08 Object Oriented Programming

Brain Teaser Copy Constructor and Operators Inheritance virtual Operations static Design Patterns

8. Object-oriented Programming

June 15, 2010


Introduction to C/C++, Tobias Weinzierl page 1 of 41

http://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierl


Outline

• Recapitulation

• Copy Constructor & Operators

• Object-oriented Programming

• Dynamic and Static Polymorphism

• The Keyword Static

• The Singleton Pattern

• Generic Programming





—8.1. Brain Teaser—

c lass S t r i n g {p r i v a t e :

/ / a in ’ t a 0−te rminated s t r i n gchar∗ data ;/ / l eng th o f s t r i n gi n t l en g t h ;

p u b l i c :S t r i n g ( ) ;vo id copyFromOtherStr ing ( const S t r i n g& s t r i n g ) ;

} ;

. . .

S t r i n g : : S t r i n g ( ) :data ( 0 ) ,l e ng th ( 0 ) {}

vo id S t r i n g : : copyFromOtherStr ing ( const S t r i n g& s t r i n g ) {data = s t r i n g . data ;l e ng th = s t r i n g . l e ng th ; s

}





—8.2. Copy Constructor and Operators—

• C++ generates a couple of default operations for any class/struct to allow you touse these datatypes as you use a built-in datatype.

i n t a ( 1 0 ) ;Date myDate ( anotherDate ) ;

• Sometimes, we have to rewrite these default operations.• For the copy constructor, it is very simple.





Copy Constructor for a String Class

c lass S t r i n g {p r i v a t e :

/ / a in ’ t a 0−te rminated s t r i n gchar∗ data ;/ / l eng th o f s t r i n gi n t l e n g t h ;

p u b l i c :S t r i n g ( ) ;S t r i n g ( const S t r i n g& s t r i n g ) ;

} ;

. . .

S t r i n g : : S t r i n g ( ) :data ( 0 ) ,l en g t h ( 0 ) {}

S t r i n g : : S t r i n g ( const S t r i n g& s t r i n g ) :data ( 0 ) ,l en g t h ( s t r i n g . l e ng th ) {data = new . . . ;

f o r ( . . . ) {. . .

}}





Excursus: C++ Strings

• Good news: You now know how to write a copy constructor for your personal stringclass.

• Bad news: Such a class does already exist.

# inc lude <s t r i n g>

std : : s t r i n g myString0 ;s td : : s t r i n g myString1 ( ” h e l l o wor ld ” ) ;s td : : s t r i n g myString2 ( myString1 ) ;s td : : s t r i n g myString3 = ” h e l l o MPG” ;s td : : s t r i n g myString4 = myString3 ;myStr ing . leng th ( ) ;myString4 += myString1 ;





Default Operations

• The following operations are generated automatically as public operations if wedon’t provide our own implementation:

• Default constructor.• Destructor.• Copy constructor.

• However, there’s still another pitfall:

MyStr ing a ;/ / do something wi th a ;MyStr ing b ( a ) ; / / worksMyStr ing c ;c = a ; / / does not work





Operators

• In C++, operators such as +,-,+=,=,¡¡ are just methods, too.• The = operators (assignment operator) is generates automatically, if we don’t

overwrite it.• It then is a bit-wise copy of the object.• We can redefine the operators.• We now know, what

std : : cout << ” s tup id ” << s td : : endl

means.





Operators Redefined—Part I

c lass Vector {p u b l i c :

/ / copy cons t ruc to rVector ( const Vector& vec to r ) ;

Vector& opera tor =( const Vector& vec to r ) ;/ / unary opera torsVector& opera tor +=( const Vector& vec to r ) ;Vector& opera tor ∗=( const double& value ) ;

vo id toStream ( std : : ostream& out ) const ;} ;

/ / b ina ry opera torsVector& opera tor +( const Vector& lhs , const Vector& rhs ) ;s td : : ostream& operator<<(s td : : ostream& out , const Vector& vec to r ) ;

. . .Vector& opera tor +( const Vector& lhs , const Vector& rhs ) {

Vector r e s u l t ( l hs ) ;r e s u l t += rhs ;r e t u r n r e s u l t ;

}





Operators Redefined—Part II

c lass Vector {p u b l i c :

/ / copy cons t ruc to rVector ( const Vector& vec to r ) ;

Vector& opera tor =( const Vector& vec to r ) ;/ / unary opera torsVector& opera tor +=( const Vector& vec to r ) ;Vector& opera tor ∗=( const double& value ) ;

vo id toStream ( std : : ostream& out ) const ;} ;

/ / b ina ry opera torsVector& opera tor +( const Vector& lhs , const Vector& rhs ) ;s td : : ostream& operator<<(s td : : ostream& out , const Vector& vec to r ) ;

. . .s td : : ostream& operator<<(s td : : ostream& out , const Vector& vec to r ) {

vec to r . toStream ( out ) ;r e t u r n out ;

}





—8.3. Inheritance—

• Sometimes, someone hands you over an object, but this object lacks somefunctionality.

