07 Object Based Programming

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Operations on Structs Constructors and Destructors Const Methods Classes

7. Object-based Programming

June 14, 2010


Introduction to C/C++,Tobias Weinzierl page 1 of 28
http://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierlhttp://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierl


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Outline

Recapitulation

Operations on Structs: Methods

Constructors and Destructors

The Const Modifier

Encapsulation & Information Hiding




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Recapitulation

i n t a ;i n t b ;a = 3 ;b = & a ;b = 4 ;b = 5 ;

Explain/sketch what the invidial lines do. What does b++;

What happens if we write

i n t a , b ;

What happens if we call a functionfoo(int*)withb.




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7.1.Operations on Structs

/ R e p r es e nt s a d a t e . Each m onth s h a l l hav e 30 d ay s ./

s t r u c t Date {i n t month , day ;

} ;

v o i d switchToNextDay (Date& date

) {. . .

}




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Class Diagrams with the Unified Modelling Language

The Unified Modelling Language is agraphical representation of yoursystem design.

Operation and the data here gohand-in-hand. UML illustrates this fact.

Object-based paradigm: Model wholesystem in terms of structs andoperations acting on these structs.

Remember: Structs can hold otherstructs asattributes.

Remember: Structs can hold pointersto other structs.




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Aggregation and Composition




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Operations on Structs Rewritten

/ R e pr es en t s a d a te . Each month s h a l l h ave 30 d ay s /s t r u c t Date {

i n t month , day ;} ;

/ / D e c l a r a t i o n

v o i d s w i tc hT oN ex t Da y ( D at e& d a t e ) { . . . }

/ R e pr es en t s a d a te . Each month s h a l l h ave 30 d ay s /s t r u c t Date {

i n t month , day ;/ / Belongs t o t h e s t r u c t i t i s embedded i n t o

v o i d switchToNextDay ( ) ;} ;




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Operations on Structs Rewritten

/ / Da te . hs t r u c t Date {

i n t month , day ;v o i d switchToNextDay ( ) ;

} ;

/ / Da te . cp p# i n c l u d e D a t e . h

v o i d Date : : switchToNextDay ( ) {day++;. . .

}

Syntax is similar to namespaces.

It is now clear, how operations and data belong together.

A good object always works solely on data of its own. If it has to manipulate datafrom another object, it should use this objects functions (operations,methods).

Internally, it is still your straightforward C realisation.




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7.2.Constructors and Destructors

/ / Da te . hs t r u c t Date {

i n t month , day ;v o i d switchToNextDay ( ) ;

} ;

. . .

Date myDate;myDate. month = 7;myDate . day = 5 ;myDate. switchToNextDay ( ) ;

The two set operations also belong to

the struct itself. It might be good design to write an

initialisation operation.

Add a methodinit(int month, intday);.




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Constructors

/ / Da te . h

s t r u c t Date {i n t m on th , d a y ;v o i d switchToNextDay ( ) ;Date( i n t month , i n t day ) ;

} ;

/ / Da te . cp pDate : : Date ( i n t month , i n t day ) :

month ( month ) ,d a y ( d a y ) {

/ / some checks}

A constructor is a special type ofoperation.

Its name equals the struct name.

It has no return type.

It uses initialisation lists.




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Creating an Instance

/ / Da te . h

s t r u c t Date {i n t m on th , d a y ;v o i d switchToNextDay ( ) ;Da te ( i n t month , i n t day ) ;D at e ( ) ;Da te ( c o n st c ha r s t r i n gR e p r e se n t a t i on ) ;

} ;

/ / Another f i l eD at e myDate1 ( 7 , 5 ) ;Date myDate2 ( ) ; / / doe sn t w or k Date myDate3 ;Date myDate4 ( 5 J u l y ) ;

We can overload constructors. Thedefault constructoris invoked without brackets.

There is no need for a default constructor.

If you dont provide a constructor at all, C++ automatically (in the background)generates a default constructor.




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Object Destruction

. . .

w h i l e ( s o m e t h i n g ) {D at e myDate ( . . . ) ;

/ / we do something

}

Instance of Date is destroyed at the end of the scope.

