Programsystemkonstruktion med C++: ¨Ovning 2

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Programsystemkonstruktion med C++: ¨ Ovning 2

Karl Palmskog palmskog@kth.se

september 2010

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Programsystemkonstruktion med C++: ¨Ovning 2 Specalfunktioner in classes

Constructor

constructors of a class initializes class members

initial values can be given in the colon list or the code block default constructor is not used if the constructor is declared default constructor runs members constructor

Copy Constructor takes an instance of the class as an argument

Copy Constructor copies the members

c l a s s V e h i c l e {

p r i v a t e : i n t w e i g h t ; p u b l i c :

V e h i c l e ( i n t w) : w e i g h t (w) {}

} ;

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Destructor

destructor takes no arguments and does not return anything default destructor is used if not defined

runs members destructor

P e r s o n : : ˜ P e r s o n ( ) {

// f r e e up memory f o r a r r a y o f heap−a l l o c a t e d s k i l l s d e l e t e [ ] s k i l l s ;

}

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Assignment operators and the Rule of Three

the class instance variable is called assignment operator function operator=

the default operator= copies the members heuristics:

1 if you need to define the destructor you need to define Copy Constructor operator=

2 if you need to define the Copy Constructor you need to define operator=

3 if you need to define the operator= you need to define the copy constructor

destructor, copy and operator= are “associated”

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An inheritance hierarchy

Animal string name

Donkey Horse

Mule

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Special functions and inheritance

Objekt Konstruktor Destruktor Animal Animal ˜Animal

Donkey Animal ˜Donkey

Donkey ˜Animal

Horse Animal ˜Horse

Horse ˜Animal

Mule Animal ˜Mule

Donkey ˜Horse

Animal ˜Animal

Horse ˜Donkey

Mule ˜Animal

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Programsystemkonstruktion med C++: ¨Ovning 2 Virtual Functions

Virtual Functions

non-virtual functions are selected statically at compile time, depending on the type of pointer or reference

virtual functions are determined dynamically during execution based on the object’s actual type

#i n c l u d e <i o s t r e a m >

u s i n g namespace s t d ; c l a s s A {

p u b l i c :

v o i d f o o ( ) { c o u t << ”A : : f o o ( ) ” << e n d l ; }

v i r t u a l v o i d b a r ( ) { c o u t << ”A : : b a r ( ) ” << e n d l ; } } ;

c l a s s B : p u b l i c A { p u b l i c :

v o i d f o o ( ) { c o u t << ”B : : f o o ( ) ” << e n d l ; } v o i d b a r ( ) { c o u t << ”B : : b a r ( ) ” << e n d l ; } } ;

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Virtual functions in practice

#i n c l u d e ” ab . h ” i n t main ( ) {

B b ;

B ∗ b P t r = &b ; A ∗ a P t r = b P t r ; b P t r −>f o o ( ) ; b P t r −>b a r ( ) ; a P t r −>f o o ( ) ; a P t r −>b a r ( ) ; r e t u r n 0 ; }

$ . / a . o u t B : : f o o ( ) B : : b a r ( ) A : : f o o ( ) B : : b a r ( )

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Implementation of virtual functions

Mule v-table {...}

V-table

*func1

*func2

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Virtual destructor

What is the problem with this code?

c l a s s A { p u b l i c :

˜A ( ) { s t d : : c o u t << ” ˜A ( ) ” << s t d : : e n d l ; } } ;

˜B ( ) { s t d : : c o u t << ” ˜B ( ) ” << s t d : : e n d l ; } } ;

i n t main ( ) {

A∗ a = new A ( ) ; A∗ b = new B ( ) ;

d e l e t e a ; d e l e t e b ; }

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Pure virtual function

if a function is pure virtual, the classes are implemented by inheritance

correspond to abstract methods in Java

c l a s s A { p u b l i c :

v i r t u a l i n t f o o ( ) = 0 ; . . .

