Introduction to Modern Scientific Programming · Introduction to Modern Scientiﬁc Programming Introduction to Modern Scientiﬁc Programming Dr. Markus Kirkilionis, Martin Eigel,

Introduction to Modern Scientific Programming


Dr. Markus Kirkilionis, Martin Eigel, Dr. Luca Sbano

Mathematics Department,University of Warwick

10/04

C++ Part C++ (4)

1


Part I

Introduction to the MatLab environment

2


Overview Part I

Basic UNIX shellUseful Commands

The MatLab EnvironmentCalculator FunctionsFunctions and plotting

Programming with MatLabProgramming LanguageAdvanced Graphics

MatLab

3


UNIX login

I Each user has his own home directory (path:"/home/username"). You start in that directory after loggingin.

I Paths can be absolute ("/usr/bin/kate") or relative("mydir/testprog") to the current directory.

I Your home directory also has the label "~" (e.g."~/moac/ex1").

I Pressing TAB once/twice triggers auto-completion on thecommand line.

4


Shell commands

command description

pwd print working directorycd <dir> change directory to dir

(".." is the parent directory,just ”cd” changes home directory)

mkdir <name> make directoryrmdir <name> remove directorycp <file1 ...> <dest> copy files/directories to destinationmv <file1 ...> <dest> move files/directories to destinatonrm <opt> <file1 ...> remove files; option ”-rf” (recursive,

force) removes entire directory treesls <opt> list directory contents; use with ”-l”

for extended infos

5


Shell commands (cont.)

command description

more <textfile> view textfile pagewiseless <textfile> scroll through textfile; exit with ”q”

man <cmd> get manual pages for command; usecursor keys for scrolling, exit with ”q”

apropos <keyw> list all commands including keyword

tar xvzf <arch.tar.gz> extract archive to current directory(flags: extract, verbose, zip-archive(”.gz”), filename)

ps process status shows list of processeskill <pid> kills process (in case it locked/crashed);

use option ”-9” for an enforced kill

6


UNIX software

Useful programs

kate easy to use text editorwith syntax highlighting

emacs, vim advanced editors

mozilla Web Browserskonqueror

opera

xmms Multimedia programsmplayerooffice OpenOffice (similar to MS Office)

7


UNIX software (cont.)

Useful programs

matlab MatLab programoctave free MatLab clonesscilab

KDevelop Integrated Development EnvironmentsAnjuta

gimp 2 2D graphics program (like Photoshop)sodipodi 2D vector graphics programblender 3D modeller and renderer

8


Before we start

Preliminaries

>> MatLab prompt, waiting for user input% starting a comment line; if a command line is ended with ”;”,

MatLab will not print the result of the command= assignment operator

Help/Info

help <cmd> command name is optionalwhos show defined variablesans last MatLab answermore on/off page-wise output on/offclear * clear all defined variables

CTRL + C cancel current calculation

9


MatLab as a calculator

Predefined functions (like sin, cos, tan, log, sqrt) can befound within the MatLab help system (help elfun).

1 >> 2∗6.4+12ˆ52 ans = 2.4884 e+053

4 >> s i n (2∗ p i )+1.4 i+sqr t (3 )5 ans = 1.7321 + 1.4000 i

10


Vectors and Matrices

Assigning values to vectors and matrices:

1 >> n = 7 % s c a l a r

2 >> A = [ 1 2 3 4 5 ] % 1 x5 mat r i x

3 >> B = 1 : 5 % same as above

4 >> C = 1 : 2 : 1 0 % 1 x5 mat r i x w i th s t e p s i z e 2

5 >> D = n : −0 . 3 : 1 % produce s a 1 x21 mat r i x

6 >> E = [ 1 ; 2 ] % 2 x1 mat r i x

7 >> F = [ 1 2 ; 3 4 ] % 2 x2 mat r i x

8 >> s = ’ s t r i n g t e s t ’ % s t r i n g as s i gnment

9 >> A ’ % con juga t e t r a n s p o s e mat r i x

11


Matrix access1 >> A = rand ( 3 , 3 )2 A =3 0 .84434 0 .28686 0.140044 0 .26009 0 .57137 0.895385 0 .96570 0 .73364 0.467776 >> A(1 , 2 )7 ans = 0.286868 >> A(2 , 2 : 3 )9 ans =

10 0 .57137 0.8953811 >> A( 2 , : )12 ans =13 0 .26009 0 .57137 0.8953814 >> s = ’ t e s t ’ ;15 >> s (2 )16 ans = e

12


Arithmetic Operators

X + Y Addition between equally size matrices or with scalar YX - Y SubtractionX * Y Matrix multiplicationX .* Y Element by element multiplicationX / Y Right division (eq: ((Y ′)−1 ∗ X ′)′)X ./ Y Element by element right divisionX \ Y Left division (eq: X−1 ∗ Y )X .\ Y Element by element left division

13


Arithmetic Operators (cont.)