• Now, it is up to you to add this functionality.• However, what happens if there is different variants of this functionality?

• Image you write a researcher management application.• Let Employee be a multi-purpose class for bookkeeping.• Task: Add a function setSalary() and getTaxesPaid .• However: There are also people getting a stipend. These guys do not pay taxes.• Writing two classes is not a variant, as all employees have to have a getWeight()

operation.

Challenge: If we duplicate the code, then multiple operations have to be rewritten:

bool isBMIAboveThreshold ( ) {. . .

}





Challenge Rewritten

• A Stipend is an extension of Employee.• A PostDoc is an extension of Employee, too.• Stipdend and PostDoc are two different things.• Whatever we do with an Employee, we can do with a Stipend and PostDoc, too.





“is a”—Relations

AbstractDB

RealDatabase HardcodedTestDB

getEntry()

• Student has all operations and attributes, Employee has.• Every operation expecting an instance of Employee could also be passed an

instance of Student.• Again, we could image this to be an extension of a cut-n-paste mechanism.





“is a”—Relations in Source Code (Part I)

c lass Employee {p r i v a t e :

double weight ; ;p u b l i c :

bool isBMIAboveThreshold ( ) ;} ;

c lass St ipend : p u b l i c Employee {p r i v a t e :p u b l i c :

double getSt ipdend ( ) const ;} ;

. . .St ipend myStipend ;myStipend . isBMIAboveThreshold ( ) ; / / t h a t ’ s f i n e





“is a”—Relations in Source Code (Part II)

c lass Employee {. . .

} ;

c lass St ipend : p u b l i c Employee {. . .

} ;

vo id foo ( const Employee& employee ) { . . . }

Stipend myStipend ;foo ( myStipend ) ;





Object-oriented Programming

• is-a relations are mapped to inheritance.• Object-based programming becomes object-oriented programming due to this

inheritance concept.• Whenever an object of type A is expected somewhere, we can also pass an object

with a subtype of B. This concept is called polymorphism.• There’s a new visibility mode protected: These attributes and operations behave

like private, i.e. they are not visible from outside, but they are visible withinsubclasses.





—8.4. virtual Operations—

c lass A {p u b l i c :

vo id foo ( ) ;} ;

c lass B : p u b l i c A {p u b l i c :

vo id foo ( ) ;} ;

c lass C: p u b l i c B {p u b l i c :

vo id foo ( ) ;} ;

A ob jec t1 ;B ob jec t2 ;C ob jec t3 ;A∗ ob jec t4 = new A ( ) ;A∗ ob jec t5 = new B ( ) ;B∗ ob jec t6 = new C ( ) ;

ob jec t1 . foo ( ) ; ob jec t2 . foo ( ) ; ob jec t3 . foo ( ) ;ob ject4−>foo ( ) ; ob ject5−>foo ( ) ; ob ject6−>foo ( ) ;





Static Polymorphism


vo id foo ( ) ;} ;


vo id foo ( ) ;} ;

A∗ ob jec t4 = new A ( ) ;

ob ject4−>foo ( ) ;

• C++ implements a static polymorphism by default.• It is called static, as the (known) object type determines which operation is to be

invoked.• There is a variant called dynamic polymorphism, too.• Java, C# support only dynamic polymorphism.





Dynamic Polymorphism


vo id foo ( ) ;v i r t u a l vo id bar ( ) ;

} ;


vo id foo ( ) ;v i r t u a l vo id bar ( ) ;

} ;

A∗ ob jec t1 = new A ( ) ;B∗ ob jec t2 = new B ( ) ;A∗ ob jec t3 = new B ( ) ;

ob ject1−>foo ( ) ;ob ject2−>foo ( ) ;ob ject3−>foo ( ) ;ob ject1−>bar ( ) ;ob ject2−>bar ( ) ;ob ject3−>bar ( ) ;





Dynamic Destructors

If you use inheritance, always make your destructor virtual.





