Inheritance - ipn · I. Hrivnacova @ Data Processing Course 2019 7 Inheritance The common behavior of objects can be expressed by means of inheritance – There is no equivalent data

I. Hrivnacova @ Data Processing Course 2019 1

Inheritance

Data Processing Course,I. Hrivnacova, IPN Orsay

● OOP, Classes – Reminder● Inheritance Relationship● Derived Class Definition● Overriding Functions● Polymorphism● Pure Virtual Functions and

Abstract Classes


Object-Oriented Programming (OOP)● The basic unit of OOP is a class● A class is an implementation of an

abstract data type:– It describes both the attributes

(data) of an object and itsoperations (methods)

● The OOP languages let you thinkin the problem space, and usesoftware objects to represent andabstract entities of the problemspace to solve the problem.

Chua Hock-Chuan: Programming Notes


A ClassA class can be visualized as a three-compartmentbox, as illustrated:

● Classname (or identifier): identifies the class.● Data Members (or variables, attributes, states,

fields): – contain the static attributes of the class.

● Member Functions (or methods, behaviors,operations): – contain the dynamic operations of the class.

● In other words, a class encapsulates the staticattributes (data) and dynamic behaviors(operations that operate on the data) in a box.

● Class Members: The data members andmember functions are collectively called classmembers.Chua Hock-Chuan: Programming Notes


Example Of Classes, Instances

● The class and instances diagrams aredrawn according to the UnifiedModeling Language (UML) notations.

Chua Hock-Chuan: Programming Notes


Class Definition

class Car { private: int yearOfManufacture; int mileage; int licensePlate;

public: int getYearOfManufacture(); void drive(float m); float getMileage();

void Car(); void ~Car();};

The members declared afterprivate are protected from beingaccessed by application programers(Data encapsulation)

For the application developeronly the public interface isof interest.


Inheritance

● The real life example of inheritance is child and parents, all theproperties of father are inherited by his son.

Source: http://www.sitesbay.com/cpp/cpp-inheritance


Inheritance

● The common behavior of objects can be expressed by meansof inheritance– There is no equivalent data model in conventional (procedural)

programming● A class A can inherit the state (variables) and behavior

(methods) of another class B– The relationship "Is a" ( "Inherits from")– We say class A is a base class (or a generalization, or a super-type)

of class B and class B is a class derived from Class A (or aspecialization, or a subtype)


Inheritance – Example● The Vehicle class describes the

properties of a vehicle as a baseclass (common properties)

● Class hierarchy arises wherecommon terms become more andmore concrete

● Inheritance is the "is-a"relationship: – a truck is a "vehicle with a

speedometer"– a "vehicle with a speedometer" is a

vehicleNicolai M. Josuttis: Object-Oriented Programming in C++


Inheritance -Class Definition

Shape

mWidthmHeight

setData()

Rectangle

getArea()

Triangle

getArea)

class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } protected: float mWidth; float mHeight;};

class Rectangle : public Shape { public: float getArea() { return mWidth * mHeight; }};

class Triangle : public Shape { public: float getArea() { return mWidth * mHeight / 2.; }};


Inheritance -Test Program

class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } protected: float mWidth; float mHeight;};

class Rectangle : public Shape { public: float getArea() { return mWidth * mHeight; }};

class Triangle : public Shape { public: float getArea() { return mWidth * mHeight / 2.; }};

#include "shapes.h"#include using namespace std;

int main(){ // Create object of derived type Rectangle r1; // Call function of base class r1.setData(3., 4.); // Call function of derived class cout


Inheritance -Test Program

#include "shapes.h"#include using namespace std;

int main(){ // Create object of derived type Rectangle r1; // Call function of base class r1.setData(3., 4.); // Call function of derived class cout


Derived Class Definition

● The syntax for definition of a derived class: class Derived : public Base { ... };

– The inheritance with private and protected member access specifierwill not be discussed in this course

● The derived class inherits data and functions of the base class● The protected members of the base class can be accessed

by the derived class, the private members are inaccessible


Overriding Functions

Shape

mWidthmHeight

setData()getArea)

Rectangle

getArea()

Triangle

getArea)

class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } virtual float getArea() { return 0; } protected: float mWidth; float mHeight;};

class Rectangle : public Shape { public: virtual float getArea() { return mWidth * mHeight; }};

class Triangle : public Shape { public: virtual float getArea() { return mWidth * mHeight / 2.; }};


class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } virtual float getArea() { return 0; } protected: float mWidth; float mHeight;};



// Test program#include "shapes.h"#include using namespace std;

int main() { // Create object of derived type Rectangle r1; // Get base class pointer to rectangle Shape* s1 = &r1; s1->setData(3., 4.); cout


// Test program#include "shapes.h"#include using namespace std;

int main() { // Create object of derived type Rectangle r1; // Get base class pointer to rectangle Shape* s1 = &r1; s1->setData(3., 4.); cout


Overriding Functions

● A member function of a class can be redefined in its derivedclasses if it is declared with keyword virtual.– The member function getArea() has been declared as virtual– The virtual keyword in derived classes is not necessary, but it is

a good to use it to make the virtual functions "visible".● The advantage of having virtual function is that we are able to

access the function of a derived class by a pointer variable ofbase class.

● Non virtual functions with the same name in a class hierarchyshould be avoided


Polymorphism// test program#include "shapes.h"#include #include using namespace std;

int main(){ // Create objects Rectangle r1; r1.setData(3., 4.); Triangle t1; t1.setData(5., 6.); Shape s3; s3.setData(6., 7.);

// Handle object via base type vector shapes; shapes.push_back(&r1); shapes.push_back(&t1); shapes.push_back(&s3); for ( Shape* shape : shapes ) { cout


Polymorphism// test program#include "shapes.h"#include #include using namespace std;


// Handle object via base type vector shapes; shapes.push_back(&r1); shapes.push_back(&t1); shapes.push_back(&s3); for ( Shape* shape : shapes ) { cout


Polymorphism

● An ability to create an object that has more than one form● The objects belonging to different types can respond to method

( field, or property) calls of the same name, each one accordingto an appropriate type-specific behavior.

● The programmer (and the program) does not have to know theexact type of the object in advance, and so the exact behavioris determined at run-time: this is called late binding or dynamicbinding


Pure Virtual Functions

Shape

mWidthmHeight

setData()getArea)

Rectangle

getArea()

Triangle

getArea)

class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } virtual float getArea() = 0;

protected: float mWidth; float mHeight;};




// test program#include "shapes.h"#include #include using namespace std;


// ...}

class Shape { public: void setData(float w, float h) { mWidth = w; mHeight = h; } virtual float getArea() = 0;

protected: float mWidth; float mHeight;};

code.cxx:41:9: error: variable type 'Shape' is an abstract class Shape s3; ^code.cxx:7:17: note: unimplemented pure virtual method 'getArea' in 'Shape' virtual float getArea() = 0; ^


Abstract Classes

● It is impossible to create an instance of the class that has notdefined all inherited pure virtual functions

● Classes whose instance cannot be created are called abstract classes.

● We can force derived classes to implement the interfacesrequired by declaring pure virtual functions– All classes derived from Shape MUST implement getArea()

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Inheritance - ipn · I. Hrivnacova @ Data Processing Course 2019 7 Inheritance The common behavior of objects can be expressed by means of inheritance – There is no equivalent data