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Inheritance Concepts

Derive a new class (subclass) from an existing class (base class or superclass).

Inheritance creates a hierarchy of related classes (types) which share code and interface.

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Inheritance Examples

Base Class Derived Classes

Student CommuterStudentResidentStudent

Shape CircleTriangleRectangle

Loan CarLoanHomeImprovementLoanMortgageLoan

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Animal class hierarchy

Wild

Animals

Domestic

Tiger Lion Bear Dog Cat horse

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Credit cardslogo

americanexpress

hologram

cardowner’s name

inheritsfrom (is a)

visacard

mastercard

pin category

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Implementing Inheritance in C++

Develop a base class called student

Use it to define a derived class called grad_student

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The Student Class Hierarchy

studentprint()

grad_student

inherits (is a)

student_id,year, name

dept,thesis

Inherits data and methods from base class

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The Student Class Hierarchy

studentprint()

grad_studentprint()

inherits (is a)

student_id,year, name

dept,thesis Override print()

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Student Class

class student {public: student(string nm, int id, int y); void print();private: int student_id; int year; string name;};

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Member functions

student::student(string nm, int id, int y){ student_id = id; year = y; name = nm;}

void student::print(){ cout << "\n STUDENT " << name;}

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Graduate Student Class grad_student.h

class grad_student: public student {public: grad_student(string nm, int id, int y, string d, string th); void print();private: string dept; string thesis;};

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grad_student.cpp

grad_student::grad_student(string nm, int id, int y, string d, string th)

:student(nm, id, y){ dept = d; thesis = th;}

void grad_student::print(){ cout << “GRAD STUDENT “ << name << dept << ", " << thesis << endl;}

Calls base class constructor

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Examples of Use grad_student * g = new grad_student(…); g->print(); // grad student print student * s = new student(…); s->print(); // student print

No concern about which print is called

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Examples of Use

student * joe; if (…) joe = new grad_student(); else joe = new student(); joe->print(); Dilemma - which print do you want?

joe->print(); //student print if NOT virtual joe->print(); //correct print if virtual “virtual” means “Make a run time decision”

Base class pointer can store any descendant class

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Examples of Use

student * all[SIZE]; all[0] = new student(…); all[1] = new grad_student(); all[2] = new student(…); all[3] = new grad_student(); … for (i=0; I < SIZE;i++) all[i]->print(); Dilemma - which print do you want?

Base class pointer can store any descendant class

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Two Types of Binding

Static Binding (the default in C++)– y->print() uses y’s print– this is known at compile time

Dynamic Binding– all[i]->print() uses the print() in the

object pointed at– this is only known at run time– coded in C++ with virtual functions

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Student Class

class student {public: student(string nm, int id, int y); void virtual print();private: int student_id; int year; string name;};

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Representing Shapes: abstract base class

shape

rectangle

square

triangle circle • • • •

inherits (is a)

shape is only a categoryand cannot be instantiated

Why do we want an abstract base class?

Helpful for organization Can use a pointer to a base class to store actual

instantiations of derived classes

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C++ Shape Classesclass shape {public: virtual double area() = 0; //abstract

// shape will NOT define area

// but forces all derived classes to define area

};

class rectangle: public shape {public: double area() const {return (height*width);}

:private: double height, width;};

Every descendant of shape MUST have an area method.

class circle: public shape {public: double area() {return (PI*radius*radius);}

:private: double radius;};

// etc

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Use:shape* p[N];circle c1,...;rectangle r1,...; :// fill in p with pointers to // circles, squares, etcp[0] = &c1; p[1] = &r1; ... : :// calculate total areafor (i = 0; i < N; ++i) tot_area = tot_area + p[i]->area();