Inheritance and Composition (aka containment)

Textbook Chapter 14

• 14.1 – 14.4

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Objectives

• Learn about inheritance

• Learn about derived and base classes

• Explore how to redefine the member functions of a base class

• Examine how the constructors of base and derived classes work

• Learn how to construct the header file of a derived class

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Objectives

• Become familiar with the C++ stream hierarchy

• Learn about containment

• Become familiar with the three basic principles of object-oriented design

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Inheritance and Composition

• The two common ways to relate two classes in a meaningful way are:

1. Inheritance (“is-a” relationship)

2. Composition/Containment (“has-a” relationship)

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Inheritance

• Inheritance is an “is-a” relationship

• For instance,“every employee is a person”

• Inheritance lets us create new classes from existing classes

• New classes are called the derived classes

• Existing classes are called the base classes

• Derived classes inherit the properties of the base classes

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Inheritance HierarchyAmong Vehicles

vehicle

wheeled vehicle boat

bicyclecar

four-door two-door

Every car is a wheeled vehicle.

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Inheritance

• is a mechanism by which one class acquires (inherits) the properties (both data and operations) of another class

• the class being inherited from is the Base Class

• the class that inherits traits is the Derived Class

• the derived class is specialized by adding properties specific to it

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Inheritance (continued)

• Single inheritance: derived class has a single base class– Multiple Inheritance possible but not used in CS215

• Can be viewed as a tree (hierarchy) where a base class is shown with its derived classes

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Inheritance (continued)

• Private members of the base class are private to the base class– Members of the derived class cannot directly

access them

• Public members of a base class can be inherited either as public members or as private members by the derived class

• The derived class can include additional data and/or function members

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Inheritance (continued)

• Derived class can redefine public member functions of base class

• Redefinition applies only to objects of the derived class, not to the base class

• All data/function members of the base class are also data/function members of the derived class (but can only be accessed via public members of the base class)

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Base Class TimeType// SPECIFICATION FILE ( timetype.h )

class TimeType{

public :

void Set ( int hours , int minutes , int seconds ) ;void Increment ( ) ;void Write ( ) const ;TimeType ( int initHrs, int initMins, int initSecs ) ; // constructor TimeType ( ) ; // default constructor

private :

int hrs ; int mins ; int secs ;

} ;12

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Class Interface Diagram

Private data:

hrs

mins

secs

Set

Increment

Write

Time

Time

TimeType class

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Header File of a Derived Class

• To define new classes– Create new header files

• To create new classes based on previously defined classes– Header files of the new classes contain commands

that specify where to look for the definitions of the base classes

• The definitions of the member functions are placed in a separate implementation file

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Using Inheritance to Add Features #ifndef EXTTIME //Always have file guard#define EXTTIME// SPECIFICATION FILE ( exttime.h)#include “TimeType.h” //include the base class

class ExtTime : public TimeType // TimeType is the base class{public :

void Set ( int hours, int minutes, int seconds , string timeZone ) ;

void Write ( ) ; ExtTime ( int initHrs , int initMins , int initSecs ,

ZoneType initZone ) ; // constructor ExtTime ( ) ; // default constructor

private :string zone ; // added data member

} ;#endif // End of file: end file guard preprocessor if statement

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class ExtTime: public Time

• says class Time is a public base class of the derived class ExtTime

• as a result, all public members of Time (except constructors) are also public members of ExtTime

• in this example, new constructors are provided, new data member “zone” is added, and member functions Set and Write are overridden

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Class Interface Diagram

Private data:

hrs

mins

secs

ExtTime class

Set

Increment

Write

Time

Time

Set

Increment

Write

ExtTime

ExtTime

Private data:zone

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Client Code Using ExtTime

#include “exttime.h” // specification file of ExtTime class . . .

ExtTime thisTime ( 8, 35, 0, “PST” ) ; ExtTime thatTime ; // default constructor called

thatTime.Write( ) ; // outputs 00:00:00 EST cout << endl ;

thatTime.Set (16, 49, 23, “CDT”) ; thatTime.Write( ) ; // outputs 16:49:23 CDT

cout << endl ;

thisTime.Increment ( ) ; thisTime.Increment ( ) ; thisTime.Write ( ) ; // outputs 08:35:02 PST cout << endl ;

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ExtTime Implementation File

• Constructors defined

• All added public member functions defined

• All changed base class public member function defined (overridden)

• Destructor (if needed) defined

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Start of Implementation File// exttime.cpp implementation file// Use includes// needed by any of the member functions#include<string> // strings used#include<iostream> // IO usedusing namespace std;// Always include header file of THIS CLASS// DO NOT INCLUDE BASE CLASS HEADER#include “exttime.h”

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Constructor Rules for Derived Classes

• at run time, the base class constructor is implicitly called first, before the body of the derived class’s constructor executes

• if the base class constructor requires parameters, they must be passed by the derived class’s constructor

