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CIS 4930 Application Development Using C++ Dr. Kun Suk Kim CISE Department, University of Florida

CIS 4930 Application Development Using C++ Dr. Kun Suk Kim CISE Department, University of Florida. Inheritance. Objectives. Inheritance Basics Derived classes, with constructors protected: qualifier Redefining member functions Non-inherited functions Programming with Inheritance

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CIS 4930 Application Development Using C++ Dr. Kun Suk Kim CISE Department, University of Florida

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  1. CIS 4930Application Development Using C++Dr. Kun Suk KimCISE Department, University of Florida Inheritance

  2. Objectives • Inheritance Basics • Derived classes, with constructors • protected: qualifier • Redefining member functions • Non-inherited functions • Programming with Inheritance • Assignment operators and copy constructors • Destructors in derived classes • Multiple inheritance

  3. Introduction to Inheritance • Object-oriented programming • Powerful programming technique • Provides abstraction dimension calledinheritance • General form of class is defined • Specialized versions then inherit properties ofgeneral class • And add to it/modify it’s functionality for it’sappropriate use

  4. Inheritance Basics • New class inherited from another class • Base class • ‘General’ class from which others derive • Derived class • New class • Automatically has base class’s: • Member variables • Member functions • Can then add additional member functionsand variables

  5. Example • Consider example: Class of ‘Person’

  6. Interface for Person Class – person.h #ifndef PERSON_H #define PERSON_H #include <string> using std::string; namespace namePerson { class Person { public: Person( ); Person(string theName, string theSsn); string getName( ) const; string getSsn( ) const; void setName(string theName); void setSsn(string theSsn); void printStatus( ) const; private: string name; string ssn; }; }//namePerson #endif //PERSON_H

  7. Implementation for Person Classes - person.cpp #include <string> #include <cstdlib> #include <iostream> #include "person.h" using std::string; using std::cout; namespace namePerson { Person::Person( ) : name("-"), ssn("-") { } Person::Person(string theName, string theSsn) : name(theName), ssn(theSsn) { } string Person::getName( ) const { return name; }

  8. Implementation for Person Classes - person.cpp string Person::getSsn( ) const { return ssn; } void Person::setName(string theName) { name = theName; } void Person::setSsn(string theSsn) { ssn = theSsn; } void Person::printStatus() const { cout << "printStatus called for undifferenctiated person.\n"; cout << "Abnormally exit the program.\n"; exit(1); } }//namePerson

  9. Derived Classes • Persons are composed of: • Students • Faculties • Each is ‘subset’ of Person

  10. Derived Classes • Don’t ‘need’ type of generic ‘person’ • Since no one’s just an ‘person’ • General concept of person helpful! • All have names • All have social security numbers • Associated functions for these ‘basics’ aresame among all persons • So ‘general’ class can contain all these‘things’ about persons

  11. Person Class • Many members of ‘person’ class applyto all types of persons • Accessor functions • Mutator functions • Most data items: • SSN • Name • We won’t have ‘objects’ of this class,however

  12. Person Class • Consider printStatus() function: • Will always be ‘redefined’ in derived classes • So different person types can havedifferent status • Makes no sense really for ‘undifferentiated’person • So function printStatus() in Person classsays just that • Error message stating “printStatus called forundifferentiated person. Abnormally exit…”

  13. Deriving from Person Class • Derived classes from Person class: • Automatically have all member variables • Automatically have all member functions • Derived class said to ‘inherit’ membersfrom base class • Can then redefine existing membersand/or add new members

  14. Interface for Student Class – student.h #ifndef STUDENT_H #define STUDENT_H #include <string> #include "person.h" using std::string; namespace namePerson { class Student : public Person { public: Student( ); Student(string theName, string theSsn, int theGrade, double theGpa); void setGrade(int theGrade); int getGrade( ) const; void setGpa(double theGpa); double getGpa( ) const; void printStatus( ) ; private: int grade; double gpa; }; }//namePerson #endif //STUDENT_H

  15. Implementation for Student Class – student.cpp #include <string> #include <iostream> #include "student.h" using std::string; using std::cout; using std::endl; namespace namePerson { Student::Student( ) : Person( ), grade(0), gpa(0.0) { } Student::Student(string theName, string theSsn, int theGrade, double theGpa) : Person(theName, theSsn), grade(theGrade), gpa(theGpa) { } void Student::setGrade(int theGrade) { grade = theGrade; }

