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COEN244: Class composition

Aishy Amer

Electrical & Computer Engineering

OutlineIntroduction

Initializing composite objects

Constant data members: where to initialize

Composite classes share objects?

Member initalization list

Objects as members of their own class?

Reference to objects

Pointers to objects

Static objects

Container class of components

Nested classes

Composition versus inheritance: a case study “Circle/Point”

c© A. Amer Class composition 1

Class composition

Class composition means a class contains an object of adifferent class

A class that has objects of other classes as their datamembers are called composite classes

An object member of a class is called component object

The class of a component object is called component class

Member function of a composite class cannot access privatemembers of included objects

For example, a class Date would be composed of a Timeobject→ Class Date is a client (or user) of class Time


Class composition: Date/Time

class Time {public :

void setTime ( int , i n t , i n t ) ;void pr in tT ime ( ) const ;

private :i n t hour , minute , second ;

} ;

class Date {public :

Date ( int , int , int , int , int , i n t ) ;void p r i n tDa te ( ) const ;

private :i n t day , month , year ;Time t ime ;

} ;


Class composition: Date/Time

/ / Think how you would design i t b e t t e r ?Date : : Date ( i n t d , i n t m, i n t y , i n t sec , i n t min , i n t hour ) {

day = d ;month = m;year = y ;t ime . setTime ( sec , min , hour ) ;

}

void Date : : p r i n tDa te ( ) {cout << month << ’ / ’ << day << ’ / ’ << year << ’ ’ ;t ime . p r in tT ime ( ) ;

}

Can class Date access private members of class Time?


Class composition: Point/Rectangle

/ / L i s t i n g 12 .1 . a composite ob jec t w i th wasted cons t ruc to r c a l l sclass Poin t {

private :i n t x , y ; / / p r i v a t e coord ina tes

public :Po in t ( i n t a=0 , i n t b=0) / / genera l cons t ruc to r

{x = a ; y = b ;cout << " Created : x= " << x << " y=" << y << endl ;

}. . . .

} ;


Class composition: Point/Rectangle

class Rectangle { / / a composi t ion o f po in t s ?private :

Po in t pt1 , pt2 ; / / top− l e f t , bottom−r i g h t corner po in t si n t th i ckness ; / / th i ckness o f the rec tang le border

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width =1) ;. . .

} ;


Testing Point/Rectangle

Poin t p1 (20 ,40) , p2 (70 ,90 ) ; / / top− l e f t , bottom−r i g h t cornersRectangle rec ( p1 , p2 , 4 ) ; / / cons t ruc t rec

/ / a l t e r n a t i v e l y ( bad choice )/ / c lass Rectangle / / a composi t ion o f po in t s ?/ / {/ / p r i v a t e :/ / i n t x1 , y1 , x2 , y2 ; / / top− l e f t , bottom−r i g h t corner po in t s/ / i n t th ickness ; / / th ickness o f the rec tang le border/ / p u b l i c :/ / Rectangle ( const i n t xx1 , const i n t yy1 ,

const i n t xx2 , const i n t yy2 , i n t width =1) ;



All data members could be specified in the memberinitialization list (MIL)

However,1. For members that are themselves class objects,

they should be initialized in the member initialization list2. Constant data members (including references) can only

be initialized inside the member initialization list

Object data members are first created before the compositeclass constructor is called



With wasted constructor calls/ / L i s t i n g 12 .1 . a composite ob jec t w i th wasted cons t ruc to r c a l l sclass Poin t {




}Po in t ( const Poin t& pt ) / / copy cons t ruc to r

. . .void operator = ( const Poin t& pt ) / / assignment opera tor

. . .. . . . .

} ;



Wasted callsclass Rectangle {

private :Po in t pt1 , pt2 ; / / top− l e f t , bottom−r i g h t corner po in t si n t th i ckness ; / / th i ckness o f the rec tang le border

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width =1) ;. . . .

