Copyright 2008 Oxford Consulting, Ltd 1 October 2008 - 1 - C++ Classes Enhanced Structures C struct only permitted to have data members Functions to

Copyright 2008 Oxford Consulting, Ltd 1 October 2008

- 1 -

C++ ClassesC++ Classes

Enhanced Structures

C struct only permitted to have data members Functions to operate on such data appeared elsewhere Association with data depended upon programmer

C++ struct may have function members as well Common package combines data and functions Combination of data and functions treated as single

object


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Classes

Word class arose in Simula 67 used to describe new data type

Means in an object oriented language for

Information hiding

Abstraction

Encapsulation


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Classes

A C++ class

Collects entities with common characteristics into objects

Is a data type

Collection of data elements

Data Members

Set of operations

Function Members


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Classesclass Name

{

body;

};

class Vehicle

{

…

};

Vehicle myVehicle;

Vehicle * rentalVehicle = &my Vehicle;


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ClassesClass Name

Identifies the class

Upper case by convention

Helps to distinguish from variable or constant

Instance NameWritten in lower case

Class BodyEnclosed in curly braces

Contains….

Data Members

Function Members

Terminated with a ;


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Class Access

public

Access

Visible to general public

Any function or class within the system

Has access

All data and function members that follow

Available to everyone to use or modify


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Class Access

protected

Access

Visible to

Class members

Classes named as friends

Members of derived classes

All data and function members that follow…...

Appear private to outsiders

Appear public to members and members of derived classes


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Class Access

private

Access

More restrictive

Visible to

Class members

Classes named as friends

All data and function members that follow

Hidden from everyone

No one can change from outside the struct or class


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Class Access

Access Restriction Increasing

Class

privateprotectedpublic Message

Message

Object


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Classes

Public Interface

Establish well defined persistent public interface

Using access specifiers

Hide implementation of the object

Implementation

Private and protected members can change

Functionality remains constant


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ClassesDeclaration

class Name

{

public:

..…

protected:

..…

private:

..…

};

private by default


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Classes

Class

BeersInstance

myBeer

Data membersEach new instance of a class gets distinct copies of all data members

Function MembersShared among all instances


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ClassesDeclaration

class Name

{

public:function membersdata members

protected:function membersdata members

private:function membersdata members

};


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Class Initialization - Constructors and Destructors

Constructor Member function with the same name as the class Responsible for initializing class data members Can be overloaded to have zero or more parameters

syntax

className (arg0, arg1, ... argn) { body }

args are optional


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Class Initialization - Constructors

The constructor is invoked….

Whenever class object declared

At that time full type checking of the definition is applied

Assumes level of accessibility of section in which declaredPublicProtectedPrivate

Invocation of new invokes the constructor for class instance

If new fails the constructor not invoked

What about malloc?


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Class Constructors

class SimpleClass{public:

SimpleClass () : myValue(10); SimpleClass (int aValue) : myValue(aValue);

private:int myValue;

};

int main (void}{

SimpleClass myClass(15), yourClass;return 0;

}

class SimpleClass{public:

SimpleClass () : myValue(10); SimpleClass (int aValue) : myValue(aValue);

private:int myValue;

};

int main (void}{

SimpleClass myClass(15), yourClass;return 0;

}


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Class Initialization - Constructors and DestructorsDestructor

Is member function

Has the same name as the class

Is preceded by the ~ symbol

Provides means to de-initialize data members

Has no parameters

Cannot specify

Return type or value

Cannot be overloaded


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Class Initialization - Constructors and Destructors

Destructor

Storage to be de-initialized must have been allocated by new

Can be called explicitly to clean up without deallocating

syntax

~className ( ) { body }

no args


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Class Initialization - Constructors and DestructorsDestructor

Destructor invoked

Whenever delete operator applied to an object.

Called by delete before freeing storage.

Class object exits scope.

Just before program terminates.

What about free?


