation and release is tied to procedure

entry/exit.I Can be performed very efficiently.I Heap-based allocation.I Amount of space used can be determined at

run-time, based upon dynamic considerations.I Memory allocation and release is not tied to

procedure entry/exit, and either must be handled byuser or by a run-time library (garbage collection).

I Generally considered to be somewhat less efficient.


Muhammad AdilRaja

Introduction


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THE PROBLEM WITH SUBSTITUTION

SUBSTITUTION

class Window {public : Window x ; / / how much space to set aside?

v i r t u a l void oops ( ) ; TextWindow y ;private : x = y ; / / what should happen here?

i n t he igh t ;i n t width ;

} ;

class TextWindow : public Window {public :

v i r t u a l void oops ( ) ;private :

char ∗ contents ;i n t cursorLoca t ion ;

} ;


Muhammad AdilRaja

Introduction


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HOW MUCH MEMORY TO SET ASIDE

How much memory should be set aside for the variable x?

1. (Minimum Static Space Allocation) Allocate theamount of space necessary for the base class only.(C++)

2. (Maximum Static Space Allocation) Allocate theamount of space for the largest subclass.

3. (Dynamic Space Allocation) Allocate for x only theamount of space required to hold a pointer.(Smalltalk, Java)


Muhammad AdilRaja

Introduction


Summary

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MINIMUM STATIC SPACE ALLOCATION

I The language C++ uses the minimum static spaceallocation approach.

I This is very efficient, but leads to some subtledifficulties.

I What happens in the following assignment?

ASSIGNMENT

Window x ;TextWindow y ;

x = y ;


Muhammad AdilRaja

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ASSIGNING A LARGER VALUE TO A SMALLER

BOX

FIGURE : Assignment

http://web.engr.oregonstate.edu/~budd/Books/oopintro3e/info/slides/chap03/slide21a.gif


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THE SLICING PROBLEM

I The problem is you are trying to take a large box andsqueeze it into a small space. Clearly this won’twork. Thus, the extra fields are simply sliced off.

I Question: Does this matter?I Answer: Only if somebody notices.I Solution: Design a language to make it difficult to

notice.


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RULES FOR MEMBER FUNCTION BINDING IN

C++

The rules for deciding what member function to executeare complicated because of the slicing problem.

1. With variables that are declared normally, the bindingof member function name to function body is basedon the static type of the argument (regardlesswhether the function is declared virtual or not).

2. With variables that are declared as references orpointers, the binding of the member function name tofunction body is based on the dynamic type if thefunction is declared as virtual, and the static type ifnot.


Muhammad AdilRaja

Introduction


Summary

References

ILLUSTRATION

ILLUSTRATION

void Window : : oops ( ){ p r i n t f ( "Window oops \ n " ) ; }

void TextWindow : : oops ( ){ p r i n t f ( " TextWindow oops %d \ n " , cursorLoca t ion ) ;

TextWindow x ;

Window a ;Window ∗ b ;TextWindow ∗ c ;

a = x ; a . oops ( ) ; / / executes Window vers ionb = &x ; b−>oops ( ) ; / / executes TextWindow or Window vers ion ;c = &x ; c−>oops ( ) ; / / executes TextWindow vers ion


Muhammad AdilRaja

Introduction


Summary

References

MINIMUM STATIC SPACE ALLOCATION

I A different approach would be to allocate theMaximum amount of space you would ever need.

I Would nicely solve the slicing problem.I Would often allocate unused space.I Maximum amount of space not known until all

classes have been seen.I For this reason, not used in practice.


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Introduction


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DYNAMIC MEMORY ALLOCATION

I In the third approach, all objects are actuallypointers.

I Only enough space for a pointer is allocated atcompile time.

I Actual data storage is allocated on the heap atrun-time.

I Used in Smalltalk, Object Pascal, and Objective-C,Java.

I Requires user to explicitly allocate new objects and,in some languages, explicitly free no longer usedstorage.

I May also lead to pointer semantics for assignmentand equality testing.


Muhammad AdilRaja

Introduction


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MEANING OF ASSIGNMENT

What does it mean when an instance of a class isassigned to another variable?

ASSIGNMENT

class Box {public i n t value ;

}

Box x = new Box ( ) ;x . value = 7;Box y = x ;y . value = 12; / / what i s x . value?