  16. Implementation for Student Class – student.cpp int Student::getGrade( ) const { return grade; } void Student::setGpa(double theGpa) { gpa = theGpa; } double Student::getGpa( ) const { return gpa; } void Student::printStatus( ) { cout << "Name: " << getName( ) << endl << "SSN Number: " << getSsn( ) << endl << "Student Records" << " Grade: " << getGrade() << " GPA: " << getGpa() << endl; } }//namePerson

  17. Interface for Faculty Class – faculty.h #ifndef FACULTY_H #define FACULTY_H #include <string> #include "person.h" using std::string; namespace namePerson { class Faculty : public Person { public: Faculty( ); Faculty (string theName, string theSsn, string theArea); string getArea( ) const; void setArea(string theArea); void printStatus( ); private: string area; }; }//namePerson #endif //FACULTY_H

  18. Implementation for Faculty Class – faculty.cpp #include <iostream> #include <string> #include "faculty.h" using std::string; using std::cout; using std::endl; namespace namePerson { Faculty::Faculty( ) : Person( ), area("-") { } Faculty::Faculty(string theName, string theSsn, string theArea) : Person(theName, theSsn), area(theArea) { } string Faculty::getArea() const { return area; }

  19. Implementation for Faculty Class – faculty.cpp void Faculty::setArea(string theArea) { area = theArea; } void Faculty::printStatus( ) { cout << "Name: " << getName( ) << endl << "SSN Number: " << getSsn( ) << endl << "Faculty Records" << endl << " Area: " << getArea() << endl; } }//namePerson

  20. Driver Program #include <iostream> #include "student.h" #include "faculty.h" using std::cout; using std::endl; using namePerson::Student; using namePerson::Faculty; int main( ) { Student st; st.setName("Study Hard"); st.setSsn("111-22-3333"); st.setGrade(4); st.setGpa(3.5); st.printStatus( ); Faculty prof("Prof. WorkHard", "444-55-6666", "C++"); prof.printStatus( ); return 0; } Name: Study Hard SSN Number: 111-22-3333 Student Records Grade: 4 GPA: 3.5 Name: Prof. WorkHard SSN Number: 444-55-6666 Faculty Records Area: C++

  21. Student Class Interface • Note definition begins same as any other • #ifndef structure • Includes required libraries • Also includes person.h • And, the heading:class Student : public Person{ … • Specifies ‘publicly inherited’ from Personclass

  22. Student Class Additions • Derived class interface only lists new or‘to be redefined’ members • Since all others inherited are already defined • i.e.: ‘all’ persons have ssn and name • Student adds: • Constructors • grade, gpa member variables • setGrade(), getGrade(), setGpa(), getGpa()member functions

  23. Student Class Redefinitions • Student redefines: • printStatus() member function • This ‘overrides’ the printStatus() functionimplementation from Person class • Its definition must be in Student class’s implementation • As do other member functions declared inStudent’s interface • New and ‘to be redefined’

  24. Inheritance Terminology • Common to simulate family relationships • Parent class • Refers to base class • Child class • Refers to derived class • Ancestor class • Class that’s a parent of a parent … • Descendant class • Opposite of ancestor

  25. Constructors in Derived Classes • Base class constructors are NOTinherited in derived classes! • But they can be invoked within derived classconstructor • Which is all we need! • Base class constructor must initialize allbase class member variables • Those inherited by derived class • So derived class constructor simply calls it • ‘First’ thing derived class constructor does

  26. Derived Class Constructor Example • Consider syntax for Student constructor:Student:: Student(string theName, string theSsn, int theGrade, double theGpa) : Person(theName, theSsn), grade(theGrade), gpa(theGpa) { } • Portion after : is ‘initialization section’ • Includes invocation of Person constructor

  27. Another Student Constructor • A second constructor:Student::Student() : Person(), grade(0), gpa(0.0) { } • Default version of base class constructoris called (no arguments) • Should always invoke one of the baseclass’s constructors

  28. Constructor: No Base Class Call • Derived class constructor should alwaysinvoke one of the base class’s constructors • If you do not: • Default base class constructor automaticallycalled • Equivalent constructor definition:Student::Student(): grade(0), gpa(0.0) { }

  29. Pitfall: Base Class Private Data • Derived class ‘inherits’ private membervariables • But still cannot directly access them • Not even through derived class memberfunctions! • Private member variables can ONLY beaccessed ‘by name’ in member functionsof the class they’re defined in

  30. Pitfall: Base Class Private Member Functions • Same holds for base class memberfunctions • Cannot be accessed outside interface andimplementation of base class • Not even in derived class member functiondefinitions