If there is a constructor, why isnt there a counterpart?



O ti St t C t t d D t t C t M th d Cl


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Destructor Syntax

/ / Da te . h

s t r u c t Date {i n t m on th , d a y ;v o i d switchToNextDay ( ) ;Da te ( i n t month , i n t day ) ;D at e ( ) ; D at e ( ) ;

} ;

/ / Da te . cp pDate : : Date ( ) {}

Destructor is called always at the end of the scope containing the object.

We cannot overload destructors.

Destructors dont have a return value.

A destructor is never called explicitly.

If you dont define a destructor, C++ automatically generates one.





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Objects on the Heap

/ / Da te . h


} ;

. . .Date myDate = new D at e ( 7 , 5 ) ;. . .

myDate>switchToNextDay ( ) ;. . .d e l e l t e myDate ;

new reserves memory (as it does in C), and new invokes the constructor. delete invokes the destructor, and it frees the memory (as it does in C). Remember of linked list: If we delete the first list entry, this entry can also free the

subsequent entries. It is much easier to enforce the consistency with these newdata structures.





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Arrays of Objects

/ / Da te . h


} ;

Date myDates [ 1 0 ] ;. . .

myDate[4]>switchToNextDay ( ) ;. . .

Arrays of objects are supported by C++.

However, such data structures have to have a standard constructor.





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Explicit Constructor Calls

/ / Da te . h


} ;

D at e m yDate1 = D at e ( 7 , 5 ) ;

Date myDate2 = Date ( ) ;. . .

Here, we need the parentheses.

Obviously, the constructor is kind of a function which returns a struct. This is bad style, as it induces a bit-wise copy.





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7.3.Const Methods

. . . why does this snippet not work?

s t r u c t I n t e g e r E n t r y {i n t value ;I n t e g e r E n t r y next ;/ / P r i n t t h i s e n t r y t o t e r m i n a l and c o n t i n u e w i t h n e x t e n t r y .v o i d p r i n t L i s t ( ) ;/ / Make argu ment n e x t ar gum ent .v o i d append ( I n t e g e r E n t r y n e x t E n t r y ) ;

} ;

. . .

v o i d doSomething ( co n st I n t e g er E n tr y& e nt r y ) {. . .e n tr y . p r i n t L i s t ( ) ;

}

Call-by-value is expensive, as it copies the whole object bit-wise. This lead to aperformance breakdown in this code. Thus, we used call-by-const-reference. However,the code now does not compile anymore.





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p

Const Keyword

s t r u c t I n t e g e r E n t r y {

i n t value ;I n t e g e r E n t r y next ;/ / P r i n t t h i s e n t r y t o t e r m i n a l and c o n t i n u e w i t h n e x t e n t r y .v o i d p r i n t L i s t ( ) co n st;/ / Make argu ment n e x t ar gum ent .v o i d append ( I n t e g e r E n t r y n e x t E n t r y ) ;

} ;

. . .

v o i d doSomething ( co n st I n t e g er E n tr y& e nt r y ) {. . .e n tr y . p r i n t L i s t ( ) ;

}





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p

Semantics of Const


i n t value ;I n t e g e r E n t r y next ;/ / P r i n t t h i s e n t r y t o t e r m i n a l and c o n t i n u e w i t h n e x t e n t r y .v o i d p r i n t L i s t ( ) co n st;

} ;

v o i d I n t eg e rE n t ry : : p r i n t L i s t ( ) co n st {v a l u e = 2 ; / / e r r or

r e t u r n value ; / / o . k .}

constoperations may not alter object state.

constoperations may not call non-const methods.

constoperatoins can be invoked on objects passed by call-by-const-reference. constoperations allow the compiler to optimise.

constoperations allow user to enforce encapsulation and to write safter apps.

use theconstmodifier whenever possible.