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Constructors of Derived and Base Classes• Derived class constructor cannot directly

access private members of the base class• Derived class can initialize private data

members of the derived class• When a derived object is declared

– It must execute one of the base class constructors

• Call to the base class constructor is specified in the heading of derived class constructor definition

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Implementation of ExtTime Default Constructor

ExtTime :: ExtTime ( )

// Default Constructor Postcondition:

// hrs == 0 && mins == 0 && secs == 0

// (via an implicit call to base class default constructor )

// && zone == EST

{

zone = “EST” ;

}

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Implementation of Another ExtTime Class Constructor

ExtTime :: ExtTime ( /* in */ int initHrs,

/* in */ int initMins,

/* in */ int initSecs,

/* in */ string initZone )

: TimeType (initHrs, initMins, initSecs) // base class constructor

// Precondition: 0 <= initHrs <= 23 && 0 <= initMins <= 59

// 0 <= initSecs <= 59 && initZone is assigned

// Postcondition:

// zone == initZone && Time set by base class constructor

{

zone = initZone ;

} 24

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Redefining (Overriding) Member Functions of the Base Class

• To redefine a public member function of a base class

– Corresponding function in the derived class must have the same name, number, and types of parameters

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Redefining (Overriding) Member Functions of the Base Class (continued)

• If derived class overrides a public member function of the base class, then to call the base class function, specify:

– Name of the base class

– Scope resolution operator (::)

– Function name with the appropriate parameter list

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Implementation of ExtTime::Set function void ExtTime :: Set ( /* in */ int hours,

/* in */ int minutes,

/* in */ int seconds,

/* in */ string timeZone )

// Precondition: 0 <= hours <= 23 && 0 <= minutes <= 59

// 0 <= seconds <= 59 && timeZone is assigned

// Postcondition:

// zone == timeZone && Time set by base class function

{

TimeType :: Set (hours, minutes, seconds);

zone = timeZone ;

}27

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Implementation of ExtTime::Write Function

void ExtTime :: Write ( )

// Postcondition:

// Time has been output in form HH:MM:SS ZZZ

// where ZZZ is the time zone abbreviation

{

TimeType :: Write ( ) ;

cout << ‘ ‘ << zone;

}28

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Class Files

Each class (including derived classes used in inheritance) has:

• Its own specification (header - .h) file to define the class for users

• Its own implementation file (.cpp) of public member functions

• For a derived class, its implementation file only contains new public member functions or base functions that are redefined

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Client Code Includes Class Definitions

• Use the preprocessor command (#include) to include a header file in a program for all classes that you will use

• If you use a derived class, you DO NOT have to include its base header file (unless you are also using its base class directly)

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Implementation File (.cpp) Compiles

• If you are the developer of a class, you also compile its implementation file (.cpp) with your program

• If you are using a class developed by someone else (string class, iostream class) they provide the compiled code of its implementation file for you (called an object module .o)

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• often several program files use the same header file containing typedef statements, constants, or class type declarations--but, it is a compile-time error to define the same identifier twice

• this preprocessor directive syntax is used to avoid the compilation error that would otherwise occur from multiple uses of #include for the same header file

#ifndef Preprocessor_Identifier

#define Preprocessor_Identifier . . .

#endif

Avoiding Multiple Inclusion of Header Files

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Using File Guards Are Required

• For user developed classes (like your own) that are not derived classes, no one else will be using them, and you can get away with not using these “file guards”

• For inherited classes, you will get errors if you do not use file guards

• FROM NOW ON YOU ARE REQUIRED TO USE THEM FOR ALL YOUR HEADER FILES

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Inheritance Example:C++ Stream Classes

• ios is the base class for all stream classes

• istream and ostream are derived from ios

• ifstream is derived from istream

• ofstream is derived from the ostream

• ios contains formatting flags and member functions to access/modify the flag settings

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C++ Stream Classes (continued)

• istream and ostream provide operations for data transfer between memory and devices

• istream defines the extraction operator (>>) and functions such as get and ignore

• ostream defines the insertion operator (<<), which is used by cout

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C++ Stream Classes (continued)

• ifstream is derived from istream for file input

• ofstream is derived from ostream for file output

• Objects of type ifstream are for file input

• Objects of type ofstream are for file output

• Header file fstream contains the definitions of ifstream and ofstream

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Protected Members of a Class

• Private members of a class cannot be directly accessed outside the class

• For a base class to give derived class access to a private member– Declare that member as protected

• The accessibility of a protected member of a class is in between public and private

• A derived class can directly access the protected member of the base class

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Public Inheritance• Public members of A (base) are public members

of B (derived) and can be directly accessed in class B

• Protected members of A are protected members of B and can be directly accessed by the member functions of B

• Private members of A are hidden in B and can be accessed by member functions of B through public or protected members of A

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Public Inheritance

Base Member Type

Become This Type in Derived

Can be Accessed by Client?

