0. CS2403 Programming Languages Support for Object-Oriented Programming
Chung-Ta King
Department of Computer Science
National Tsing Hua University
(Slides are adopted from Concepts of Programming Languages, R.W. Sebesta)

1. Software Productivity
Pressure on software productivity vs. continual reduction in hardware cost
Productivity increases can come from reuse
Abstract data types (ADTs) for reuse?
Mere ADTs are not enough
ADTs are difficult to reuse—always need changes for new uses, e.g., circle, square, rectangle, ...
All ADTs are independent and at the same level  hard to organize program to match problem space
More, in addition to ADTs, are needed 1

2. What Are Needed?
Given a collection of related ADTs, need to factor out their commonality and put it in a new type  abstraction
The collection of ADTs then inherit from the new type and add their own
class Shape {
private:
int x; int y;
public:
Shape(int a, int b);
...
virtual void draw(); };
class Circle: public Shape {
private:
int radius;
public:
Circle(int x1, y2, r1);
...
void draw(); }; 2

3. What Are Needed? (cont.)
For classes in an inheritance relationship, a method call may need to be bound to a specific object of one of the classes at run time
Shape *shape_list[3]; // array of shape objects
shape_list[0] = new Circle;
shape_list[1] = new Square;
shape_list[2] = new Triangle;
for(int i = 0; i < 3; i++){
shape_list[i].draw(); }
Polymorphism and dynamic binding
Dynamic Class Loading for C++ on Linux ( 3

4. Object-Oriented Programming
An object-oriented language must provide supports for three key features:
Abstract data types
Inheritance: the central theme in OOP and languages that support it
Polymorphism and dynamic binding
What does it mean in a pure OO program, where x, y, and 3 are objects?
y = x + 3; 4

5. Outline Object-Oriented Programming (Sec. 12.2)
Design Issues for OO Languages (Sec. 12.3)
Support for OOP (Sec ~ 12.9)
Smalltalk
C++
Java C#
Ada 95
Ruby
Implementation of OO Constructs (Sec ) 5

6. Object-Oriented Concepts
ADTs are usually called classes, e.g. Shape
Class instances are called objects, e.g. shape_list[0]
A class that inherits is a derived class or a subclass, e.g. Circle, Square
The class from which another class inherits is a parent class or superclass, e.g. Shape
Subprograms that define operations on objects are called methods, e.g. draw() 6

7. Object-Oriented Concepts (cont.)
Calls to methods are called messages, e.g. shape_list[i].draw();
Object that made the method call is the client
The entire collection of methods of an object is called its message protocol or message interface
Messages have two parts--a method name and the destination object
In the simplest case, a class inherits all of the entities of its parent 7

8. Inheritance
Allows new classes defined in terms of existing ones, i.e., by inheriting common parts
Can be complicated by access controls to encapsulated entities
A class can hide entities from its subclasses (private)
A class can hide entities from its clients
A class can also hide entities for its clients while allowing its subclasses to see them
A class can modify an inherited method
The new one overrides the inherited one
The method in the parent is overridden 8

9. Inheritance (cont.) There are two kinds of variables in a class:
Class variables - one/class
Instance variables - one/object, object state
There are two kinds of methods in a class:
Class methods – accept messages to the class
Instance methods – accept messages to objects
Single vs. multiple inheritance
One disadvantage of inheritance for reuse:
Creates interdependencies among classes that complicate maintenance 9

10. Dynamic Binding
A polymorphic variable can be defined in a class that is able to reference (or point to) objects of the class and objects of any of its descendants
Shape *shape_list[3]; // array of shapes
shape_list[0] = new Circle;
shape_list[1] = new Square;
shape_list[2] = new Triangle;
for(int i = 0; i < 3; i++){
shape_list[i].draw(); } 10

11. Dynamic Binding
When a class hierarchy includes classes that override methods and such methods are called through a polymorphic variable, the binding to the correct method will be dynamic, e.g., shape_list[i].draw();
Allows software systems to be more easily extended during both development and maintenance, e.g., new shape classes are defined later 11

