1. 1 Another Way to Define A Class - Inheritance

2. 2 Inheritance Concept Rectangle Triangle Polygon class Polygon { private: int width, length; public: void set(int w, int l); } class Rectangle{ private: int width, length; public: void set(int w, int l); int area(); } class Triangle{ private: int width, length; public: void set(int w, int l); int area(); }

3. 3 Rectangle Triangle Polygon class Polygon { protected: int width, length; public: void set(int w, int l); } class Rectangle : public Polygon { public: int area(); } class Rectangle{ protected: int width, length; public: void set(int w, int l); int area(); } Inheritance Concept

4. 4 Rectangle Triangle Polygon class Polygon { protected: int width, length; public: void set(int w, int l); } class Triangle : public Polygon { public: int area(); } class Triangle{ protected: int width, length; public: void set(int w, int l); int area(); } Inheritance Concept

5. 5 Point Circle3D-Point class Point { protected: int x, y; public: void set(int a, int b); } class Circle : public Point { private: double r; } class 3D-Point: public Point { private: int z; } xyxy xyrxyr xyzxyz

6. 6 Augmenting the original class Specializing the original class Inheritance Concept RealNumber ComplexNumber ImaginaryNumber Rectangle Triangle Polygon Point Circle real imag real imag 3D-Point

7. 7 Why Inheritance ? Inheritance is a mechanism for building class types from existing class types defining new class types to be a –specialization –augmentation of existing types

8. 8 Define a Class Hierarchy Syntax: class DerivedClassName : access-level BaseClassName where –access-level specifies the type of derivation private by default, or public Any class can serve as a base class –Thus a derived class can also be a base class

9. 9 Class Derivation Point 3D-Point class Point{ protected: int x, y; public: void set(int a, int b); } class 3D-Point : public Point{ private: double z; … } class Sphere : public 3D-Point{ private: double r; … } Sphere Point is the base class of 3D-Point, while 3D-Point is the base class of Sphere

10. 10 What to inherit? In principle, every member of a base class is inherited by a derived class – just with different access permission

11. 11 Access Control Over the Members Two levels of access control over class members –class definition –inheritance type class Point{ protected: int x, y; public: void set(int a, int b); } class Circle : public Point{ … }

12. 12 The type of inheritance defines the minimum access level for the members of derived class that are inherited from the base class With public inheritance, the derived class follow the same access permission as in the base class With protected inheritance, only the public members inherited from the base class can be accessed in the derived class as protected members With private inheritance, none of the members of base class is accessible by the derived class Access Rights of Derived Classes privateprotectedpublic private protectedprivate protected publicprivateprotectedpublic Type of Inheritance Access Controlfor Members

13. 13 Class Derivation mother daughterson class mother{ protected: int x, y; public: void set(int a, int b); private: int z; } class daughter : public mother{ private: double a; public: void foo ( ); } void daughter :: foo ( ){ x = y = 20; set(5, 10); cout<<“value of a ”<<a<<endl; z = 100; // error, a private member } daughter can access 3 of the 4 inherited members

14. 14 Class Derivation mother daughterson class mother{ protected: int x, y; public: void set(int a, int b); private: int z; } class son : protected mother{ private: double b; public: void foo ( ); } void son :: foo ( ){ x = y = 20; // error, not a public member set(5, 10); cout<<“value of b ”<<b<<endl; z = 100; // error, not a public member } son can access only 1 of the 4 inherited member

15. 15 What to inherit? In principle, every member of a base class is inherited by a derived class – just with different access permission However, there are exceptions for –constructor and destructor –operator=() member –friends Since all these functions are class-specific

16. 16 Constructor Rules for Derived Classes The default constructor and the destructor of the base class are always called when a new object of a derived class is created or destroyed. class A { public: A ( ) {cout<< “A:default”<<endl;} A (int a) {cout<<“A:parameter”<<endl;} } class B : public A { public: B (int a) {cout<<“B”<<endl;} } B test(1); A:default B output:

17. 17 Constructor Rules for Derived Classes You can also specify an constructor of the base class other than the default constructor class A { public: A ( ) {cout<< “A:default”<<endl;} A (int a) {cout<<“A:parameter”<<endl;} } class C : public A { public: C (int a) : A(a) {cout<<“C”<<endl;} } C test(1); A:parameter C output: DerivedClassCon ( derivedClass args ) : BaseClassCon ( baseClass args ) { DerivedClass constructor body }

