The Object Concept. Object-Oriented Programming Spring 2008

Transcription

1 The Object Concept Object-Oriented Programming Spring 2008

2 The Basics Of Objects Identity It must be possible to determine whether two objects are the same. State Set of characteristics Behavior Set of operations

3 Object = ID + State + Behaviour An object has state, exhibits some well-defined behaviour and has a unique identity.

4 Some Object Examples The complex number e 3+2I Characteristics: Real part, Imaginary part, argument, absolute value,... Operations: add, multiply,... The list (13, hello, true) Characteristics: Elements, length,... Operations: Add elements, remove elements, print, iterate over, sort,... The vehicle My old GMC, license plate # Characteristics: Speed, position,... Operations: Move, stop, accelerate,...

5 Objects in C? Ans. #1/3 C, unlike Pascal, allows functions to possess some state information, hence we can create (very) degenerate objects in C: Behaviour: of one sort only Identity: single object int counter(void) { static int n = 0; return ++n;

6 Objects in C? Ans. #2/3 More interesting behaviour can be achieved by using files and separate compilation in C (or units in Turbo Pascal). Still, Behaviour: several functions Identity: single object /* stack.h: interface file */ extern void push(int val); extern int pop(void); /* stack.c: implementation file */ #include "stack.h" static int stack[100], sp; void push(int val) { stack[sp++] = val; int pop(void) { return stack[--sp];

7 Objects in C? Ans. #3/3 FILE * is as close as you can get to an object in C. Behaviour: many functions Identity: the file pointer State: the file contents, flags, status,... #include <stdio.h> main(int ac, char **av) { FILE *fin; for (++av; *av!= (char *)0; av++) { if ((fin = fopen(*av, r )) == (FILE *)0) continue; while (!feof(fin)) putc(fgetc(fin)); return 0;

8 Identity the property of an object which distinguishes it from all other objects. Identity name An object may have several different names. An object may exist without any particular name bound to it. Identity state State may change while identity may not Lifetime of an Object the time span extending from the time an object is first created and consumes space until that space is reclaimed. An object can continue to exist even if all references to it are lost. Identity is tightly related to lifetime

9 Object s Lifetime Objects may live As long as the program is alive (e.g., global objects in C++) While a certain block is running (e.g., C++ stack objects) From creation (instantiation) till destructions (e.g., C++ heap objects) From creation till no longer needed (e.g., objects in Java, C#, smalltalk) Garbage Collected languages

10 Common Garbage Collection Schemes Three basic methods for garbage collection: Reference counting Mark and Sweep Stop and Copy Many heuristics and methods enhance these basic techniques

11 Reference Counting [Collins 1960] Add a reference count field for every object. how many pointers reference this object. Update field when the #references changes New objects get RC of 1 When p that points to o1 is modified to point to o2 o1.rc-- and o2.rc++ When nothing points to an object, it can be deleted. If o1.rc == 0 delete o1 decrement RC for all children of o1 Recursively delete objects whose RC is decremented to 0.

12 Mark and Sweep [McCarthy 1960] Mark Phase Explore pointers starting from the program variables, and mark all reachable objects Sweep Phase Sweep through all objects in the heap and reclaim the unmarked ones

13 Stop and Copy Divide the heap into two parts. Part 1 takes all allocations Part 2 is reserved When Part 1 runs full, copy live (reachable) objects to Part 2 Switch the roles of the two parts

14 Self Reference to Identity In many cases, it becomes necessary for a method of an object to explicitly reference the object identity. Examples: Name space ambiguity resolution Return reference to self Store address of self Notation: self in Smalltalk Current in Eiffel this in C++, Java, C#

15 Identity: Equality & Assignment Two Kinds of Equality: Is this the same object? Names equal if designate same object. Are they in the same state? Names equal if designate objects of the same state. Two Kinds of Assignment: Identity Assignment Reference is duplicated. Only one identity is involved. After the assignment: Two names refer to the same object State Assignment Copy the state information. After the assignment: Two names refer to distinct objects Address 0x Foo Bar 0x196C 12 Memory Contents