12. Dynamic Binding (cont.)
An abstract method is one that does not include a definition (it only defines a protocol)
An abstract class is one that includes at least one virtual method
An abstract class cannot be instantiated
class Shape {
private:
int x; int y;
public:
...
virtual void draw(); };
class Circle: public Shape {
private:
int radius;
public:
...
void draw() {...}; }; 12

13. Outline Object-Oriented Programming (Sec. 12.2)
Design Issues for OO Languages (Sec. 12.3)
Support for OOP (Sec ~ 12.9)
Smalltalk
C++
Java C#
Ada 95
Ruby
Implementation of OO Constructs (Sec ) 13

14. Design Issues for OOP Languages
The exclusivity of objects
Are subclasses subtypes?
Type checking and polymorphism
Single and multiple inheritance
Object allocation and deallocation
Dynamic and static binding
Nested classes
Initialization of objects 14

15. The Exclusivity of Objects
Option 1: Everything is an object
Advantage: elegance and purity
Disadvantage: slow operations on simple objects
Option 2: Add objects to existing typing system
Advantage: fast operations on simple objects
Disadvantage: confusing typing (2 kinds of entities)
Option 3: Imperative-style typing system for primitives and everything else objects
Advantage: fast operations on simple objects and a relatively small typing system
Disadvantage: still confusing by two type systems 15

16. Are Subclasses Subtypes?
Does an “is-a” relationship hold between a parent class object and an object of subclass?
If a derived class is-a parent class, then objects of derived class behave same as parent class object
subtype small_Int is Integer range ;
Every small_Int variable can be used anywhere Integer variables can be used
A derived class is a subtype if methods of subclass that override parent class are type compatible with the overridden parent methods
A call to overriding method can replace any call to overridden method without type errors 16

17. Type Checking and Polymorphism
Polymorphism may require dynamic type checking of parameters and the return value
Dynamic type checking is costly and delays error detection
If overriding methods are restricted to having the same parameter types and return type, the checking can be static 17

18. Single and Multiple Inheritance
Multiple inheritance allows a new class to inherit from two or more classes
Disadvantages of multiple inheritance:
Language and implementation complexity: if class C needs to reference both draw() methods in parents A and B, how to do? If A and B in turn inherit from Z, which version of Z entry in A or B should be ref.?
Potential inefficiency: dynamic binding costs more with multiple inheritance (but not much)
Advantage:
Sometimes it is quite convenient and valuable 18

19. Object Allocation and Deallocation
From where are objects allocated?
If behave like ADTs, can be allocated from anywhere
Allocated from the run-time stack
Explicitly created on the heap (via new)
If they are all heap-dynamic, references can be uniform thru a pointer or reference variable
Simplifies assignment: dereferencing can be implicit
If objects are stack dynamic, assignment of subclass B’s object to superclass A’s object is value copy, but what if B is larger in space?
Is deallocation explicit or implicit? 19

20. Dynamic and Static Binding
Should all binding of messages to methods be dynamic?
If none are, you lose the advantages of dynamic binding
If all are, it is inefficient
Alternative: allow the user to specify 20

21. Nested Classes
If a new class is needed by only one class, no reason to define it to be seen by other classes
Can the new class be nested inside the class that uses it?
In some cases, the new class is nested inside a subprogram rather than directly in another class
Other issues:
Which facilities of the nesting class should be visible to the nested class and vice versa 21

22. Initialization of Objects
Are objects initialized to values when they are created?
Implicit or explicit initialization
How are parent class members initialized when a subclass object is created?