} ;

Rectangle : : Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width ){ pt1 = p1 ; pt2 = p2 ; th ickness = width ; } / / se t data members



Wasted callsPoin t p1 (20 ,40) , p2 (70 ,90 ) ;/ / 1 . p1 and p2 created , Po in t genera l cons t ruc to r c a l l e d

Poin t po i n t (100 ,120) ;/ / 2 . po i n t i s created , Po in t genera l cons t ruc to r c a l l e d

Rectangle rec ( p1 , p2 , 4 ) ;/ / 3 . Creat ion o f Rectangle ob jec t rec/ / 3 . 1 . c rea t i on o f pt1 and pt2 , c a l l i n g Po in t d e f a u l t cons t ruc to r s/ / Note : Po in t d e f a u l t cons t ruc to r s are c a l l e d before the/ / the Rectangle ob jec t i s created !/ / 3.2 assignment opera tor i s c a l l e d



Wasted calls

→ For large composite objects, the process of creation canbecome quit wasteful. What to do?

The solution: Member initialization list (MIL)

Rectangle : : Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t w): th ickness (w) , pt1 ( p1 ) , pt2 ( p2 ) / / i n i t i a l i z a t i o n l i s t

{ } / / empty member body



Saving calls/ / L i s t i n g 12 .2 . A composite ob jec t w i thou t wasted cons t ruc to r c a l l sclass Poin t {




}Po in t ( const Poin t& pt ) / / copy cons t ruc to r

. . .void operator = ( const Poin t& pt ) / / assignment opera tor

. . .. . . .

} ;



Saving callsclass Rectangle {

private :Po in t pt1 , pt2 ; / / top− l e f t , bottom−r i g h t corner po in t si n t th i ckness ; / / th i ckness o f the rec tang le border

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width =1) ;. . .

} ;

/ / Be t te r choice : copy cons t ruc to r c a l l sRectangle : : Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t w)

: th ickness (w) , pt1 ( p1 ) , pt2 ( p2 ) / / i n i t i a l i z a t i o n l i s t{ } / / empty member body



Saving callsPoin t p1 (20 ,40) , p2 (70 ,90 ) ;/ / 1 . p1 and p2 created , Po in t genera l cons t ruc to r c a l l e d

Poin t po i n t (100 ,120) ;/ / 2 . po i n t i s created , Po in t genera l cons t ruc to r c a l l e d

Rectangle rec ( p1 , p2 , 4 ) ;/ / 3 . Creat ion o f Rectangle ob jec t rec/ / 3 . 1 . pt1 and pt2 created , Po in t copy cons t ruc to r c a l l e d/ / 3.2 No assignment opera tor i s c a l l e d ! Saving !



Summary

The purpose of the member initialization list is to avoida call to the default constructor of the component classbefore the call to the composite class constructor

The member initialization list replaces the call

to the default constructor of the component class

with the call to the constructor specified in the initialization list


Outline

Introduction


Constant data members: where toinitialize





Pointers to objects

Static objects


Nested classesComposition versus inheritance: a case study “Circle/Point”


Constant variable?

Many usage of "const":const i n t x ; / / constant i n tx = 2; / / i l l e g a l − can ’ t modify x

const i n t ∗ pX ; / / changeable p o i n t e r to constant i n t∗pX = 3; / / i l l e g a l − can ’ t use pX to modify an i n tpX = &someOtherIntVar ; / / l e g a l − pX can po in t somewhere e lse

i n t ∗ const pY ; / / constant p o i n t e r to changeable i n t∗pY = 4; / / l e g a l − can use pY to modify an i n tpY = &someOtherIntVar ; / / i l l e g a l − can ’ t make pY po in t anywhere e lse

const i n t ∗ const pZ ; / / const p o i n t e r to const i n t∗pZ = 5; / / i l l e g a l − can ’ t use pZ to modify an i n tpZ = &someOtherIntVar ; / / i l l e g a l − can ’ t make pZ po in t anywhere e lse


Constant variable?