16 Root: New root Duplicating an Object Use existing one Depth: 0-level (aka: pointer copying) 1-level (aka: shallow copying) 2-level... Deep copy Default depth: Determined by parent Determined by Children

17 class PList { public Point head; public PList tail; 1-Level Copy PList onelevelcopy() { PList res = new PList(); res.head = head; res.tail = tail; return res;

18 class PList { public Point head; public PList tail; 2-Level Copy onelevelcopy() {... PList twolevelcopy() { PList res = new PList(); res.head = new Point(head.x, head.y); if(tail!= null) res.tail = tail.onelevelcopy(); return res;

19 Deep Copy public class PList { public Point head; public PList tail; PList deepcopy() { PList res = new PList(); PList to = res; for(plist from = this; from!= null; from = from.tail) { to.head = new Point(from.head.x, from.head.y); to = to.tail = (from.tail == null)? null : new PList(); return res;

20 Depth Determined By Children public class Point { public final int x, y; public Point(int a, int b) { x = a; y = b; public Point copy() { return this; public class PList { public Point head; public PList tail; public PList copy() { PList res = new PList(); res.head = head.copy(); if(tail!= null) res.tail = tail.copy(); return res;

21 1-Level Copy Into an Existing Root public class PList { public Point head; public PList tail; void copyinto(plist res) { res.head = head; res.tail = tail;

22 Value vs. Reference Semantics Suppose that the state of a certain object includes another object... Value Semantics: Objects are stored as part of other objects Expanded in Eiffel, non pointers in C++, primitives in Java Reference Semantics: include reference to other objects Non atomic classes in Smalltalk Pointers and references in C++, Object types in Java Comparison: Access Speed: VS is faster Shallow comparison and copy: RS is faster Ownership: undefined in RS Sharing: difficult in VS Automatic garbage collection: easier in RS

23 Penalties of Reference Semantics Indirect access: more machine instructions in each access to value Loss of caching opportunities Free store allocation: search the heap in object creating and add to the heap in destruction. Memory management: extra burden on the programmer shoulders in non garbagecollecting languages Memory allocation though is easier, since objects tend to be smaller and more uniformly sized. Dynamic binding overhead: object kind is usually only known in run-time Loss of optimization opportunities

24 What's the Output Under // Psuedo code: // VS, RS? //? denotes identity comparison // == denotes value comparison for primitives class Counter { public int n = 0; void f(counter c0, Counter c1) { System.out.println(c0? c1); // Identity comparison c0.n += 1; // Mutation System.out.println(c0? c1); // Identity comparison System.out.println(c0.n == c1.n); // State comparison Counter c = new Counter(); f(c,c);

25 Case Study: Equality and Assignment in Eiffel Value semantics: Primitive types: Char, Integer, Real, and Boolean, and object attributes marked as expanded. Reference semantics: Object types (all classes) Reference (identity) operations: a := b -- Reference assignment a = b -- Reference equality testing a /= b -- Reference inequality testing Shallow (state) operations: a.copy(b) -- Attribute by attribute shallow copy a := clone(b) -- Create new clone object equal(a,b) -- Attribute by attribute comparison Deep (state) operations: a.deep_copy(b) -- Attribute by attribute copy & cloning of inner objects a := deep_clone(b) -- Create a full clone of a complex structure deep_equal(a,b) -- Attribute by attribute recursive comparison

26 Static Aspect of Objects Attributes: place holders in which the state is stored Nicknames: Fields, Properties, Instance Variables, Data Members Kinds of attributes: Immutable (static attributes) / Mutable (dynamic attributes) Structure: the set of all attributes State: the current value of all of the object attributes Immutable Objects : structure comprises only immutable attributes Example: Integer in Smalltalk, String in Java Kinds of values Scalars: Integer, Real, Boolean,... Aggregates: Array, List, Tree,... Objects: Invoice, Reservation,... Kinds of representations: value or reference/pointer