23. Outline Object-Oriented Programming (Sec. 12.2)
Design Issues for OO Languages (Sec. 12.3)
Support for OOP (Sec ~ 12.9)
Smalltalk
C++
Java C#
Ada 95
Ruby
Implementation of OO Constructs (Sec ) 23

24. Inheritance in C++ Inheritance Multiple inheritance is supported
A class need not be the subclass of any class
Access controls for members are
Private: accessible only from within other members of the same class or from their friends.
Protected: accessible from members of the same class and from their friends, but also from members of their derived classes.
Public: accessible from anywhere that object is visible
Multiple inheritance is supported
If two inherited members with same name, they can both be referenced using scope resolution operator 24 24

25. Inheritance in C++ (cont.)
In addition, the subclassing process can be declared with access controls (private or public), which define potential changes in access by subclasses
Private derivation: inherited public and protected members are private in the subclasses  members in derived class cannot access to any member of the parent class
Public derivation: public and protected members are also public and protected in subclasses 25 25

26. Inheritance Example in C++
class base_class {
private:
int a; float x;
protected:
int b; float y;
public:
int c; float z; };
class subclass_1 : public base_class {…};
// b, y protected; c, z public
class subclass_2 : private base_class {…};
// b, y, c, z private; no access by derived 26

27. Inheritance Example 2 in C++
class base{
int x;
public:
void setx(int n) {x = n;}
void showx(void) {cout << x << "\n“;}};
class derived : private base{
int y;
void setxy(int n) {y = n; x = n; }
showxy(void) {
cout<<x<<"\n"; cout<<y<<"\n"; } };
Where are the errors?
void setxy(int n) { y=n; x=n; } // Error: private member of base
cout << x << "\n"; // Error: private member of base ( 27 27

28. Inheritance Example 2 (cont.)
class base{
int x;
public:
void setx(int n) {x = n;}
void showx(void) {cout<<x<< "\n"; } };
class derived : private base{
int y;
void setxy(int n, m) {setx(n); y = m; }
void showxy(void) {showx(); cout<<y; } }; ( 28 28

29. Reexportation in C++
A member that is not accessible in a subclass (because of private derivation) can be visible there using scope resolution operator (::), e.g.,
class subclass_3 : private base_class {
base_class :: c;
... }
One motivation for using private derivation
A derived class adds some new members, but does not want its clients to see members of the parent class, even though they had to be public in the parent class definition 29

30. Another Example in C++ class single_LL { private: class node {
public: node *link; int contents; };
node *head;
public:
single_LL() {head = 0};
void insert_at_head(int);
void insert_at_tail(int);
int remove_at_head();
int empty(); } 30 30

31. Another Example in C++ (cont.)
class stack: public single_LL {
public:
stack() {};
void push(int value) {
single_LL::insert_at_head(value); };
int pop() {
return single_LL::remove_at_head();}; };
class queue: public single_LL {
... enqueue(int value) ... dequeue() ... 31 31

32. Problem with Public Derivation
Clients of stack can access all of the public members of the parent class, single_LL, including insert_at_tail  not desirable
Public derivation is used when one wants subclass to inherit the entire interface of the base class
Let subclass inherit only the implementation of the base class and make it not subtype of the parent

33. Private Derivation in C++
class stack2: private single_LL {
public:
stack2() {};
void push(int value) {
single_LL::insert_at_head(value); };
int pop() {
return single_LL::remove_at_head();};
single_LL::empty(); //reexport }; 33 33

34. Private Derivation in C++ (cont.)
class queue2: private single_LL {
public:
queue2() {};
void enqueue(int value) {
single_LL::insert_at_tail(value); };
int dequeue() {
return single_LL::remove_at_head();};
single_LL::empty(); //reexport }; 34 34

35. Dynamic Binding in C++
A method can be defined to be virtual, and can be called through polymorphic variables and dynamically bound to messages
A pure virtual function has no definition at all
A class that has at least one pure virtual function is an abstract class
class Shape {
public:
...
virtual void draw()=0; };
class Circle: public Shape {
public:
...
void draw() {...}; }; 35

36. Dynamic Binding in C++ (cont.)
square* sq = new square;
rectangle* rect = new rectangle;
shape* shape_ptr;
shape_ptr = sq;
shape_ptr->draw(); // dynamically bound
rect = sq;
rect->draw(); // statically bound
square sq1; // sq1 on stack
rectangle rect1;
rect1 = sq1; // copy data member values
rect1.draw(); 36 36