Finally: Super Constructor


A( i n t v a r i a b l e ) ;} ;


B ( ) ;} ;

. . .

B : : B ( ) :A( 23 ) {

}

This is the reason, C++ introduced initialisation lists, and treats both attributes andsuper types the same way. Please take care of the order: First, you have to call thesupertype’s constructor, then you have to initialise your attributes in the right order.





Excursus: Multiple Inheritance

c lass A {} ;c lass B {} ;

c lass C: p u b l i c A,B {} ;

• Multiple inheritance is supported by C++.• However, today it is considered to be a bad smell.• There’s also a concept called virtual inheritance which is also considered to be a

bad smell nowadays.





Excursus: Private Inheritance

c lass A {} ;c lass B {} ;

c lass C: p r i v a t e A,B {} ;

• Private inheritance is supported by C++.• Here, the “is-a” relationship does not hold anymore, i.e. it is a pure implementation

inheritance, not a behavioural.• However, today it is considered to be a bad smell. I nevertheless find is useful.





Abstract Operations

c lass A {} ;

c lass B : p u b l i c A {} ;

c lass C: p u b l i c A {} ;

• Let B and C be classes that belong together logically. An example: B writesmeasurements into a database, C plots data to Matlab.

• However, B and C have to implementation in common. They share solely thesignature.

• Consequently, all operations (in A) have to be virtual.• However, it does not make sense to provide any default implementation of an

operation in A.





Abstract Operations

c lass A {v i r t u a l vo id foo ( ) = 0 ;

} ;

c lass B : p u b l i c A {v i r t u a l vo id foo ( ) ;

} ;

c lass C: p u b l i c A {v i r t u a l vo id foo ( ) ;

} ;

• vitual operations can be made abstract.• A class with at least one abstract method is an abstract class.• We cannot instantiate abstract types.• This way, we enforce subclasses to implement them.• Classes with solely abstract operations are called interface.





—8.5. static—

Variable Lifecycle

vo id foo ( ) {i n t myInt ;. . .

}

vo id bar ( ) {s t a t i c i n t myInt ;. . .

}

• Variables belong to a scope.• Whenever we encounter a variable definition, we reserve memory.• At the end of the scope, the variable is destroyed (invoke destructor, free memory).





Static Variables

vo id foo ( ) {i n t myInt ;. . .

}

vo id bar ( ) {s t a t i c i n t myInt ;. . .

}

• Variables belong to a scope.• Whenever we encounter a static variable definition for the first time, we reserve

memory.• A static variable is freed when the application terminates (invoke destructor, free

memory).





An Example for Static Variables

vo id bar ( ) {s t a t i c i n t myInt = 20;

myInt ++;}

. . .bar ( ) ; / / f i r s t c a l l o f bar ( ) everbar ( ) ;bar ( ) ;

• Static variables are bind to the application, not to the scope.• However, they are still accessible only within the scope.• Usuall, (good?) procedural programmers do not use static.





Object Variables

c lass A {i n t f i e l d ;vo id foo ( ) ;

} ;

vo id A : : foo ( ) { f i e l d ++; }

A instance1 , ins tance2 ;ins tance1 . foo ( ) ;ins tance2 . foo ( ) ;

Both instances work on different instantiations of field.





Class Variables (Static)

c lass A {s t a t i c i n t f i e l d ;vo id foo ( ) ;

} ;

vo id A : : foo ( ) { f i e l d ++; }

A instance1 , ins tance2 ;ins tance1 . foo ( ) ;ins tance2 . foo ( ) ;

A : : f i e l d ++;

Here, static binds the attribute to the class. It is a class attribute. Depending on thevisibility, one can either access the static attribute within any operation of A, or evenoutside of the class (not recommended as it violates the encapsulation idea). If the fieldis public, the class acts (from a syntax point of view) similar to a namespace.





Example: Instance Counter

c lass A {p r i v a t e :

s t a t i c i n t ins tances ;p u b l i c :

A ( ) ;} ;

vo id A : : A ( ) {i ns tances ++;

}

This code keeps track how many instances of A are created.





Static Operations


vo id foo ( ) ;s t a t i c vo id bar ( ) ;

} ;

vo id A : : foo ( ) { . . . }vo id A : : bar ( ) { . . . }

• We can also declare methods as static.• Static methods (class methods) belong to the class, not to an object.• Static methods may work on class attributes only.