27 Dynamic Aspect of Objects Active Object: encompasses its own thread of control; may undergo spontaneous state changes. Example: Ada s tasks, Java Runnable Quite rare in sequential OOPLs. Passive Object: can change state only if operated by other objects. Client: may query an object for its state or request it to change it. Should not have direct access to attributes. State encapsulation is enforced in many OOPLs. Message Passing: the means of communication between a client and object. Client: send a message. Methods + Messages are called function members in C++. Message: a symbolic name (called selector in Smalltalk) + optional arguments. Object: invoke a method, optionally return an answer.

28 Examples of Messages The object vehicle can receive messages such as: Move forward at a given speed Accelerate Decelerate Stop The object list can receive messages such as: Add an element X Remove an element Y Return the length of yourself Sort yourself according to a given criteria Concatenate yourself with another list object Print yourself

29 Different Objects May Have Different Methods Bound to the Same Message In Object Oriented Programming: The message rotate left n degrees would be implemented differently by different objects: Shape: degrees. Circle: Do nothing if n == 0. Otherwise Rotate left n Do nothing. Rectangle: Do nothing if n == 180, 360,... Otherwise shape rotate. Square: Do nothing if n == 90, 270,... Otherwise rectangle rotate.

30 Protocol and Behavior Behavior: the way an object acts and reacts. internal state changes message sent to other objects returned value Protocol: the envelope of the behavior. Features: elements of the protocol. Operations that the object recognizes Attributes accessible from outside

31 Features and Encapsulation Given an expression a.x, What is x? Java: x is a field Eiffel: x is either a field or a parameter-less method. The Eiffel approach stems from the Uniform Access Principle: All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation.

32 Encapsulation using C# Properties class MyClass{ private int _x; public int x{ get{return _x; set{ _x = value; class MyClient{ public static void Main(){ MyClass mc = new MyClass(); mc.x = 10; int xval = mc.x; Console.WriteLine(xVal);//Displays 10

33 Kinds of Operations Mutator (SETter): alters state Iterator: changes the externally accessible parts of the state Moving in a linked list Exposing a register in hardware Inspector (selector, GETter): a query on the object s state. Convertor (to other): returns an equivalent object of different structure. Constructor: creates a new object and/or initializes its state. Convertor (from other): creates and initializes an object as an equivalent of another Destructor: frees object s space and/or does other clean-up operations. Revealer: exposes an internal part of an object for direct external manipulation.

34 Example: Operations in a Date Class class Date { public: // Constructor Date(int day = -1, int month = -1, int year = -1); // Inspectors int day(void) const { return d; int month(void) const { return m; int year(void) const { return y; // Mutators int day(int day); int month(int month); int year(int year); In C++, as in most other OOPLs all objects are instances of a class. // Destructor ~Date(void) { cout << "Sic transit gloria mundi"; private: int d; int m; int y; ; oop

35 Mutators of the Date Class int Date::day(int day) { int res = d; d = day; return res; int Date::month(int month) { int res = m; m = month; return res; int Date::year(int year) { int res = y; y = year; return res; Note that the mutators are not pure; they also serve an inspection purpose. Such mixed mode programming is not recommended.

36 Constructor of the Date Class Date::Date(int day, int month, int year): d(day), // Constructor header: m(month), // Initialize all data members y(year) { // Additional initialization steps // Error checking // Accounting for leap year //...

37 Example Operations in an Array Class class Array { public: // Constructor Array(int n_): n(n_), buff(new float[n]) { // Destructor ~Array(void) { delete[] data; // Inspector int n(void) { return n; // Revealer float& elem(int i) { return buff[i]; private: int n; float *buff; ; Revealers can serve the purpose of both inspectors and mutators.