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Private Constructors...

class Date{friend class Holiday;public:….private:

Date (int aDay=15, int aMonth=8, int aYear=1971);int day, month, year;

};

class Holiday{public:

Date *createHoliday(int day, int month) {

myHoliday = new Date (day, month); return myHoliday;

}

private:Date *myHoliday;

};

class Date{friend class Holiday;public:….private:

Date (int aDay=15, int aMonth=8, int aYear=1971);int day, month, year;

};

class Holiday{public:

Date *createHoliday(int day, int month) {

myHoliday = new Date (day, month); return myHoliday;

}

private:Date *myHoliday;

};


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Class Initialization - Constructors and Destructor

Important to remember…...

Constructor

Initializes storage

Does not allocate

Destructor

Deinitializes storage

Does not delete

Sequence

new constructor destructor delete


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Class Member Functions

Manager

Implementer

Helper

Access


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Manager Functions Manage the class objects

Initialization / deinitialization

Assignment

Memory management

Type conversion Examples of Manager Functions

Constructor / destructor Invoked

Explicitly by the compiler


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Implementer Functions

Define and provide

Capabilities associated with class

Invoked

Explicitly by the programmer

Provide

Significant portion of the public interface to the class


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class Stack

{

public:

Stack() { head = NULL; }

void push (Entry aValue) { add the value and set new head}

Entry pop() { set new head and return value}

private:

Entry* head;

};


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Helper Functions

Carry out auxiliary tasks

Not invoked (typically)

Explicitly by programmer

Typically dependent upon the underlying data structure

Generally declared private


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Access Functions

Provide access to otherwise protected or private data

Invoked

Explicitly by the programmer

Provide

Remaining component of the public interface


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Classes - Overloading Class Member Functions

Class Member Functions can use

Same name

In same scope

If signature is unique

• Same rules as non-member functions applied to disambiguate

• Must provide implementation for each overloaded function


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Classes - Name Scope

In C++

Have 3 kinds of scope….

File

Local

Class


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File

Portion of the program not contained within the definition of

Function

Class

Outer most scope of the program

Encloses

Local Scope

Class Class


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Classes - Name ScopeLocal

Portion of the program (in general)

Contained within the definition of a function

Each function

Represents distinct local scope

Within a function

Each block maintains an associated local scope

Those variables are local to the enclosing scope

Local scopes can be nested


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Class

Each class represents a distinct scope

Member functions

Treated as within the scope of their class


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Classes - UnionsUnion syntax is based upon that for structures and classessyntax

union Name {

type1 var1;type2 var2;

...typen varn;

};Name - union variable name

serves as the type specifiertypei - any of the typesvari - variable of typei


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Classes - UnionsIn comparison with a class…..

Union Can Only hold one type of variable at a timeDeclare

ConstructorDestructor

Union CannotDeclare static data membersDeclare as a member instance of a class that defines a

ConstructorDestructor


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Classes - Unions

union Mine

{

int intValue;

float floatValue;

char charValue;

};

Mine aMine;

char

int

float


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Classes - UnionsMember Access…..

Access union member exactly like struct or classunionName.memberunionPtr -> member

ExampleMine me;me.intVal = 10; // me will hold an intme.charVal = ‘a’; // me will hold a char

Members can bepublic - like a struct is the defaultprotectedprivate


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Classes - Unions

Initialization

Union can be initialized only with value of type of first member

Anonymous Union

Has no name tag

Not followed by object definition

Cannot have private or protected members

Cannot have function members

If defined at file scope … Must be declared static


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Summary

Now have defined 3 variations on the basic class data structure

<classKey> <className> [:baseList] {< member list>};

classKey class, struct, union

className name of the class

baseClass list of classes the class is derived from

member list data and function members

Documents

Copyright 2008 Oxford Consulting, Ltd 1 October 2008 - 1 - C++ Classes Enhanced Structures C struct only permitted to have data members Functions to