i n t y ;const i n t ∗ pConstY = &y ; / / l e g a l − but can ’ t use pConstY to modify yi n t ∗ pMutableY = &y ; / / l e g a l − can use pMutableY to modify y∗pMutableY = 42;

const i n t x ; / / x cannot be modi f ied

const i n t ∗ pX = &x ; / / pX i s the address o f a const i n t/ / and can ’ t be used to change an i n t

∗pX = 4; / / i l l e g a l − can ’ t use pX to change an i n t

i n t ∗ p I n t ; / / address o f normal i n tp I n t = pX ; / / i l l e g a l − cannot conver t from const i n t ∗ to i n t ∗

i n t ∗ p I n t ; / / address o f a normal i n tp I n t = &x ; / / i l l e g a l − cannot conver t from const i n t ∗ to i n t ∗


Constant data members

Where to initialize?

What is common in the following data?account number, employee date of hire, price paid, ID, weightof one unit of area

How often they change? Does it make sense to changethem?

Recall: with const int x=10;x cannot be assigned any value later

In analogy: a constant data memberhas to be initialized immediately after it is created

How can we do it?



Member initialization listclass Rectangle {

private :Po in t pt1 ;Po in t pt2 ;i n t th i ckness ;const double weight ; / / weight o f one u n i t o f area

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , double wt , i n t width =1) ;

. . . .} ;

Rectangle : : Rectangle ( const Poin t& p1 , const Poin t& p2 , double wt , i n t width ): th ickness ( width ) , pt1 ( p1 ) , pt2 ( p2 ) , weight ( wt )/ / i n i t i a l i z a t i o n o f weight ( wt ) here i s mandatory

{ }



Another exampleclass MyClass {

private :i n t hours ; / / I s ’ hours ’ declared here?i n t minutes ;const i n t s ize ;

public :MyClass ( i n t h , i n t m, i n t s )

: hours ( h ) , minutes (m) , s ize ( s ) / / member i n i t i a l i z a t i o n l i s t{ }

void p r i n t ( ) const{

cout << " your c lass data are \ n " ;cout << " Hours : " << hours << ’ ’

<< " Minutes : " << minutes << ’ ’ << " Size : " << s ize << endl ;}



void set ( i n t a , i n t b , i n t c ) {/ / s i ze=a ; / / ∗∗∗TRY IT ∗∗∗∗ : uncomment t h i s l i n e to t e s t/ / so you want to change s ize ?/ / Well , s ince s ize i s constant the compi ler w i l l not l e t t h i s go/ / cons t_ tes t . cpp : 5 6 : assignment o f read−only member ‘ MyClass : : s i ze ’hours = b ;minutes = c ;

}

void operator = ( const MyClass &c ) {hours = c . hours ;minutes=c . minutes ;/ / s i ze = c . s ize ; / / ∗∗∗TRY IT ∗∗∗∗ : uncomment t h i s l i n e to t e s t/ / the compi ler w i l l not agree ! ! s i ze i s const => cannot be changed !/ / E r ro r : assignment o f read−only member ‘ MyClass : : s i ze ’

}} ; / / end c lass



MyClass mc(5 ,10 ,60 ) ;/ / w i th t h i s you are dec la r i ng & i n i t i a l i z i n g/ / the ob jec t mc i n c l u d i n g hours , minutes and s ize ./ / They are being created ( " cons t ruc ted " ) w i th a s p e c i f i e d value ./ / Thus they are being i n i t i a l i z e d . I f you change t h e i r/ / value l a t e r w i th opera tor = t h a t i s an assignment ./ / l i k e mc=mc1 ;mc. p r i n t ( ) ; / / p r i n t s 5 ,10 ,60mc. set ( 6 , 5 , 7 ) ;mc. p r i n t ( ) ; / / p r i n t s 6 ,5 ,60

MyClass mc1(15 ,20 ,70) ;/ / so you want to assign mc1 to mc/ / t h i s imp l i es you would change the const data member s ize ! ! !mc=mc1 ; / / s i ze i s not / cannot be changed !mc. p r i n t ( ) ; / / p r i n t s 15 ,20 ,60