X ^ Y Power operatorX .^ Y Element by element power operator-X NegationX’ Complex conjugate transposeX.’ Transpose

See also help matfun for further matrix functions.Comparison operators are the usual suspects:X < Y, X <= Y, X == Y, X<>Yand so on.

14


Creating matrices

zeros(m,n) zero m × n matrixones(m,n) m × n matrix of oneseye(n) n × n identity matrixdiag(v) n × n matrix with v (an n-vector) on its diagonal

1 >> A = rand ( 2 , 3 ) % random 2 x3 mat r i x

2 A =3 0 .466753 0 .415569 0.3938084 0 .749506 0 .020459 0.5287075

6 >> b = rand ( 1 , 2 ) % random ve c t o r o f s i z e 2

7 b =8 0 .62804 0.36830

15


Basic function plotting

1 >> x = −5:5;2 >> y = x . ˆ 3 ;3 >> p l o t ( x , y ) % p l o t as connected graph

4 >> p l o t ( x , y , ’ o ’ ) % p l o t on l y p o i n t s

-150

-100

-50

0

50

100

150

-6 -4 -2 0 2 4 6

line 1

Other useful functions:hold on/off, title(string)x/ylabel(string)axis(range)text(x,y,string)legend(name1, name2,...)

16


Basic function plotting (cont.)

1 % 100 p o i n t s i n [ 0 , 1 ]

2 >> x1 = l i n s p a c e ( 0 , 1 , 1 0 0 ) ;3 % 100 p o i n t s i n [0 ,10ˆ −1]

4 >> x2 = l o g s p a c e (0 ,−1 ,100) ;5 >> p l o t ( x1 , x1 . ˆ 3 . ∗ s i n ( x1 ) , x2 , tan ( x2 ) )6 >> x l a b e l ( ’ x ’ ) , y l a b e l ( ’ f ( x ) ’ )7 % se t t i t l e , d i s p l a y g r i d , a l l ow a d d i t i o n a l p l o t s i n graph

8 >> t i t l e ( ’ s i n ( ) and tan ( ) ’ )9 >> g r i d

10 >> ho ld on11 >> p l o t ( x2 , z e r o s ( s i z e ( x2 ) ) , ’+ ’ )12 % se t v i ewpo r t to 1 <= x <= 5, −10 <= y <= 10

13 >> a x i s ( [ 1 5 −10 10 ] )

17


-1

-0.5

0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1

f(x)

x

line 1line 2line 3

18


Other basic commands

max(x) return largest entry of xmin(x) return smallest entry of xabs(x) return absolute valuessize(A) return 1× k vector of As dimensionslength(A) length of array is max(size(A))save <fname> save variables to fileload <fname> load variables from filequit exits MatLab

19


Script files

I Matlab files have suffix ”.m”.

I They include scripts (list of commands like in a user session)or a single function of the same name as the file.

I Script/Function files have to be in working directory (pwd) orin one of the directories included in the search path.

I Scripts/Functions are executed by typing their filename(without suffix).

I Matlabs own text editor can be started with the commandeditor.

20


Control structures: Conditions

The if <exp> <...> elseif <exp> <...> else <...>construct is used to execute code depending on given conditions.

1 >> i f b > 02 c = a/b ;3 e l s e i f b < 04 c = −a/b ;5 e l s e6 c = 0 ;7 end

21


Control structures: Loops

for loops typically involve a running variable like i=1:n,

1 >> n = 13 ;2 >> f a c = 1 ;3 >> f o r i =2:n4 f a c = i ∗ f a c ;5 end6 >> fac , c l e a r f a c

while loops are executed while a certain condition is fulfilled.

1 >> n = 0 ;2 >> wh i l e n+1 <= x3 n = n+1;4 end5 >> n

The break command can be used to terminate loops.

22


Iterations in MatLab

Algorithm to compute the square root of 2, implemented inMatLab:

1 >> n = 1 ;2 >> x (1)=1;3 >> wh i l e n+1 <= 104 x ( n+1) =x (n)+(2−x ( n )ˆ2)/(2∗ x ( n ) ) ;5 n=n+1;6 end7 >> x

23


Iterations in Octave

Algorithm to compute the square root of 2, implemented inOctave:

1 octave > n = 1 ;2 octave > x (1)=1;3 octave > wh i l e ( n+1 <= 10)4 > x ( n+1)=x (n)+(2−x ( n )ˆ2)/(2∗ x ( n ) ) ;5 > n++6 > endwh i l e7 octave > x

24


Differential Equations in MatLab

Euler Algorithm to numerically integrate the ODE:

dx(t)

dt= x(t)(1− x(t)), x(0) = a

implemented in MatLab:

1 >> n = 1 ;2 >>h=0.01;3 >> x (1)=a ;4 >> wh i l e n+1 <= 1005 x ( n+1) =x (n)+h∗( x ( n)∗(1−x ( n ) ) ) ;6 n=n+1;7 end8 >> p l o t ( x )

25


Differential Equations in Octave

Euler Algorithm to numerically integrate the ODE:

dx(t)

dt= x(t)(1− x(t)), x(0) = a

implemented in Octave:

1 octave > n = 1 ;2 octave>h=0.01;3 octave > x (1)=a ;4 octave > wh i l e ( n+1 <= 10)5 > x ( n+1)=x (n)+h∗( x ( n)∗(1−x ( n ) ) ) ;6 > n++7 > endwh i l e8 octave > gp l o t ( x )

26


Functions

By writing function files (”.m” suffix and function keyword) onecan define custom functions that behave just like the built in ones.