} ;

v i r t u a l i n t f o o ( ) {

. . . }

. . . } ;

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Programsystemkonstruktion med C++: ¨Ovning 2 Templates

Templates

templates (models) used to write type independednt code inheritance is an alternative to shared behaviors

functions and classes parameterized on a given type

compiler replaces occurrences of type parameters and create

parameter free function/class

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c l a s s S t a c k { p r i v a t e :

s t a t i c c o n s t i n t STK SIZE = 1 0 0 ; i n t m s t k [ STK SIZE ] ;

i n t m top ; p u b l i c :

S t a c k ( ) : m top ( 0 ) {}

b o o l i s E m p t y ( ) { r e t u r n ( m top == 0 ) ; } b o o l i s F u l l ( ) { r e t u r n ( m top >= STK SIZE ) ; } b o o l p u s h ( i n t v a l ) {

i f ( i s F u l l ( ) ) r e t u r n f a l s e ; m s t k [ m top++] = v a l ; r e t u r n t r u e ;

}

b o o l pop ( i n t &v a l ) {

i f ( i s E m p t y ( ) ) r e t u r n f a l s e ; v a l = m s t k [−−m top ] ;

r e t u r n t r u e ; }

} ;

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t e m p l a t e <typename T>

c l a s s S t a c k { p r i v a t e :

s t a t i c c o n s t i n t STK SIZE = 1 0 0 ; T m s t k [ STK SIZE ] ;

i n t m top ; p u b l i c :

S t a c k ( ) : m top ( 0 ) {}

b o o l i s E m p t y ( ) { r e t u r n ( m top == 0 ) ; } b o o l i s F u l l ( ) { r e t u r n ( m top >= STK SIZE ) ; } b o o l p u s h ( c o n s t T &v a l ) {

i f ( i s F u l l ( ) ) r e t u r n f a l s e ; m s t k [ m top++] = v a l ; r e t u r n t r u e ;

}

b o o l pop (T &v a l ) {

i f ( i s E m p t y ( ) ) r e t u r n f a l s e ; v a l = m s t k [−−m top ] ;

r e t u r n t r u e ; }

} ;

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Use of template class

#i n c l u d e ” s t a c k . h ” i n t main ( ) {

S t a c k <i n t > i s t k ; // s t a c k o f i n t

S t a c k <double> f s t k ; // s t a c k o f d o u b l e s i s t k . p u s h ( 4 7 1 1 ) ;

i s t k . p u s h ( 4 7 . 1 1 ) ; f s t k . p u s h ( 4 7 . 1 1 ) ; f s t k . p u s h ( 4 7 1 1 ) ; i n t i t o p ;

i s t k . pop ( i t o p ) ;

s t d : : c o u t << i t o p << s t d : : e n d l ; r e t u r n 0 ;

}

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A template maxfunction

Exam Assignment: write a template function max that returns the larger of two arguments.

c o n s t & T max ( c o n s t T & x , c o n s t T & y ) { i f ( x < y ) {

r e t u r n y ; } e l s e {

r e t u r n x ; }

}

F¨ oljdfr˚ agor:

what type parameter functionof the arguments is required?

why is the return type of reference?

why can not you give arguments of different types?

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Meta Programming with templates

t e m p l a t e<i n t N>

s t r u c t f a c t {

enum { v a l u e = N ∗ f a c t <N−1 >:: v a l u e } ; } ;

t e m p l a t e<> s t r u c t f a c t <0> { enum { v a l u e = 1 } ;

} ;

i n t main ( ) {

s t d : : c o u t << f a c t <5 >:: v a l u e << s t d : : e n d l ; r e t u r n 0 ;

}

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Compilation of templatiserade classes and functions

template classes/functions has to be defined before use usually occurs because both the declaration and definition of .h-file

compiler need not be given the name of the class file

support for separate production of the definition/declaration

varies between compilers

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Programsystemkonstruktion med C++: ¨Ovning 2