Can a data member of a class be shared by two composite classobjects?

|−−−−−|| ||−−−−−x−−−−−−−| 2 po in t s o f a rec tang le shared wi th two other ob jec ts

| ||−−−−−−−x−−−−−−|

|−−−−−−|

An object can be shared using reference or pointer to an object


Outline

Introduction







Pointers to objects

Static objects




Reference data members

Recall: all references in C++ are constant:1. they cannot change after they are initialized2. but their content can change

Hence,reference data members must be initialized only in a memberinitialization list

Note:Point& pt1;→ pt1 is a reference (an alias) that cannot change but itscontent can be changedconst Point& pt1;→ pt1 is a reference that cannot change; its contentscannot change either


Reference data members: Example

class Rectangle {private :

Po in t& pt1 ; / / shared : re ference cannot be changedPoin t& pt2 ; / / shared : re ference cannot be changedi n t th i ckness ; / / th i ckness o f the rec tang le borderconst double weight ; / / weight o f one u n i t o f area

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , double wt , i n t width =1) ;. . . .

} ;


Reference data members: Example

Rectangle : : Rectangle ( const Poin t& p1 , const Poin t& p2 , double wt , i n t width ): pt1 ( p1 ) , pt2 ( p2 ) , weight ( wt )

/ / pt1 ( p1 ) i s mandatory because r e f i s constant{ th ickness=width ; }

Q: What about the rightmost parameters in a constructor?Can you tell what is wrong with ...

Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width =1 , double wt ) ;/ / H in t : Rectangle R( p1 , p2 , width , wt ) ; or Rectangle R( p1 , p2 , wt , width ) ;


Pointers members of composite classes


Po in t ∗pt1 ; / / can be shared and changedPoin t ∗pt2 ;i n t th i ckness ; / / th i ckness o f the rec tang le borderconst double weight ; / / weight o f one u n i t o f area

public :Rectangle ( const Poin t∗ p1 , const Poin t∗ p2 , double wt , i n t width =1) ;. . . .

} ;

Rectangle : : Rectangle ( const Poin t∗ p1 , const Poin t∗ p2 , double wt , i n t width ): th ickness ( width ) , pt1 ( p1 ) , pt2 ( p2 ) , weight ( wt )

/ / pt1 ( p1 ) not mandatory here/ / Note : pt1 ( p1 ) i s p o i n t e r i n i t i a l i z a t i o n −− not copy cons t ruc to r/ / You may pass Po in te rs as not const ; Then they can be changed{ }


Pointers members of composite classes

Poin t∗ p= new Poin t (20 ,40 ) ;Po in t∗ q=new Poin t (70 ,90 ) ;

Rectangle rec ( p , q , 4 ) ;/ / 3 . Creat ion o f Rectangle ob jec t rec/ / 3 . 1 . pt1 and pt2 po in te r s are assigned They Po in t to p and q .


Outline

Introduction







Pointers to objects

Static objects




Member initialization list

Mandatory:

Constant data memebrs including references (which areconstant), constant pointers, ...

Call to base class constructor

Reference to a component object

Prefered:

Initialize component objects(i.e., objects as member of other classes)it saves extra calls

Optional:

A pointer to a component object

Other data members (non-static)c© A. Amer Class composition 33


Practical programmers:

emphasize initializing ALL data members in ALL constructors(primitive or user-defined)

initalize most of the data memebrs in the MIL

initialzie data members IN ORDER of declaration unless youABSOLUTELY cannot (see example below)



Examples of the “absolutely cannot”

When the initialization procedure is complexdata = new Sample ∗ [MAX_ELEMENT ] ; / / Two dimensional a r rayfor ( unsigned i n t i = 0 ; i < MAX_ELEMENT ; i ++)data [ i ] = new Sample [MAX_ELEMENT ] ;

When there is an important comment to make before aparticular initialization line