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Const Variants


i n t value ;I n t e g e r E n t r y next ;

c on st i n t g e tV a lu e ( ) ;c on st i n t g e tVa lu e ( ) co n st;

/ / v a r i a n t Ai n t g e tVa lu e ( ) co n st;

i n t g e tV a lu e ( ) ;

/ / v a r i a n t Bc on st i n t & g e t V al u e ( ) co n st;c on st i n t & g e tV a lu e ( ) ;

} ;





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Const Variants


i n t value ;I n t e g e r E n t r y next ;

c on st i n t g e tV a lu e ( ) ;c on st i n t g e tVa lu e ( ) co n st;

/ / v a r i a n t Ai n t g e tVa lu e ( ) co n st;

i n t g e tV a lu e ( ) ;

/ / v a r i a n t Bc on st i n t & g e t V al u e ( ) co n st;c on st i n t & g e tV a lu e ( ) ;

} ;

constbelongs to the signature, i.e. we can overload with respect to const. constafter the operation enforces the operation not to manipulate object state.

constbefore the return argument does not allow programmer to manipulate result.

The compiler tries to use constoperations before it falls back to non-constoperations.





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7.4.Classes

With the new technique at hand, we can encapsulate data and operations asthese two things go hand in hand.

We can write a couple of setter and getter operations to allow the user tomanipulate our brand new data structure.

However, the use still can reset attributes manually. We can not forbid this.

Consequently, we need an alterantive, new technique to forbid this.

Furthermore, it would be nice if the user doesnt even see the attributes, as

it might be reasonable that the user cant even read attributes if we dont providethe corresponding getters (next pointer in our list example, e.g.).





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Our Beloved Colleagues

/ R e p r es e n ts a d a t e .

Each m onth s h a l l ha ve 30 d ay s ./

s t r u c t Date {i n t month , day ;v o i d switchToNextDay ;

} ;

/ / t h i s i s t h e co de our c o l l e a g u e wroteDa te myDate ( . . . ) ;myDate. day++;





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Encapsulation



c l a s s Da te {p r i v a t e :

i n t month , day ;p u b l i c :


In principle, classis an alias forstruct,

i.e. we can do all the things we can do with structs with classes, too.

However, for classes we can create publicand privatesections. Only add themin the header.

Private attributes and operations are not available from outside, but only within theoperations of the class (encapsulation).





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Encapsulation at Work




i n t m on th , d a y ;p u b l i c :


. . .v o i d Date : : switchToNextDay ( ) {

d a y++; / / o . k .}

Date myDate;

myDate. day ++; / / doe sn t w or k .





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Encapsulation and the Object Lifecycle




i n t m on th , d a y ;D at e ( ) ;

p u b l i c :v o i d switchToNextDay ( ) ;

Da te ( i n t month , i n t day ) ;} ;

We can make constructors private and, thus, forbid everybody to create aninstance of our object.

We could make the destructor private, too. However, this never makes sense.





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Remark: Classes and Namespaces

namespace cal end ar {

/ R e p r es e n ts a d a t e . Each m onth s h a l l ha ve 30 d ay s ./

c l a s s D at e {p r i v a t e :

i n t m on th , d a y ;D at e ( ) ;

p u b l i c :

v o i d switchToNextDay ( ) ;Da te ( i n t month , i n t day ) ;} ;

}

v o i d cal enda r : : Date : : switchToNextDay ( ) {. . .

}

/ / f u l l y q u a l i f i e d ar gum ent here n o t necessary/ / we ca n access p r i v a t e arguments o f o t h e r i n s t a n c e sv o i d cal enda r : : Date : : copyFromOtherDate (co n st ca le n d a r : : Date& d a te ) {

. . .}





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Remark: Classes and Recursion

c l as s I n t e g e rE n t r y {

p r i v a t e :i n t value ;I n t e g e r E n t r y next ;

p u b l i c :v o i d append ( I n t e g e r E n t r y newEntry ) ;

} ;

v o i d I n t e g e r E n t r y : : append ( I n t e g e r E n t r y newEntry) {

i f ( next ==0) {n e xt = n e wEn try ;

}e l s e {

next>append(newEntry );}

}

This is not recursion, as the operation is invoked on another object.

However, recursion and object-based programming work together.



07 Object Based Programming

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