1 f u n c t i o n d = d i s t ( a , b )2 % DIST . C a l c u l a t e d i s t a n c e .

3 % DIST(a , b ) r e t u r n s d i s t a n c e between a and b .

4

5 ba = b − a ; % ba and dd a r e l o c a l v a r i a b l e s !

6 dd = ba ∗ ba ’ ;7 d = s q r t ( dd ) ;8 % end o f f u n c t i o n d i s t

By providing an appropriate comment directly after the functiondefinition, help square will show exactly that.Source of several routines can be examined by using type, e.g.type rank.

27


Functions (cont.)

nargin equals the number of arguments given to the function.error can be use to print an error message to the user.

1 f u n c t i o n d = d i s t ( a , b )2 % DIST . C a l c u l a t e d i s t a n c e .

3 % DIST(A,B) r e t u r n s d i s t a n c e between A and B.

4 i f ( na r g i n ˜= 2)5 e r r o r ( ’ Not enough i npu t arguments ’ ) ;6 end7 i f ( s i z e ( a , 1 ) ˜= s i z e (b , 1 ) )8 e r r o r ( ’A and B not o f same d im e n s i o n a l i t y ’ ) ;9 end

10

11 ba = b − a ;12 d = ba ∗ ba ’ ;13 d = s q r t ( dd ) ;14 % end o f f u n c t i o n d i s t

28


Functions (cont.)

Multiple return values are simply defined as a row vector.

1 f u n c t i o n [ vo l , d i a ] = c u b e p r o p e r t i e s (q , d )2 % CUBEPROPERTIES . Volume and l e n g t h o f d i a g on a l o f cube .

3 % CUBEPROPERTIES(Q,D) r e t u r n s volume VOL and l e n g t h o f

4 % d i a g ona l DIAG o f a D−d imen s i o n a l cube wi th s i d e l e n g t h s Q.

5 % D i s op t i o n a l , d e f a u l t i n g to 2 .

6 i f n a r g i n < 27 d = 2 end8 v o l = qˆd ;9 d i a = q∗ s q r t ( d ) ;

10 % end o f f u n c t i o n c u b e p r o p e r t i e s

The function can be called like this:[vv,dd] = cubeproperties(1.4,3)[vv,dd] = cubeproperties(2.2) (implicitly q = 2)

29


Functions: recursion

Functions can of course call themselves. For calculating theFibonacci sequence given by

fn = fn−1 + fn−2, f0 = f1 = 1

we define function fibonacci.

1 f u n c t i o n f n = f i b o n a c c i2 n = i n p u t ( ’ Type i n f i b o n a c c i number : ’ ) ;3 i f n == 04 f n = 1 ;5 e l s e i f n == 16 f n = 1 ;7 e l s e8 f n = f i b o n a c c i ( n−1) + f i b o n a c c i ( n−2);

input requests user input, disp displays a string.

30


3D plots

meshgrid generates the product set X × Y elements of which canthen be used for evaluating functions at.

1 >> x = (0 :2∗ p i /20 :2∗ p i ) ;2 >> y = ( 0 : 1 / 4 0 : 1 ) ;3 >> [X ,Y] = meshgr id ( x , y ) ;4 >> Z = s i n (X) . ∗ exp (Y ) ;5 >> mesh (X,Y, Z ) ; % unshaded

6 >> s u r f (X,Y, Z ) ; % shaded

7 >> con tou r (X,Y, Z ) ; % contou r p l o t

With subplot(m,n,i) you can create several (i.e. m ∗ n) plots ina single window.

31


3D plots (cont.)

peak(n) generates a n × n demo data set.

1 >> s = peaks ( 5 0 ) ;2 >> mesh ( s )3 >> c on t ou r f ( s ) % f i l l e d contou r p l o t

4 >> co lormap ( hot )5 >> s had i ng i n t e r p6 >> r o t a t e 3d on

Other options:colormap ([flag | gray | hsv | jet | prism | ...])shading [interp | flat | faceted]

32


Parametric plots

Parametric plots (f (t), g(t)) are straightforward:

1 >> t = (0 :2∗ p i /100:2∗ p i ) ’ ;2 >> p l o t ( cos ( t ) , s i n ( t ) ) % pa r ame t r i c p l o t

3 >> a x i s ( ’ s qua r e ’ ) % make s c a l i n g equa l

Other useful commands (use help)loglog, semilogx, semilogyprint, printopt

Also browse the online doc!

33


Part II

Introduction to Object Oriented Programming withC++

34


Overview Part II

Execution ControlConditionals and LoopsWriting and Compiling Your Source Code

Object Oriented ProgrammingConcepts of OO

More Advanced C++

35


Introduction

Some prerequisites before we can start with Object Orientation(OO):

I Definition of variables

I Definition of functions

I Execution control statements.