His to r y ( ) : s i ze ( 3 ) , count ( 0 ) , i dx ( 0 ) {/ / The f o l l o w i n g l i n e i s the source o f BUG 3876data = new Sample [MAX_ELEMENT ] ;i f ( data == NULL) {cout << ’ ’No memory a l l o c a t e d ’ ’ <<endl ; e x i t ( 1 ) ;

}cout << ’ ’ user created defaul t cons t ruc to r o f H i s to r y c a l l e d ’ ’ <<endl ;

}


Outline

Introduction







Pointers to objects

Static objects





Recall:

Data members are allocated in the order in which they arelisted in the class declaration

Static data (members) are one time allocated at the beginningof program execution

Static data (members) are destroyed after the programterminate



/ / An ob jec ts as member o f i t s own c lassclass Poin t {

private :i n t x , y ; / / p r i v a t e coord ina tesPoin t o r i g i n ; / / can I do t h i s ?

. . .} ;

Po in t p ; / / P i s created means/ / memory f o r x and y i s a l l o c a t e d/ / memory f o r o r i g i n i s a l l o c a t e d/ / memory f o r x and y of o r i g i n i s a l l o c a t e d/ / memory f o r o r i g i n o f o r i g i n i s a l l o c a t e d . ./ / . . . e tc . i n f i n i t e recu rs i ve loop !

→ (non-static) objects as members of their own class aredisallowed


Reference to objects as members of their own class?

/ / An ob jec ts as member o f i t s own c lassclass Poin t {

private :i n t x , y ; / / p r i v a t e coord ina tesPoin t &o r i g i n ; / / can I do t h i s ?

public :Po in t ( Po in t &org , i n t a=0 , i n t b=0) : o r i g i n ( org ){ x=a ; y=b ; } / / o r i g i n ( org ) mandatory

. . .} ;


Reference to objects as members of their own class?

Poin t ∗p=0; / / p set to n u l lPoin t ∗p = new Poin t (∗p , 8 0 , 9 0 ) ; / / a l l o c a t e memory and set to 80 ,90Poin t p1 ( p ) ; / / ∗p i s used as the o r i g i n f o r p1

→ Do not use (non-static) objects as members of their own classunless you have toBut what is the solution then?


Static objects as members of their own class

/ / L i s t i n g 12.3 Using s t a t i c data members and a s t a t i c member f u n c t i o n .class Poin t {

private :i n t x , y ; / / p r i v a t e coord ina tess t a t i c i n t count ;s t a t i c Poin t o r i g i n ;


{x = a ; y = b ; count ++;cout << " Created : x=" << x << " y=" << y

<< " count= " << count << endl ;}



s t a t i c i n t q u a n t i t y ( ) / / const i s not al lowed{ return count ; }

. . .~Po in t ( ) { / / d e s t r u c t o r

count−−;cout <<" Po in t destroyed : x=" <<x <<" y=" <<y <<endl ;

}} ;



i n t Poin t : : count = 0 ; / / i n i t i a l i z a t i o nPoin t Po in t : : o r i g i n (640 ,0 ) ; / / i n i t i a l i z a t i o n/ / ou tput : ’ Created : x=640 y=0 count =1 ‘

cout << " Number o f po in t s : " << Po in t : : q u a n t i t y ( ) << endl ;/ / ou tput : Number o f po in t s : 1

Poin t p1 , p2 (30 ) , p3 (50 ,70 ) ;/ / ou tput : 0 ,0 ,2 30 ,0 ,3 and 50 ,70 ,4

cout << " Number o f po in t s : " << p1 . q u a n t i t y ( ) << endl ;/ / ou tput : number o f po in t s : 4/ / Output from hidden c a l l s :/ / destroyed 50 ,70 destroyed 30 ,0 destroyed 0 ,0/ / But why ‘ destroyed 640 ,0 ’ i s not output ?/ / ==> s t a t i c data are destroyed a f t e r the program termina te


A note to return types

Any difference between:1. Polynomial Polynomial::operator = (Polynomial& p);//with return *this;2. Polynomial& Polynomial::operator = (Polynomial& p);//with return *this

There is a BIG difference!