A Factory Method

c lass A {p r i v a t e :

i n t a t t r i b u t e ;p u b l i c :

s t a t i c A createAnAFromTerminal ( ) ;} ;

s t a t i c A createAnAFromTerminal ( ) {A r e s u l t ;s td : : c in >> r e s u l t . a t t r i b u t e ;r e t u r n A ;

}

. . .A myA;myA = A : : createAnAFromTerminal ( ) ;

• Static methods may manipulate any attribute of the class as they are part of theclass.

• Static methods are called similar to methods wrapped by a namespace.• Static methods typically represent operations on sets while methods represent

operations on an element of a set.





—8.6. Design Patterns—

Standard book by the gang of four.

• Design Patterns: Stem from reocurringsolutions for certain technicalchallenges.

• More complex than idioms.• Something different than an algorithm:

It describes the technical realisation.• Famous book by the gang of four.





A Singleton

/∗∗∗ Implements a connect ion to a∗ database ( b ig f i l e ) w i th l o t s o f∗ mate r i a l constants .∗ /

c lass Mater ia lConstants {} ;

We want to ensure that MaterialConstantsis created only once throughout the applica-tion lifecycle. Here are some hints:• MaterialConstants should not have a

public (default) constructor.• As anyone could need the database, a

static getDatabase() operation mightbe helpful that returns an instance ofMaterialConstants.

• This static operation might hold aninstance of MaterialConstants.However, it should not create it eachtime it is invoked.





Singleton Implementation - Part I

c lass Mater ia lConstants {p r i v a t e :

/∗∗∗ noone should be able i n s t a n t i a t e the database on h is own∗ /

Mater ia lConstants ( ) ;/∗∗∗ noone should be able to copy a database∗ /

Mater ia lConstants ( const Mater ia lConstants &) ; / / noone should be ablep u b l i c :

i n t ge tEnt ry ( ) const ;vo id addEntry ( . . . ) ;

} ;

. . .Mater ia lConstants myDatabase ; / / fo rb idden

/ / however , we need something l i k emyDatabase . ge tEnt ry ( ) ;

We could make all the operations static. However, then we don’t have control of theopen process. Even worse, this won’t fit to our fancy new object-oriented paradigm.





Singleton Implementation - Part II


/∗∗∗ noone should be able i n s t a n t i a t e the database on h is own∗ /

Mater ia lConstants ( ) ;/∗∗∗ noone should be able to copy a database∗ /

Mater ia lConstants ( const Mater ia lConstants &) ; / / noone should be ablep u b l i c :

. . .

/∗∗∗ Provide access to one s i n g l e database ( s i ng l e t on )∗ /

s t a t i c Mater ia lConstants& getDatabase ( ) ;} ;

. . .Mater ia lConstants : : getDatabase ( ) . ge tEnt ry ( ) ; / / f i n eMater ia lConstants myConstants =

Mater ia lConstants : : getDatabase ( ) ; / / fo rb idden





Singleton Implementation - Part III


. . .p u b l i c :

. . .



Mater ia lConstants& Mater ia lConstants : : getDatabase ( ) {Mater ia lConstants r e s u l t ;r e t u r n r e s u l t ;

}

This simple realisation of the static operation works. However, it creates asegmentation fault.

If we removed the reference symbol, it would create one instanceand one copy per call. That is not what we want.





Singleton Implementation - Part III


. . .p u b l i c :

. . .



Mater ia lConstants& Mater ia lConstants : : getDatabase ( ) {Mater ia lConstants r e s u l t ;r e t u r n r e s u l t ;

}

This simple realisation of the static operation works. However, it creates asegmentation fault. If we removed the reference symbol, it would create one instanceand one copy per call. That is not what we want.





Singleton Implementation - Part IV


. . .p u b l i c :

. . .



Mater ia lConstants& Mater ia lConstants : : getDatabase ( ) {s t a t i c Mater ia lConstants r e s u l t ;r e t u r n r e s u l t ;

}





A Factory Pattern

• Study our simple MaterialConstantstype.

• This shall be part of a project wherewe are promised to get access to thereal database by the end of theproject. Meanwhile, we have to workwith a small number of artificialmaterial constants.

• Could be switch from one database toanother without changing anything inthe code using the material database?





A Factory Pattern

AbstractDB

RealDatabase HardcodedTestDB

getEntry()

• We define an abstract Database type. It is an interface (see UML diagram).• This type is delivered by our singleton, i.e. codes work only with the interface.• We implement the interface with a stupid class where we hardcode some material

constants (basically a big case statement).• Within the static operation, we then decide whether we deliver a real database or

our artificial one.




08 Object Oriented Programming

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