I The compiler model.

We will then move on to the central idea of OO: the class.

36


IntroductionSome General Remarks

I A block/group of code (i.e. a compound statement) isdenoted by braces ”{ }”.When the term statement is used in this notes, it can be asingle statement or a compound one.

I Variables declared within "{ }"-groups are only accessiblewithin the nearest group (scope). This is called scoping.

I Variables declared outside any compound statements areglobally accessible. This should be avoided however, as itmakes code hard to understand and error prone.

I To compile (i.e. generate an executable file) and test code,you will need to define at least a main() function for yourprogram. This will be explained shortly.

37


IntroductionDeclaration and Definition

A C++ program is a collection of variables, functions, definitionsand function calls.

I A declaration introduces a new identifier to the compiler:”This function or this variable exists somewhere and here iswhat it should look like”.

I A definition means: ”Make this variable or this functionhere”.In this case the compiler actually allocates storage for thefunction or variable.

Declarations have to occur before you access the respectiveidentifiers!

38


IntroductionDeclaration & Definition of Variables

In contrast to most scripting languages C++ is strongly typed, i.e.every variable is of a fixed type.Declaring a variable: TYPE variablename;Basic types are int, unsigned int, long, unsigned long,float, double, char.

1 i n t i n t e g e r 1 = 2 ;2 i n t i n t e g e r 2 , i n t e g e r 3 ;3 i n t e g e r 2 = 5 ;4 i n t e g e r 3 = ( i n t e g e r 1 + i n t e g e r 2 ) ∗ i n t e g e r 2 ;5

6 double mydouble = 3 . 141 ;7 char mycharac te r = ’ a ’ ;

Note: Each statement has to be terminated by a ”;”.

39


IntroductionBoolean operators

A boolean variable can be either false or true:"bool my flag = true;"

logical op sample

&& AND true && false == false|| OR true || false == true! NOT !false == true

Note: For bitwise AND, OR, XOR and the complement theoperators "&, |, ^, ~" have to be used1: "5 & 4 == 4".

1valid for cardinal types char, int, long

40


IntroductionType Casting

I Sometimes it is necessary to explicitely change the type of avariable.

I This is called type casting.

I The syntax is: (NEW TYPE)variable

1 i n t i = 2 ;2 double d1 , d2 , d3 ;3 d1 = i /3 ; / / p e r f o r m s i n t e g e r d i v i s i o n

4 d2 = i / 3 . ; / / p e r f o r m s d o u b l e d i v i s i o n

5 d3 = ( double ) i /3 ; / / d i t o

Note: Casting is considered bad coding style and should beprevented whenever possible. C++ offers casting templates thatare more type safe.

41


IntroductionArrays

I An Array is a sequential list of a certain type.

I It can be defined by "TYPE arrayname[SIZE];". The size isfixed at compile time, i.e. it has to be a constant number, nota variable!

I Access an element of the array by "arrayname[POS]".

I The index starts at 0 and ends at SIZE-1.

1 char cAr r [ 4 ] = { ’ a ’ , ’ c ’ , ’ g ’ , ’ t ’ } ;2 s t d : : cout << ” Las t : ” << cAr r [ 3 ] ;3

4 i n t i A r r [ 1 0 ] ;5 f o r ( i n t i = 0 ; i < 10; ++ i )6 i A r r [ i ] = 10− i ;

Note: Arrays defined like this can not change their size dynamically.We will get to better methods when discussing STL containers!

42


IntroductionDeclaration of Methods and FunctionsDeclaring a function(we’ll come to details later!):return TYPE functionname(param TYPE varname[, ...]);

1 / / f u n c t i o n d e c l a r a t i o n

2 void p r i n tSometh i ng ( ) ;3

4 / / c o u l d u s e p r i n t S o m e t h i n g ( ) h e r e a l r e a d y ,

5 / / b u t n o t s q u a r e ( x ) !

6

7 / / f u n c t i o n d e f i n i t i o n

8 void p r i n tSometh i ng ( ){9 s t d : : cout<<” I got c a l l e d ” ;

10 }11

12 / / d e c l a r a t i o n a n d d e f i n i t i o n o f f u n c t i o n

13 unsigned i n t squa r e ( i n t number ){14 return number∗number ; }

Note: std::cout is the standard output stream printing to theconsole. For using it, the iostream classes have to be included atthe top of the file: #include <iostream>

43


Introduction Scope Example

1 i n t i 1 = 3 ; / / g l o b a l v a r i a b l e i 1

2

3 void func ( ){4 i n t i 2 = i 1 ; / / c a n s e e i 1

5 }6

7 i n t main ( ){8 i n t i 3 = i 1 ; / / c a n s e e i 1

9

10 {11 i n t i 4 ;12 }13

14 / / c a n n o t s e e i 2 , i 4 , i 5 h e r e !