18. 18 Define its Own Members Point Circle class Point{ protected: int x, y; public: void set(int a, int b); } class Circle : public Point{ private: double r; public: void set_r(double c); } xyxy xyrxyr protected: int x, y; private: double r; public: void set(int a, int b); void set_r(double c); The derived class can also define its own members, in addition to the members inherited from the base class

19. 19 Even more … A derived class can override methods defined in its parent class. With overriding, –the method in the subclass has the identical signature to the method in the base class. –a subclass implements its own version of a base class method. class A { protected: int x, y; public: void print () {cout<<“From A”<<endl;} } class B : public A { public: void print () {cout<<“From B”<<endl;} }

20. 20 class Point { protected: int x, y; public: void set(int a, int b) {x=a; y=b;} void foo (); void print(); } class Circle : public Point{ private: double r; public: void set (int a, int b, double c) { Point :: set(a, b); //same name function call r = c; } void print(); } Access a Method Circle C; C.set(10,10,100); // from class Circle C.foo (); // from base class Point C.print(); // from class Circle Point A; A.set(30,50); // from base class Point A.print(); // from base class Point

21. 21 Putting Them Together Time is the base class ExtTime is the derived class with public inheritance The derived class can –inherit all members from the base class, except the constructor –access all public and protected members of the base class –define its private data member –provide its own constructor –define its public member functions –override functions inherited from the base class ExtTimeTime

22. 22 class Time Specification class Time { public : void Set ( int h, int m, int s ) ; void Increment ( ) ; void Write ( ) const ; Time ( int initH, int initM, int initS ) ; // constructor Time ( ) ; // default constructor protected : int hrs ; int mins ; int secs ; } ; // SPECIFICATION FILE ( time.h)

23. 23 Class Interface Diagram Protected data: hrs mins secs Set Increment Write Time Time class

24. 24 Derived Class ExtTime // SPECIFICATION FILE ( exttime.h) #include “time.h” enum ZoneType {EST, CST, MST, PST, EDT, CDT, MDT, PDT } ; class ExtTime : public Time // Time is the base class and use public inheritance { public : void Set ( int h, int m, int s, ZoneType timeZone ) ; void Write ( ) const; //overridden ExtTime (int initH, int initM, int initS, ZoneType initZone ) ; ExtTime (); // default constructor private : ZoneType zone ; // added data member } ;

25. 25 Class Interface Diagram Protected data: hrs mins secs ExtTime class Set Increment Write Time Set Increment Write ExtTime Private data: zone

26. 26 Implementation of ExtTime Default Constructor ExtTime :: ExtTime ( ) { zone = EST ; } The default constructor of base class, Time(), is automatically called, when an ExtTime object is created. ExtTime et1; hrs = 0 mins = 0 secs = 0 zone = EST et1

27. 27 Implementation of ExtTime Another Constructor ExtTime :: ExtTime (int initH, int initM, int initS, ZoneType initZone) : Time (initH, initM, initS) // constructor initializer { zone = initZone ; } ExtTime *et2 = new ExtTime(8,30,0,EST); hrs = 8 mins = 30 secs = 0 zone = EST et2 5000 ??? 6000 5000

28. 28 Implementation of ExtTime void ExtTime :: Set (int h, int m, int s, ZoneType timeZone) { Time :: Set (hours, minutes, seconds); // same name function call zone = timeZone ; } void ExtTime :: Write ( ) const // function overriding { string zoneString[8] = {“EST”, “CST”, MST”, “PST”, “EDT”, “CDT”, “MDT”, “PDT”} ; Time :: Write ( ) ; cout <<‘ ‘<<zoneString[zone]<<endl; }

29. 29 Working with ExtTime #include “exttime.h” … int main() { ExtTime thisTime ( 8, 35, 0, PST ) ; ExtTime thatTime ; // default constructor called thatTime.Write( ) ; // outputs 00:00:00 EST thatTime.Set (16, 49, 23, CDT) ; thatTime.Write( ) ; // outputs 16:49:23 CDT thisTime.Increment ( ) ; thisTime.Write ( ) ; // outputs 08:35:02 PST }

30. 30 Inheritance Summary Inheritance is a mechanism for defining new class types to be a specialization or an augmentation of existing types. In principle, every member of a base class is inherited by a derived class with different access permissions, except for the constructors