Without the ’&’ a copy of the object to be returned is made,and this copy is subsequently returned

With the ’&’ you are returning a reference to the object, whichsimply means you are not making a copy of the object toreturn

There are several implications related to this which we havediscussed in class


Outline

Introduction







Pointers to objects

Static objects




Class composition: container class


Po in t pt1 , pt2 ; / / top− l e f t , bottom−r i g h t corner po in t si n t th i ckness ; / / th i ckness o f the rec tang le border

public :Rectangle ( const Poin t& p1 , const Poin t& p2 , i n t width =1) ;

. . . .} ;

Rectangle conta ins a f i x e d c o l l e c t i o n o f components : two

A container class includes a ‘dynamically’ created collectionof components

Useful: when we need a data type that contains a dynamiccollection of values

How often we need such thing?c© A. Amer Class composition 46


While the program executes, we would like to add moreobjects to the container

Why? for temporary storage or for processing

Example:Container class: customer charge accountcomponents: customer credit card transactions to beprocessedProcessing: totals, taxes, printing, reports, searching, etc.

But why not using an array for that?



With an array we cannot perform high level tasks such asaddition, search, validation, setting values of subcomponents, checking the index, etc.

Container classes are designed to perform these operation forthe client code

The client code does not need to deal with low-levelimplementations


Container classes: an example

For simplicity: a Sample component class with one datamember

The Sample objects are produced for example by a stockexchange ticker tape, a patient monitoring device, or apressure observation.

For simplicity, we assume the values for the Sample class arehard-coded in an array


Container classes: an example

/ / L i s t i n g 12.7 A con ta ine r c lass w i th dynamical ly−a l l o c a t e d memory

class Sample { / / component c lassprivate :double value ; / / sample value

public :Sample ( double x = 0) / / d e f a u l t and convers ion cons t ruc to r

{ value = x ; }void set ( double x ) / / mod i f i e r method

{ value = x ; }double get ( ) const / / s e l e c t o r method

{ return value ; }} ;



class His to r y { / / con ta ine r c lass : set valueprivate :

i n t size , count , i dx ;Sample ∗data ; / / could be an ar ray o f data

public :H i s to r y ( ) : s i ze ( 3 ) , count ( 0 ) , i dx ( 0 ) / / make ar ray empty

{data = new Sample [ s ize ] ; / / a l l o c a t e new spacei f ( data == NULL)

{ cout << " Out o f memory \ n " ; e x i t ( 1 ) ; }}

~ H i s to r y ( ) { delete [ ] data ; } / / f r ee dynamic memory

Note: Since "Sample *data" is not a constant it must not beinitialize in the MIL



void add ( double ) ; / / add a sample a t the end

Sample& getComponent ( ) / / r e t u r n re ference to Sample{ return data [ i dx ] ; } / / can be a message t a r g e t

void g e t F i r s t ( ) { i dx = 0; }bool getNext ( ) { return ++ idx < count ; }void average ( ) const ; / / p r i n t average

} ;



void His to r y : : add ( double s ) {i f ( count == s ize ) {

s ize = s ize ∗ 2; / / double s ize i f out o f spaceSample ∗p = new Sample [ s ize ] ;i f ( p == NULL)

{ cout << " Out o f memory \ n " ; e x i t ( 1 ) ; } / / t e s t f o r successfor ( i n t i =0; i < count ; i ++)

p [ i ] = data [ i ] ; / / copy e x i s t i n g elementsdelete [ ] data ; / / de le te e x i s t i n g ar raydata = p ; / / rep lace i t w i th new ar raycout << " new s ize : " << s ize << endl ; / / debugging p r i n t

}data [ count ++ ] . se t ( s ) ; / / use next space a v a i l a b l e

}



void His to r y : : average ( ) const {cout << " \ n Average value : " ;double sum = 0;for ( i n t i = 0 ; i < count ; i ++)

sum += data [ i ] . get ( ) ;cout << sum / count << endl ;