15 i n t i 5 ;16 }

44


IntroductionExecution Control

Classical control structures are inherited from C:

I if-else statement

I while and do-while loops

I for loop

I switch-case selection statement.

These are similar to other imperative programming languages2.They depend on a boolean expression that can be either true orfalse.

Boolean Comparison

Use the common operators < > <= >= != and pay attention tothe equality operator ==. Don’t confuse it with the assignment =.

Moreover, nonzero values are always considered being true(i.e. 0 == false).

2Pascal, Basic, Ada, Matlab, Fortran, Java, . . .

45


IntroductionExecution Control: if-else

Two forms are possible:

1 i f ( e x p r e s s i o n )2 s ta tement3

4

5 i f ( e x p r e s s i o n )6 s ta tement7 e l s e8 s ta tement

Note: It is common to indent your code according to its logicalstructure3. This makes the code much easier to read.

3pay attention to the provided examples!

46


Introductionif-else example

I The standard function rand() returns an integer number in[0,RAND MAX ]. Requires: "include <stdlib.h>"

I std::cout and std::cin denote the console output andinput streams.

I Use operators << and >> for sending and receiving data.

1 unsigned i n t r a n d i n t = rand ( ) ;2 i f ( r a n d i n t <= 1000){3 s t d : : cout << ” r a n d i n t i s : ” << r a n d i n t ;4 } e l s e s t d : : cout << ” r a n d i n t i s l a r g e r than 1000” ;

47


IntroductionExecution Control: while

The expression can be evaluated at the beginning or at the end ofthe loop:

1 whi le ( e x p r e s s i o n )2 s ta tement3

4

5 do6 s ta tement7 whi le ( e x p r e s s i o n )

Note: You can break out of a while and for loop or jump to thenext iteration by using

break quit the loop

continue stops current iteration and goes back to beginningfor next iteration.

48


Introductionwhile example

1 unsigned i n t r a n d i n t = rand ( ) ;2 whi le ( r a n d i n t < RAND MAX/2)3 r a n d i n t = rand ( ) ;4

5

6 whi le (1){7 i f ( ( double ) rand ( )/RAND MAX > 0 .9)8 break ; / / l e a v e u n c o n d i t i o n a l l o o p

9 e l s e s t d : : cout << ” . ” ;10 }11

12

13 i n t i ;14 do { / / n u m b e r 0 w i l l e x i t l o o p

15 s t d : : c i n >> i ;16 }whi le ( i != 0 ) ;

Note: do-while is executed at least once, even if the initialcondition is false.

49


IntroductionExecution Control: for

Normally applied for ”counting” loops:

I Initialisation before the first iteration.

I Conditional testing.

I Some form of ”stepping” to alter the conditional expression.

1 f o r ( i n i t i a l i s a t i o n ; c o n d i t i o n a l ; s t e p )2 s ta tement

50


Introductionfor example

1 / / p r i n t n u m b e r s 1 . . 1 0

2 f o r ( i n t i = 0 ; i < 10; ++ i )3 s t d : : cout << ” i : ” <> c ) ;

Note: ++ and -- denote the increment and decrement operators,i.e. ++i means i = i + 1.

51


IntroductionExecution Control: switch-caseMulti-way selection:

I Selector gives an integral value.

I This is compared to the defined values.

I If a matching value is found, the statement is executed.

I If no match is found, default is executed.

1 switch ( s e l e c t o r ){2 case i n t e g r a l −va l u e1 : s ta t ement ; break ;3 case i n t e g r a l −va l u e2 : s ta t ement ; break ;4 . . .5 de fau l t : s t a t ement ;6 }

Note: The trailing break quits the execution in the switchstatement. Otherwise execution would run though all followingstatements.

52


Introductionswitch-case example

1 char c ;2 bool end = f a l s e ;3 do{4 s t d : : c i n >> c ;5 switch ( c ){6 case ’ h ’ :7 s t d : : cout << ”Hi t h e r e ! ” ;8 break ;9 case ’Q ’ :

10 case ’ q ’ :11 end = true ;12 break ;13 de fau l t :14 s t d : : cout << ”Unknown command . ” ;15 }16 whi le ( ! end ) ;

53


IntroductionThe main function

I Function int main(int argc, char** args) is startingpoint when running your program.

I args is an array of argc command line arguments.

I int main() is another possibility when no arguments arerequired.

1 #inc lude 2

3 i n t main ( ){4 s t d : : cout<<”Easy Going ! ”<<s t d : : e nd l ;5 return 0 ;6 }

54


The Compiler Model

Steps for writing software

I Open text editor of your choice (e.g. kate). Source files oftenget the extension ".cpp", declarations ".h".

I Every program must have exactly one main function4. This isthe entry point for program execution.

I We will be using the GNU C++ compiler g++. Simple codecan be compiled by typing "g++ my program.cpp".

I The compiler produces an executable file. This can be run bytyping "./my program".

I In case your software contains bugs, you might want to debugit, e.g. by using ddd.

4libraries dont need to

55


The Compiler Model (cont.)