Public Public Yes

Protected Protected N/A

Private N/A N/A

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BasePrivateData

Overriding DerivedMembers

Client Code Derived Class

Overridden BaseMembers

Non-OverriddenBase MembersNon-Overridden

Base Members

BaseConstructors

DerivedConstructors

DerivedPrivateData

Call

Call

Base Class

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Other Types of Inheritance• There are three types of inheritance

– Public

– Protected

– Private

• Different types treat base class information differently

• We will focus on Public Inheritance for CS215

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Why Use Inheritance?

Inheritance is used quite frequently:

• It saves coding: you only have to add or redefine the public member functions unique for your class

• It helps organize and simplify large programs

• The client doesn’t have to know anything about the hierarchy – just uses the derived class

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Composition (or Containment)

• is a mechanism by which an object of one class contains an object of another class

• Called a “has-a” relationship

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A TimeCard object has a Time object #include “TimeType.h”

class TimeCard { public: void Punch ( /* in */ int hours,

/* in */ int minutes, /* in */ int seconds ) ; void Print ( ) ;

TimeCard ( /* in */ int idNum,

/* in */ int initHrs,

/* in */ int initMins,

/* in */ int initSecs ) ; TimeCard ( ) ; private: long id ; TimeType timeStamp ; } ;

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TimeCard ClassTimeCard has a TimeType object

Private data:

hrs

mins

secs

Punch

Private data:

id

timeStamp

Increment

SetPrint . . .

TimeCard

TimeCard

Write . . .

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TimeCard Constructor TimeCard :: TimeCard ( /* in */ int idNum,

/* in */ int initHrs,

/* in */ int initMins,

/* in */ int initSecs )

: timeStamp (initHrs, initMins, initSecs) // constructor initializer

// Precondition: 0 <= initHrs <= 23 && 0 <= initMins <= 59

// 0 <= initSecs <= 59 && initNum is assigned

// Postcondition:

// id == idNum && timeStamp set by its constructor

{

id = idNum ;

} 47

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Default Constructor

TimeCard :: TimeCard ( )

// Default constructor:

// timeStamp default constructor is called

{

id = 0;

}

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Punch Print Member Functions

void TimeCard::Punch (int hrs, int min, int sec){ timeStamp.Set(hrs, min, sec);}

void TimeCard::Print(){ cout << "For ID: " << id << endl; cout << "Timestamp is: "; timeStamp.Write(); cout << endl;}

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Sample Client Code

cout << "Enter employee 1 ID: "; cin >> empID1; cout << endl; TimeCard timeCard1(empID1, 8, 1, 2); TimeCard timeCard2; timeCard1.Print(); timeCard2.Print(); timeCard2.Punch(9,3,4); timeCard2.Print();

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Order in Which Constructors are Executed

Given a class X,

• if X is a derived class its base class constructor is executed first

• next, constructors for member objects (if any) are executed (using their own default constructors if none is specified)

• finally, the body of X’s constructor is executed

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Two Programming Paradigms

Structural (Procedural) Object-Oriented PROGRAM (C) PROGRAM (C++)

FUNCTION

FUNCTION

FUNCTION

OBJECT

Operations

Data

OBJECT

Operations

Data

OBJECT

Operations

Data

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Structured Programming

• Structured programming

– Function is a fundamental entity

– Debug functions

– Program is a collection of interacting functions

– Programmer is action-oriented

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OOD and OOP

• OOD

– Object is a fundamental entity

– Debug objects

– Program is a collection of interacting objects

– Programmer is object-oriented

– OOD encourages code reuse

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What is an object?

OBJECT

Operations

Data

set of methods(public member functions)

internal state(values of private data members)

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OOD and OOP

• The fundamental principles of Object-Oriented Design (OOD) are:

– Encapsulation: combine data and operations on data in a single unit

– Inheritance: create new objects from existing objects

– Polymorphism: the ability to use the same expression to denote different operations

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Summary

• Inheritance and composition are meaningful ways to relate two or more classes

• Inheritance is an “is-a” relation

• Composition is a “has-a” relation

• Single inheritance: a derived class is derived from one class, called the base class

• Multiple inheritance: a derived class is derived from more than one base class

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Summary

• Derived class can redefine function members of a base class

– Redefinition applies only to objects (instances) of derived class

• Base class objects will still use the base class functionality.

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Summary

• When initializing object of a derived class, the base class constructor is executed first

• A call to a base class constructor (with parameters) is specified in the heading of the definition of the derived class constructor

– When none is specified, the default is used

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Summary

• In composition– Class member is an instance of another class– Call to constructor of member objects is

specified in heading of the definition of class’s constructor

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Summary

• Three basic principles of OOD are

– Encapsulation

– Inheritance

– Polymorphism

• Finding classes: describe the problem and choose classes from the list of nouns and operations from the list of verbs