31. 31 More C++ Concepts Operator overloading Friend Function This Operator Inline Function

32. 32 Review There are different types of member functions in the definition of a class –Accessor int Str :: get_length(); –implementor/worker void Rectangle :: set(int, int); –helper void Date :: errmsg(const char* msg); –constructor Account :: Account(); Account :: Account(const Account& a); Account :: Account(const char *person); –destructor Account :: ~Account();

33. 33 Operator overloading Programmer can use some operator symbols to define special member functions of a class Provides convenient notations for object behaviors

34. 34 int i, j, k; // integers float m, n, p; // floats k = i + j; // integer addition and assignment p = m + n; // floating addition and assignment Why Operator Overloading The compiler overloads the + operator for built-in integer and float types by default, producing integer addition with i+j, and floating addition with m+n. We can make object operation look like individual int variable operation, using operator functions Date a,b,c; c = a + b;

35. 35 Operator Overloading Syntax Syntax is: operator@(argument-list) --- operator is a function --- @ is one of C++ operator symbols (+, -, =, etc..) Examples: operator+ operator- operator* operator/

36. 36 class CStr { char *pData; int nLength; public: // … void cat(char *s); // … friend CStr operator+(CStr str1, CStr str2); friend CStr operator+(CStr str, char *s); friend CStr operator+(char *s, CStr str); //accessors char* get_Data(); int get_Len(); }; Example of Operator Overloading void CStr::cat(char *s) { int n; char *pTemp; n=strlen(s); if (n==0) return; pTemp=new char[n+nLength+1]; if (pData) strcpy(pTemp,pData); strcat(pTemp,s); pData=pTemp; nLength+=n; }

37. 37 The Addition (+) Operator CStr operator+(CStr str1, CStr str2) { CStr new_string(str1); //call the copy constructor to initialize an entirely new CStr object with the first operand new_string.cat(str2.get_Data()); //concatenate the second operand onto the end of new_string return new_string; //call copy constructor to create a copy of the return value new_string } new_string str1 strlen(str1) strcat(str1,str2) strlen(str1)+strlen(str2)

38. 38 How does it work? CStr first(“John”); CStr last(“Johnson”); CStr name(first+last); CStr operator+(CStr str1,CStr str2) { CStr new_string(str1); new_string.cat(str2.get()); return new_string; } “John Johnson” Temporary CStr object Copy constructor name

39. 39 Implementing Operator Overloading Two ways: –Implemented as member functions –Implemented as non-member or Friend functions the operator function may need to be declared as a friend if it requires access to protected or private data Expression obj1@obj2 translates into a function call –obj1.operator@(obj2), if this function is defined within class obj1 –operator@(obj1,obj2), if this function is defined outside the class obj1

40. 40 1.Defined as a member function Implementing Operator Overloading class Complex {... public:... Complex operator +(const Complex &op) { double real = _real + op._real, imag = _imag + op._imag; return(Complex(real, imag)); }... }; c = a+b; c = a.operator+ (b);

41. 41 2.Defined as a non-member function Implementing Operator Overloading class Complex {... public:... double real() { return _real; } //need access functions double imag() { return _imag; }... }; Complex operator +(Complex &op1, Complex &op2) { double real = op1.real() + op2.real(), imag = op1.imag() + op2.imag(); return(Complex(real, imag)); } c = a+b; c = operator+ (a, b);

42. 42 3.Defined as a friend function Implementing Operator Overloading class Complex {... public:... friend Complex operator +( const Complex &, const Complex & );... }; Complex operator +(Complex &op1, Complex &op2) { double real = op1._real + op2._real, imag = op1._imag + op2._imag; return(Complex(real, imag)); } c = a+b; c = operator+ (a, b);

43. 43 What is ‘Friend’? Friend declarations introduce extra coupling between classes – Once an object is declared as a friend, it has access to all non-public members as if they were public Access is unidirectional – If B is designated as friend of A, B can access A’s non-public members; A cannot access B’s A friend function of a class is defined outside of that class's scope

44. 44 More about ‘Friend’ The major use of friends is –to provide more efficient access to data members than the function call –to accommodate operator functions with easy access to private data members Friends can have access to everything, which defeats data hiding, so use them carefully Friends have permission to change the internal state from outside the class. Always recommend use member functions instead of friends to change state