Write Source Compile Files

Run Preprocessor Run Compiler Run Linker

ErrorOK

Generating object code Generating binaryResolving Macros,

Templates, Defines

ErrorOK

Correct Code

C++ Compiler

Test Code

Start End

Use Debugger

Using Makefiles

for larger projects

56


MakefilesI A Makefile lets you define all commands required for

building a program in an easier to manage and repeatable way.I You normally only have to type "make" which automatically

executes the building process defined in the file "Makefile".

The structure is (commands have to be preceded by a TAB !):

TARGET T: REQUIRES X, Y, ZCOMMAND

Example:

# link command:my_program: my_source.o

g++ my_source.o -o my_program

# compilation commands:my_source.o: my_source.cpp

g++ -c my_source.cpp -o my_source.o57


A Better make: SCons

I SCons5 is a modern replacement for make.

I It is based on the Python scripting language, making it morepowerful and comprehensible.

I SCons files are named "SConstruct".

I Start building process by typing "scons".

1 # B u i l d ” m y p r o g r a m ” f r o m s o u r c e ” m y s o u r c e . c p p ”

2 env=Envi ronment ( )3 env . Program ( ’ my program ’ , ’ my source . cpp ’ )

or

1 # B u i l d ” m y p r o g r a m ” f r o m l i s t o f s o u r c e s

2 env . Program ( ’ my program ’ , [ ’my A . cpp ’ , ’my B . cpp ’ ] )

5www.scons.org

58


C++ overview

I First commercial availability in 1985.

I C++ has evolved into being the most widely used - and assuch the certainly best supported - programming language.

I Operating systems, word processors, games, scientificapplications etc. are written in C++.

I These days freely available on literally every platform (e.g.GNU compiler).

I Hundrets of books, thousands of libraries and many tools likeIDEs6, compilers, debuggers, profilers, editors etc.

6integrated development environments

59


C++ overview (cont.)

Conceptually C++[4]

I is a general purpose programming language

I is a better C

I supports data abstraction

I supports object oriented programming

I facilitates generic programming.

60


OO Principles

Data Abstraction/Encapsulation Objects communicate throughtheir interfaces, i.e. data and logic are not revealedto other objects.

Polymorphism Objects can appear in ”multiple forms” while at thesame time providing identical interfaces.

Inheritance Properties (i.e. attributes and methods) can beinherited/derived from one object to others.

public Methods

Data

Methods

Methods

Object Interface

class A

class B

Objects are called classes in C++,defined by the class keyword.

61


OO Principles (inheritance/polymorphism)

Shape is base class (generalisation), Circle and Rectangle areconcretisations:

Shape

#color :Color

+ getArea ():double

+ draw ():void

+ setColor ():void

Circle

+ getArea ():double

Rectangle

+ getArea ():double

+ rotate (angle :double ):void

Generalisation

I They inherit methods andattributes of Shape.

I Existing methods can beoverloaded for providing a newimplementation of a derivedinterface: Is-A relationship.

I New attributes and methodscan be added: Is-Like-Arelationship.

62


OO Principles (cont.)

Some OO advantages

I Reusability of code by abstraction, component models,inheritance.

I Maintainability: Easier extension of data and methods.

I Working with objects supports clean software engineering:Data and methods combined.

63


A Gentle Introduction to C++

Note

C++ is not C.Learn C++ (i.e. design classes!), not C (i.e. procedural design).

Most important entity is a class:

1 c l a s s Tes tC l a s s {2 } ;

It can contain member variables and methods. These can bedeclared being private or public7.Public methods and variables define the interface of a class.Private methods and variables can only be accessed by methods ofthe class.

7or protected

64


C++ Class example

I C++ classes can be used like normal built-in types.

I Access public methods or variables of a class by "." qualifier.

1 c l a s s Tes tC l a s s {2 pub l i c :3 void p r i n t S t r i n g ( s t d : : s t r i n g s ){4 s t d : : cout<<”This i s the s t r i n g : ” << s + ”\n” ;5 }6 } ;7

8 i n t main ( ){9 Tes tC l a s s t c ; / / d e f i n e v a r i a b l e t c

10 t c . p r i n t S t r i n g ( ”Hi ! ” ) ; / / c a l l m e t h o d o f c l a s s

11 return 0 ; / / e n d p r o g r a m

12 }

65


C++ Class example (cont.)

I For nontrivial classes, declaration and implementation shouldbe separated.

I Put declaration in "YourClass.h", definition in"YourClass.cpp".

I Include declaration in all files that need to use your class with"#include <YourClass.h>". Esp. "YourClass.cpp" needs it!

1 / / d e c l a r a t i o n i n ” t e s t c l a s s . h ”

2 c l a s s Tes tC l a s s {3 pub l i c :4 void p r i n t S t r i n g ( s t d : : s t r i n g s ) ;5 } ;

1 / / d e f i n i t i o n i n ” t e s t c l a s s . c p p ”

2 void Tes tC l a s s : : p r i n t S t r i n g ( s td : : s t r i n g s ){3 s t d : : cout<<”This i s the s t r i n g : ” << s + ”\n” ;4 }

66


C++ Class example (cont.)