37. Support for OOP in C++ Evaluation
C++ provides extensive access controls (unlike Smalltalk)
C++ provides multiple inheritance
Programmer must decide at design time which methods will be statically or dynamically bound
Static binding is faster!
Smalltalk type checking is dynamic (flexible, but somewhat unsafe and is ~10 times slower due to interpretation and dynamic binding 37

38. Support for OOP in Java Focus on the differences from C++
General characteristics
All data are objects except the primitive types
All primitive types have wrapper classes to store data value, e.g., myArray.add(new Integer(10));
All classes are descendant of the root class, Object
All objects are heap-dynamic, are referenced through reference variables, and most are allocated with new
No destructor, but a finalize method is implicitly called when garbage collector is about to reclaim the storage occupied by the object, e.g., to clean locks 38

39. Inheritance in Java
Single inheritance only, but interface provides some flavor of multiple inheritance
An interface like a class, but can include only method declarations and named constants, e.g.,
public interface Comparable <T> {
public int comparedTo (T b); }
A class can inherit from another class and “implement” an interface for multiple inheritance
A method can have an interface as formal parameter, that accepts any class that implements the interface  a kind of polymorphism
Methods can be final (cannot be overridden) 39 39

40. Dynamic Binding in Java
In Java, all messages are dynamically bound to methods, unless the method is final (i.e., it cannot be overridden, and thus dynamic binding serves no purpose)
Static binding is also used if the method is static or private, both of which disallow overriding 40 40

41. Nested Classes in Java Several varieties of nested classes
All are hidden from all classes in their package, except for the nesting class
Nonstatic classes nested directly are called innerclasses
An innerclass can access members of its nesting class, but not a static nested class
Nested classes can be anonymous
A local nested class is defined in a method of its nesting class  no access specifier is used 41 41

42. Evaluation of Java Design decisions to support OOP are similar to C++
No support for procedural programming
No parentless classes
Dynamic binding is used as “normal” way to bind method calls to method definitions
Uses interfaces to provide a simple form of support for multiple inheritance 42 42

43. Support for OOP in Ruby General characteristics
Everything is an object and all computation is through message passing
Class definitions are executable, allowing secondary definitions to add members to existing definitions
Method definitions are also executable
All variables are type-less references to objects
Access control is different for data and methods
It is private for all data and cannot be changed
Methods can be either public, private, or protected
Method access is checked at runtime 43

44. Outline Object-Oriented Programming (Sec. 12.2)
Design Issues for OO Languages (Sec. 12.3)
Support for OOP (Sec ~ 12.9)
Smalltalk
C++
Java C#
Ada 95
Ruby
Implementation of OO Constructs (Sec ) 44

45. Implementing OO Constructs
Most OO constructs can be implemented easily by compilers
Abstract data types  scope rules
Inheritance
Two interesting and challenging parts:
Storage structures for instance variables
Dynamic binding of messages to methods 45 45

46. Instance Data Storage
Class instance records (CIRs) store the state of an object
Static (built at compile time and used as a template for the creation of data of class instances)
Every class has its own CIR
CRI for the subclass is a copy of that of the parent class, with entries for the new instance variables added at the end
Because CIR is static, access to all instance variables is done by constant offsets from beginning of CIR 46 46

47. Dynamic Binding of Methods Calls
Methods in a class that are statically bound need not be involved in CIR; methods that are dynamically bound must have entries in the CIR
Calls to dynamically bound methods can be connected to the corresponding code thru a pointer in the CIR
Storage structure for the list of dynamically bound methods is called virtual method tables (vtable)
Method calls can be represented as offsets from the beginning of the vtable 47 47

48. Example of CIR in Java public class A { public int a, b;
public void draw() {...}
public int area() {...} }
public class B extends A {
public int c, d;
public int sift() {...} 48 48

49. Example CIR

50. Summary
OO programming involves three fundamental concepts: ADTs, inheritance, dynamic binding
Major design issues:
exclusivity of objects, subclasses and subtypes, type checking and polymorphism, single and multiple inheritance, dynamic binding, explicit and implicit de-allocation of objects, and nested classes
C++ has two distinct type system (hybrid)
Java is not a hybrid language like C++; it supports only OO programming 50