45. 45 Assignment Operator Assignment between objects of the same type is always supported –the compiler supplies a hidden assignment function if you don’t write your own one –same problem as with the copy constructor - the member by member copying –Syntax: class& class::operator=(const class &arg) { //… }

46. 46 Example: Assignment for CStr class CStr& operator=(const CStr &source){ //... Do the copying return *this; } Assignment operator for CStr: CStr& operator=(const CStr & source) Return type - a reference to (address of) a CStr object Argument type - a reference to a CStr object (since it is const, the function cannot modify it) Assignment function is called as a member function of the left operand =>Return the object itself str1=str2; str1.operator=(str2)

47. 47 Overloading stream-insertion and stream-extraction operators In fact, cout > are operator overloading built in C++ standard lib of iostream.h, using operator " >" cout and cin are the objects of ostream and istream classes, respectively We can add a friend function which overloads the operator << friend ostream& operator<< (ostream &ous, const Date &d); ostream& operator<<(ostream &os, const Date &d) { os<<d.month<<“/”<<d.day<<“/”<<d.year; return os; } … cout<< d1; //overloaded operator ostream& operator<<(ostream &os, const Date &d) { os<<d.month<<“/”<<d.day<<“/”<<d.year; return os; } … cout<< d1; //overloaded operator cout ---- object of ostream

48. 48 Overloading stream-insertion and stream-extraction operators We can also add a friend function which overloads the operator >> istream& operator>> (istream &in, Date &d) { char mmddyy[9]; in >> mmddyy; // check if valid data entered if (d.set(mmddyy)) return in; cout<< "Invalid date format: "<<d<<endl; exit(-1); } friend istream& operator>> (istream &in, Date &d); cin ---- object of istream cin >> d1;

49. 49 The “this” pointer Within a member function, the this keyword is a pointer to the current object, i.e. the object through which the function was called C++ passes a hidden this pointer whenever a member function is called Within a member function definition, there is an implicit use of this pointer for references to data members pData nLength this Data member referenceEquivalent to pDatathis->pData nLengththis->nLength CStr object (*this)

50. 50 Inline functions An inline function is one in which the function code replaces the function call directly. Inline class member functions –if they are defined as part of the class definition, implicit –if they are defined outside of the class definition, explicit, I.e.using the keyword, inline. Inline functions should be short (preferable one- liners). –Why? Because the use of inline function results in duplication of the code of the function for each invocation of the inline function

51. 51 class CStr { char *pData; int nLength; … public: … char *get_Data(void) {return pData; }//implicit inline function int getlength(void); … }; inline void CStr::getlength(void) //explicit inline function { return nLength; } … int main(void) { char *s; int n; CStr a(“Joe”); s = a.get_Data(); n = b.getlength(); } Example of Inline functions Inline functions within class declarations Inline functions outside of class declarations In both cases, the compiler will insert the code of the functions get_Data() and getlength() instead of generating calls to these functions

52. 52 Inline functions An inline function can never be located in a run-time library since the actual code is inserted by the compiler and must therefore be known at compile-time. It is only useful to implement an inline function when the time which is spent during a function call is long compared to the code in the function.

53. 53 Overloading Summary Operator overloading provides convenient notations for object behaviors There are three ways to implement operator overloading –member functions –normal non-member functions –friend functions

54. 54 Polymorphism

55. 55 Object-Oriented Concept Encapsulation –ADT, Object Inheritance –Derived object Polymorphism –Each object knows what it is

56. 56 Polymorphism – An Introduction noun, the quality or state of being able to assume different forms - Webster An essential feature of an OO Language It builds upon Inheritance

57. 57 Before we proceed…. Inheritance – Basic Concepts –Class Hierarchy Code Reuse, Easy to maintain –Type of inheritance : public, private –Function overriding

58. 58 Class Interface Diagram Protected data: hrs mins secs ExtTime class Set Increment Write Time Set Increment Write ExtTime Private data: zone Time class

59. 59 Why Polymorphism?--Review: Time and ExtTime Example by Inheritance void Print (Time someTime ) //pass an object by value { cout << “Time is “ ; someTime.Write ( ) ; cout << endl ; } CLIENT CODE Time startTime ( 8, 30, 0 ) ; ExtTime endTime (10, 45, 0, CST) ; Print ( startTime ) ; Print ( endTime ) ; OUTPUT Time is 08:30:00 Time is 10:45:00 // Time :: write()