MyClass.h

MyClass 1 declaration

MyClass 2 declaration

MyClass1.cpp

MyClass 1 implementation

MyClass2.cpp

MyClass 2 implementation

MyProgram.cpp

int main(){

MyClass2 mc2;

...

}

MyClass1.obj

MyClass2.obj

MyProgram.obj

MyProgram

Compile Link

67


Class Inheritance

I Inherited methods may be overloaded, i.e. a newimplementation is provided.

1 c l a s s A{2 pub l i c :3 s t d : : s t r i n g getName ( ) { return ” Ba s e c l a s s ” ; }4 i n t Counter ;5 } ;6

7 c l a s s B: pub l i c A{8 s t d : : s t r i n g getName ( ) { return ” Der i v ed C l a s s ” ; }9 } ;

10 / / m a i n ( ) o m i t t e d

11 B b ;12 b . Counter = 1 ;13 s t d : : cout << b . getName ( ) ;

68


Class Lifecycle

I The definition of a variable of a class is called instantiation.

I The variable itself is an instance of the respective class.

I When a class is instantiated, one of its constructors is called.

I A constructor should setup the classes state, i.e. its variables.

I When an instance, i.e. a variable of class type, is destroyed,the class’ destructor is called.

I A destructor should clean up a classes state, e.g. write itsvariables to a file.

I In case you do not provide either or both of these methodtypes for your class, the compiler will implicitely define adefault constructor and/or destructor.

69


Class Lifecycle: Constructor

I A class can have several constructors.

I They must be distinguished by their parameter list.

I Constructors do not have a return type.

I They qualify as constructors by having the same name as theclass.

1 c l a s s Coun t i ngC l a s s {2 pub l i c :3 Coun t i ngC l a s s ( ) : Counter ( 0 ) {}4

5 Coun t i ngC l a s s ( i n t c ) : Counter ( c ) {}6

7 pr i va te :8 i n t Counter ;9 } ;

70


Class Lifecycle: Constructor (cont.)

The syntax for variable initialisationCountingClass():Counter(0) {}

is8 equivalent toCountingClass() { Counter = 0; }

Constructors - and all other functions! - can be given defaultargumentsCountingClass(int c = 0):Counter(c) {}

The last example combines the two previously defined constructorsinto one constructor.

When providing a default parameter, all other parameters to theright also need to have a default argument.void myfunc(int i, int j = 4, int k = 10);

8most of time

71


Class Lifecycle: Destructor

I A class has exactly one destructor.

I The destructor does not have a return type and also has noarguments.

I It gets the same name as the class, headed by a tilde ~.

1 c l a s s Coun t i ngC l a s s {2 pub l i c :3 ˜ Coun t i ngC l a s s ( ){4 s t d : : cout << ”Counter : ” << Counter ;5 s t d : : cout << ” Terminat ing .\ n” ;6 }7

8 pr i va te :9 i n t Counter ;

10 } ;

72


Inheritance and Construction/Destruction

I The base class’ ctor9 is called automatically before the derivedclass’ ctor is executed.

I The base class’ dtor is called automatically after the derivedclass’ dtor is executed10.

I If you want a different constructor than the default one of thebase class to be called, use:

1 c l a s s B : pub l i c A{2 pub l i c :3 / / B ’ s c t o r c a l l s A ’ s c t o r A ( s t r i n g , i n t )

4 B( s td : : s t r i n g s ) :A( s , 3 )5 { }6 } ;

9ctor is a common abbreviation for constructor, dtor means destructor10this can be circumvented by using a virtual destructor (will discuss later)

73


The C++ Class: OverviewFeatures of a class TestClass:

I When instantiated, one of the class’ constructors are called.

I When disposed the unique class’ destructor is called.

I Each class can obtain several constructors namedTestClass(YOUR-PARAMS) and one destructor~TestClass(). These methods have no return type.

I A class can be inherited from a base class.

I Methods and data are contained in it!

instantiate class access class’ members destroy class (implicitely)

constructor called destructor called

74


Using Library Classes

For many common tasks C++ already provides functions, classesand data structures in the standard library.

I Classes of the standard library are included in thenamespace11 "std".

I Namespaces provide a logical naming organisation.

I Namespace qualifier is "::".

I You access classes of the standard library by a preceding"std::".

I Recall: To use classes defined in other files, use "#include<otherclass>" to include the declarations you intend toaccess.

I "using namespace std;" can be used to automaticallyaccess the standard namespace.

11Namespaces will (somewhen. . . ) be explained in the appendix in more detail.

75


stdlibThe std::string Class

I Provides a poweful character aggregation type.I Operators "=, +=, +, []" are overloaded, see example.I Some methods: string substr(uint from, uint to),

int find(string s, frompos = 0),uint size(), insert(uint pos, string s).