}


Testing container class

double a [ ] = {3 , 5 , 7 , 11 , 13 , 17 , 19 , 23 , 29 } ; / / i npu t dataHis to r y h ;for ( i n t i =0; i < 9 ; i ++)

h . add ( a [ i ] ) ; / / add h i s t o r ycout << " \ n Measurement h i s t o r y : " << endl << endl ;h . g e t F i r s t ( ) ; / / work i s pushed updo {

cout << " " << h . getComponent ( ) . get ( ) ;/ / p r i n t each component

} while ( h . getNext ( ) ) ;h . average ( ) ;

/ / Output :new s ize : 6new s ize : 12Measurement h i s t o r y : 3 5 7 11 13 17 19 23 29Average : 14.1111


Container classes

Container classes with more complex design includesorting, search, removal, insertion, update, comparison

A significant number of container classes are availablefrom the Standard Template Library


Nested classes

In C++, we can define a server class inside a client class

This way the nested class name is not visible outside of thecomposite class

Nested class definitions can appear in either the private orpublic section

In both cases Nested class is hidden from the rest of theprogram

Nested class cannot be used to declare variables of its type inother scopes outside the composite class


Nested classes

class His to r y {private :

class Sample{ / / component c lass

private :double value ; / / sample value

public :Sample ( double x = 0) / / d e f a u l t and convers ion cons t ruc to r

{ value = x ; }void set ( double x ) / / mod i f i e r method

{ value = x ; }double get ( ) const / / s e l e c t o r method

{ return value ; }} ;

i n t size , count , i dx ;Sample ∗data ;


Nested classes

public :H i s to r y ( ) : s i ze ( 3 ) , count ( 0 ) , i dx ( 0 ) { / / make ar ray empty

data = new Sample [ s ize ] ; / / a l l o c a t e new spacei f ( data == NULL)

{ cout << " Out o f memory \ n " ; e x i t ( 1 ) ; }}. . . .

} ;

/ / Usedouble a [ ] = {3 , 5 , 7 , 11 , 13 , 17 , 19 , 23 , 29 } ; / / i npu t data

His to r y h ;for ( i n t i =0; i < 9 ; i ++)

h . add ( a [ i ] ) ; / / add h i s t o r y. . .Sample s =5; / / OK?His to r y : : Sample s =5; / / OK? when?


Outline

Introduction







Pointers to objects

Static objects




Composition or inheritance?

Composition: the existence of a "has-a" relationshipaggregate objects in a class

Inheritance: the existence of a “is-a” relationship conceptuallyconnect classes in an inheritance hierarchy

In most problems, inheritance is very useful and should beoften considered, however,

Inheritance forces the client programmer to learn aconsiderable amount about the server design, speciallywhen the hierarchy is tallDependencies between classes in inheritance are verystrong


Composition or inheritance?

Composition is conceptually simpler & links between classesare not as strong

Composition is, in general, easier to maintain

Choice depends on degree of similarity between relatedclasses

If the number of common methods is small & the number ofmethods to be is large, composition is a good alternative

If a “is-a” relationship is not clear then use composition:A Rectangle "has a" Point but it is not a Point;A Cylinder "has a" Circle or "is a" Circle?An Address "is a" Name or "has a" Name?


Circle/Cylinder: composition

/ / L i s t i n g 14.2 code reuse through c lass composi t ionclass C i r c l e { / / o r i g i n a l code f o r reuse

protected : / / i n h e r i t a n c e i s one of the opt ionsdouble rad ius ; / / i n t e r n a l data

public :s t a t i c const double PI ; / / i t must be i n i t i a l i z e dC i r c l e ( double r ) ; / / convers ion cons t ruc to rdouble getLength ( ) const ; / / compute c i rcumferencedouble getArea ( ) const ; / / compute areadouble getRadius ( ) const ;void set ( double r ) ; / / change s ize

} ;const double C i r c l e : : PI = 3.1415926536;



class Cy l inder { / / new c lass Cy l inderprotected :