Two possibilities:I Copy semantics. x and y are independent of each

other, a change in one has no effect on the other.I Pointer semantcs. x and y refer to the same object,

and hence a change in one will alter the other.


Muhammad AdilRaja

Introduction


Summary

References

COPY SEMANTICS VS POINTER SEMANTICS

I If a value is indirectly accessed through a pointer,when an assignment is performed (or equality test ismade) is the quantity assigned simply the pointer oris it the actual value?


Muhammad AdilRaja

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PROBLEMS WITH POINTER SEMANTICS

I If x is assigned to y and then changes are made to x,are these changes reflected in y?

I If x is explicitly freed, what happens if the user triesto access memory through y?

I In C++, programmer can make assignment (equalitytesting) mean anything they want.

I Object Pascal, Java uses pointer semantics, nobuilt-in provision for copies.

I Smalltalk and Objective-C use pointer semantics,have several techniques for making copies.


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Introduction


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AN OLD JOKE CONCERNING EQUALITY

I There is an old joke that goes something like this: Aman walks into a pizza parlor and sits down.

I A waiter comes to the table and asks the man whathe would like to order. The man looks around theroom, then points to the woman sitting at the nexttable, and says “I’ll have what she is eating”.

I The waiter thereupon walks to the womans table,picks up the half-eaten pizza from in front of her, andplaces it before the startled customer.

I A classic confusion between equality and identity.


Muhammad AdilRaja

Introduction


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EQUALITY AND IDENTITY

I A test for identity asks whether two variables refer toexactly the same object.

I A test for equality asks whether two variables refer tovalues that are equivalent.

I Of course, the meaning of equivalent is inheritentlydomain specific.

I Object-oriented languages allow the programmer tocontrol the meaning of the equality test by allowingthe redefinition of a standard method. (for example,equals in Java).


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PARADOXES OF EQUALITY I

I But child classes cannot change the type signatureof overridden methods.

I This means the argument must often be moregeneral than one would like:

EQUALITY

class Object {public boolean equals ( Object r i g h t ) {

. . .}

}

class PlayingCard extends Object {public boolean equals ( Object r i g h t ) {

. . . / / r i g h t must be ob jec t even i f we are only

. . . / / i n t e r e s t e d i n comparing cards to cards}

}

And if you add inheritance into the mix, the possibilitiesfor paradoxical behavior increase even more.


Muhammad AdilRaja

Introduction


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PARADOXES OF EQUALITY II

EQUALITY

class Foo {boolean equals ( Object r i g h t ) { . . . }

}

Foo a , b ;

i f ( a . equals ( b ) ) / / even i f t h i s i s t r ue

i f ( b . equals ( a ) ) / / no guarantee t h a t t h i s i s t r ue


Muhammad AdilRaja

Introduction


Summary

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PARADOXES OF EQUALITY III

And if you add inheritance into the mix, the possibilitiesfor paradoxical behavior increase even more.

EQUALITY

class Parent {boolean equals ( Object x ) { . . . }

}

class Chi ld extends Parent {boolean equals ( Object x ) { . . . }

}

Parent p ;Ch i ld c ;

i f ( p . equals ( c ) ) / / w i l l execute using the parent method

i f ( c . equals ( p ) ) / / w i l l execute using the c h i l d s method


Muhammad AdilRaja

Introduction


Summary

References

SUMMARY I

I We have explored the implications that result fromthe inclusion of the principle of substitution in anobject oriented programming language.

I Because values are not known until run time, youeither have complex semantics (as in C++) or objectsare dynamic (as in Java and most other languages).

I Because objects are dynamic, most object-orientedlanguages end up using a garbage collection system.

I Dynamic semantics naturally lean to pointersemantics for assignment

I Pointer semantics mean that equality and identity aretwo different concepts

I Since equality is domain specific, the programmermust be free to redefine the meaning as appropriate.


Muhammad AdilRaja

Introduction


Summary

References

SUMMARY II

I Because the programmer can redefine equalityarbitrarily, there is no guarantee that semantics ofequals is preserved.


Muhammad AdilRaja

Introduction


Summary

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REFERENCES I

I Images and content for developing these slides havebeen taken from the follwoing book with thepermission of the author.

I An Introduction to Object Oriented Programming,Timothy Budd.

I This presentation is developed using Beamer:I CambridgeUS, dove.

http://web.engr.oregonstate.edu/~budd/Books/oopintro3e/info/ReadMe.html

Implications of Substitution

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