  31. Pitfall: Base Class Private Member Functions Impact • Larger impact here vs. member variables • Member variables can be accessed indirectlyvia accessor or mutator member functions • Member functions simply not available • This is ‘reasonable’ • Private member functions should be simply‘helper’ functions • Should be used only in class they’re defined

  32. The protected: Qualifier • New classification of class members • Allows access ‘by name’ in derived class • But nowhere else • Still no access ‘by name’ in other classes • In class it’s defined  acts like private • Considered ‘protected’ in derived class • To allow future derivations • Many feel this ‘violates’ information hiding

  33. Redefinition of Member Functions • Recall interface of derived class: • Contains declarations for new memberfunctions • Also contains declarations for inheritedmember functions to be changed • Inherited member functions NOT declared: • Automatically inherited unchanged • Implementation of derived class will: • Define new member functions • Redefine inherited functions as declared

  34. Redefining vs. Overloading • Very different! • Redefining in derived class: • SAME parameter list • Essentially ‘re-writes’ same function • Overloading: • Different parameter list • Defined ‘new’ function that takes differentparameters • Overloaded functions must have differentsignatures

  35. A Function’s Signature • Definition of a ‘signature’: • Function’s name • Sequence of types in parameter list • Including order, number, types • Signature does NOT include: • Return type • const keyword • &

  36. Accessing Redefined Base Function • When redefined in derived class, baseclass’s definition not ‘lost’ • Can specify it’s use:Person p;Student s; p.printStatus();  calls Person’s printStatus function s.printStatus();  calls Student’s printStatus function s.Person::printStatus();  Calls Person’s printStatus function! • Not typical here, but useful sometimes

  37. Functions Not Inherited • All ‘normal’ functions in base class areinherited in derived class • Exceptions: • Constructors (we’ve seen) • Destructors • Copy constructor • But if not defined, generates ‘default’ one • Recall need to define one for pointers! • Assignment operator • If not defined  default

  38. Assignment Operators and Copy Constructors • Recall: overloaded assignment operatorsand copy constructors NOT inherited • But can be used in derived class definitions • Typically MUST be used! • Similar to how derived class constructorinvokes base class constructor

  39. Assignment Operator Example • Given ‘Derived’ is derived from ‘Base’:Derived& Derived::operator =(const Derived & rightSide){ Base::operator =(rightSide); …} • Notice code line • Calls assignment operator from base class • This takes care of all inherited member variables • Would then set new variables from derivedclass…

  40. Copy Constructor Example • Consider:Derived::Derived(const Derived& Object) : Base(Object), …{…} • After : is invocation of base copyconstructor • Sets inherited member variables of derivedclass object being created • Note Object is of type Derived; but it’s also oftype Base, so argument is valid

  41. Destructors in Derived Classes • If base class destructor functions correctly • Easy to write derived class destructor • When derived class destructor is invoked: • Automatically calls base class destructor! • So no need for explicit call • So derived class destructors need only beconcerned with derived class variables • And any data they ‘point’ to • Base class destructor handles inherited dataautomatically

  42. Destructor Calling Order • Consider:class B derives from class Aclass C derives from class B A  B  C • When object of class C goes out of scope: • Class C destructor called first • Then class B destructor called • Finally class A destructor is called • Opposite of how constructors are called

  43. “Is a” vs. “Has a” Relationships • Inheritance • Considered an “Is a” class relationship • e.g.: An Student “is a” Person • A Convertible “is a” Automobile • A class contains objects of another classas it’s member data • Considered a “Has a” class relationship • e.g.: One class “has a” object of anotherclass as it’s data

  44. Protected and Private Inheritance • New inheritance ‘forms’ • Both are rarely used • Protected inheritance:class Faculty : protected Person{…} • Public members in base class becomeprotected in derived class • Private inheritance:class Faculty : private Person{…} • All members in base class become privatein derived class

  45. Multiple Inheritance • Derived class can have more than onebase class! • Syntax just includes all base classesseparated by commas:class derivedMulti : public base1, base2{…} • Possibilities for ambiguity are endless! • Dangerous undertaking! • Some believe should never be used • Certainly should only be used be experiencedprogrammers!

  46. Summary 1 • Inheritance provides code reuse • Allows one class to ‘derive’ from another,adding features • Derived class objects inherit members ofbase class • And may add members • Private member variables in base classcannot be accessed ‘by name’ in derived • Private member functions are not inherited

  47. Summary 2 • Can redefine inherited member functions • To perform differently in derived class • Protected members in base class: • Can be accessed ‘by name’ in derived classmember functions • Overloaded assignment operator notinherited • But can be invoked from derived class • Constructors are not inherited • Are invoked from derived class’s constructor

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