C i r c l e c ; / / no PI , no rad iusdouble he igh t ; / / new code

public :Cy l inder ( double r , double h ) : c ( r ) / / i n i t i a l i z e r l i s t ( no PI )

{ he igh t = h ; }double getLength ( ) const { return c . getLength ( ) ; } / / from c lass C i r c l edouble getRadius ( ) const { return c . getRadius ( ) ; } / / from c lass C i r c l evoid set ( double r ) { c . se t ( r ) ; } / / from c lass C i r c l edouble getVolume ( ) const / / no getArea ( )

{double rad ius = c . getRadius ( ) ;return C i r c l e : : PI ∗ rad ius ∗ rad ius ∗ he igh t ;

}} ;



i n t main ( ) {Cy l inder cy l1 ( 2 . 5 , 6 . 0 ) , cy l2 ( 5 . 0 , 7 . 5 ) ; / / i n i t i a l i z e datadouble l eng th = cy l1 . getLength ( ) ; / / s i m i l a r to C i r c l ecy l1 . se t ( 3 . 0 ) ;double diam = 2 ∗ cy l1 . getRadius ( ) ; / / no c a l l to getArea ( )double vo l = cy l2 . getVolume ( ) ; / / not i n C i r c l ecout << " Circumference of f i r s t c y l i n d e r : " << leng th << endl ;cout << " Volume of the second c y l i n d e r : " << vo l << endl ;cout << " Diameter o f the f i r s t c y l i n d e r : " << diam << endl ;return 0;}


Circle/Cylinder: inheritance

/ / L i s t i n g 14.3 code reuse through i n h e r i t a n c eclass C i r c l e { / / o r i g i n a l code f o r reuse

protected : / / i n h e r i t a n c e i s one of the opt ionsdouble rad ius ; / / i n t e r n a l data

public :s t a t i c const double PI ; / / i t must be i n i t i a l i z e dC i r c l e ( double r ) ; / / convers ion cons t ruc to rdouble getLength ( ) const / / compute c i rcumference

{ return 2 ∗ PI ∗ rad ius ; }double getArea ( ) const ; / / compute areadouble getRadius ( ) const ;void set ( double r ) ; / / change s ize

} ;const double C i r c l e : : PI = 3.1415926536;



class Cy l inder : public C i r c l e { / / new c lass Cy l inderprotected :

double he igh t ; / / o ther data i s i n C i r c l epublic :

Cy l inder ( double r , double h ) : C i r c l e ( r ) / / i n i t i a l i z e r l i s t ( no PI ){ he igh t = h ; } / / new code

double getVolume ( ) const / / no getArea ( ){ return he igh t ∗ getArea ( ) ; } / / a d d i t i o n a l c a p a b i l i t y

} ;



i n t main ( ) {Cy l inder cy l1 ( 2 . 5 , 6 . 0 ) , cy l2 ( 5 . 0 , 7 . 5 ) ; / / i n i t i a l i z e datadouble l eng th = cy l1 . getLength ( ) ; / / s i m i l a r to C i r c l ecy l1 . se t ( 3 . 0 ) ;double diam = 2 ∗ cy l1 . getRadius ( ) ; / / no c a l l to getArea ( )double vo l = cy l2 . getVolume ( ) ; / / not i n C i r c l ecout << " Circumference of f i r s t c y l i n d e r : " << leng th << endl ;cout << " Volume of the second c y l i n d e r : " << vo l << endl ;cout << " Diameter o f the f i r s t c y l i n d e r : " << diam << endl ;return 0;

}


Review


Po in t p ;. . .

}/ / which Const ruc torRectangle ( Po in t& q , . . . ) { p=q ; . . . } / / = d e f a u l t cons t r . +assignment op/ / orRectangle ( Po in t& q , . . . ) : p ( q ) { . . . } / / copy cons t ruc to r

/ / What i s i n a member i n i t i a l i z a t i o n l i s t ?


aishy amer - users.encs.concordia.caamer/teach/coen244/notes/composition.pdf · / / 3. creation of...

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