60. 60 Static Binding When the type of a formal parameter is a parent class, the argument used can be: the same type as the formal parameter, or, any derived class type. Static binding is the compile-time determination of which function to call for a particular object based on the type of the formal parameter When pass-by-value is used, static binding occurs

61. 61 Polymorphism – An Introduction noun, the quality or state of being able to assume different forms - Webster An essential feature of an OO Language It builds upon Inheritance Allows run-time interpretation of object type for a given class hierarchy –Also Known as “Late Binding” Implemented in C++ using virtual functions

62. 62 Dynamic Binding Is the run-time determination of which function to call for a particular object of a derived class based on the type of the argument Declaring a member function to be virtual instructs the compiler to generate code that guarantees dynamic binding Dynamic binding requires pass-by-reference

63. 63 Virtual Member Function // SPECIFICATION FILE ( time.h ) class Time { public :... virtual void Write ( ) ; // for dynamic binding virtual ~Time(); // destructor private : int hrs ; int mins ; int secs ; } ;

64. 64 This is the way we like to see… void Print (Time * someTime ) { cout << “Time is “ ; someTime->Write ( ) ; cout << endl ; } CLIENT CODE Time startTime( 8, 30, 0 ) ; ExtTime endTime(10, 45, 0, CST) ; Time *timeptr; timeptr = &startTime; Print ( timeptr ) ; timeptr = &endTime; Print ( timeptr ) ; OUTPUT Time is 08:30:00 Time is 10:45:00 CST Time::write() ExtTime::write()

65. 65 Virtual Functions Virtual Functions overcome the problem of run time object determination Keyword virtual instructs the compiler to use late binding and delay the object interpretation How ? –Define a virtual function in the base class. The word virtual appears only in the base class –If a base class declares a virtual function, it must implement that function, even if the body is empty –Virtual function in base class stays virtual in all the derived classes –It can be overridden in the derived classes –But, a derived class is not required to re-implement a virtual function. If it does not, the base class version is used

66. 66 Polymorphism Summary: When you use virtual functions, compiler store additional information about the types of object available and created Polymorphism is supported at this additional overhead Important : –virtual functions work only with pointers/references –Not with objects even if the function is virtual –If a class declares any virtual methods, the destructor of the class should be declared as virtual as well.

67. 67 Abstract Classes & Pure Virtual Functions Some classes exist logically but not physically. Example : Shape –Shape s; // Legal but silly..!! : “Shapeless shape” –Shape makes sense only as a base of some classes derived from it. Serves as a “category” –Hence instantiation of such a class must be prevented class Shape //Abstract { public : //Pure virtual Function virtual void draw() = 0; } A class with one or more pure virtual functions is an Abstract Class Objects of abstract class can’t be created Shape s; // error : variable of an abstract class

68. 68 Example Shape virtual void draw() Circle public void draw() Triangle public void draw()

69. 69 A pure virtual function not defined in the derived class remains a pure virtual function. Hence derived class also becomes abstract class Circle : public Shape { //No draw() - Abstract public : void print(){ cout << “I am a circle” << endl; } class Rectangle : public Shape { public : void draw(){ // Override Shape::draw() cout << “Drawing Rectangle” << endl; } Rectangle r; // Valid Circle c; // error : variable of an abstract class

70. 70 Pure virtual functions : Summary Pure virtual functions are useful because they make explicit the abstractness of a class Tell both the user and the compiler how it was intended to be used Note : It is a good idea to keep the common code as close as possible to the root of you hierarchy

71. 71 Summary..continued It is still possible to provide definition of a pure virtual function in the base class The class still remains abstract and functions must be redefined in the derived classes, but a common piece of code can be kept there to facilitate reuse In this case, they can not be declared inline class Shape { //Abstract public : virtual void draw() = 0; }; // OK, not defined inline void Shape::draw(){ cout << “Shape" << endl; } class Rectangle : public Shape { public : void draw(){ Shape::draw(); //Reuse cout <<“Rectangle”<< endl; }

72. 72 Polymorphism Summary Polymorphism is built upon class inheritance It allows different versions of a function to be called in the same manner, with some overhead Polymorphism is implemented with virtual functions, and requires pass-by-reference