1 #inc lude < s t r i n g >2 using namespace s t d ;3

4 i n t main ( ){5 s t r i n g s1 ( ” s t r i n g 1 ” ) ;6 s t r i n g s2 = ” s t r i n g 2 ” ;7 s t r i n g s3 = s1 + s2 ;8 s2 += s1 ;9 cout << s2 [ s2 . f i n d ( ”1” ) ] ;

10 return 0 ; }

76


stdlibThe iostreams ClassStandard C++’s iostreams provide functions for reading andwriting to files and the console.

I Include declarations of functions and data:"#include<iostreams>"

I Streams declare operators << for writing and >> for reading.I Stream names: cout, cin (console output and input).

1 / / p r e p r o c e s s o r d i r e c t i v e f o r d e c l a r a t i o n i n c l u s i o n

2 #inc lude 3 #inc lude < s t r i n g >4 using namespace s t d ;5 s t r i n g s = ”whats up ” ;6 i n t main ( ){7 s t r i n g name ;8 c i n >> name ;9 cout << s << name << ”?” << end l ; }

77


stdlibReading and Writing Files

File streams are declared in the fstream classes.

I Include declarations by "#include <fstream>".

I In analogy to std::cin, std::cout use std::ifstream,std::ofstream (input/output file stream) with operators<<, >>.

I Constructors of ifstream, ofstream take string containingfilename to be used, e.g.ifstream in("testfile.txt");

I Use "bool getline(ifstream, string)" for reading a fileline-wise: Read line from ifstream into string, returningtrue when successful, false otherwise (e.g. ”end of file”).

78


stdlibReading and Writing Files: Example

1 #inc lude < s t r i n g >2 #inc lude < f s t r eam>3 using namespace s t d ;4

5 i n t main ( ){ / / c o p y f i l e

6 / / o p e n f o r r e a d i n g

7 i f s t r e am i n ( ” F i l e . t x t ” ) ;8 / / o p e n f o r w r i t i n g

9 o f s t r e am out ( ”Copy . t x t ” ) ;10 s t r i n g s ;11 whi le ( g e t l i n e ( in , s ) )12 out << s << ”\n” ;13 return 0 ;14 }

Note: getline() discards newline character so we add it again.

79


stdlibThe Container std::vectorThe standard library contains generic implementations forcommonly used containers named vector, list, deque, set,map, hash, queue, stack and string.

I Containers provide dynamic memory management at runtime!I std::vector is the probably best known container.I Use of conventional arrays should be discarded in favour of

std::vector.I As a template class it can contain objects of arbitrary type.I Usage: "#include <vector>" first, then

"std::vector<TYPE> my var;".

1 #inc lude < vec to r >2 using namespace s t d ;3 / / m a i n ( ) o m i t t e d

4 vec to r <int > i n t v e c ;5 vec to r <s t r i n g > s t r i n g v e c ;6 vec to r <Tes tC la s s > t e s t c l a s s v e c ;

80


stdlibThe Container std::vector (cont.)

I uint size() returns the number of elements,

I push back(x) inserts an element x ,

I clear() removes all elements,

I index operator [pos] returns element at pos.

1 / / i n c l u d e s a n d m a i n ( ) o m i t t e d

2 s t r i n g s1 = ” f e ed ” ;3 s t r i n g s2 = ”me” ;4 vec to r <s t r i n g > s t r v e c ;5 s t r v e c . push back ( s1 ) ;6 s t r v e c . push back ( s2 ) ;7 s t r v e c . push back ( s t r i n g ( ” ! ” ) ) ;8 f o r ( unsigned i n t i = 0 ; i < s t r v e c . s i z e (); ++ i )9 cout << s t r v e c [ i ] << ” ” ;

81


stdlibThe Container std::map

I Recall: The std::vector can be used as a versatilesubstitute for conventional arrays.

I It is a sequential container, i.e. a linear list of elements thatcan be accessed by their index.

I A std::map is an associative container.

I It contains pairs of <key,value> elements.

I A map can be accessed by element keys.

I We will use the std::pair<keyType,valueType> templatefor creating elements to put in a std::map.

82


stdlibThe Container std::map (cont.)

1 / / i n c l u d i n g , < s t r i n g > , < map >

2 using namespace s t d ;3 / / m a i n ( ) o m i t t e d

4 map<s t r i n g , int > my map ;5 pa i r <s t r i n g , int > pr1 ( ”A” , 1 0 ) ;6 pa i r <s t r i n g , int > pr2 ;7 pr2 . f i r s t = ”B” ;8 pr2 . second = 100 ;9 my map . i n s e r t ( pr1 ) ;

10 my map . i n s e r t ( pr2 ) ;11 my map . i n s e r t ( pa i r <s t r i n g , int >(”AnotherWay” ,−10)) ;12 my map [ ” C r e a t e I t ” ] = 1234 ;13

14 cout << my map [ ”B”] << my map [ ” C r e a t e I t ” ]15 << my map [ ”C” ] ;

83


stdlibIterators

I Iterators provide a generic way for accessing containerelements.

I They generalise the notion of pointers12.

I Each standard container implicitely defines appropriateiterator types, e.g. std::vector<int>::iterator orstd::map<string,int>::iterator.

I One gets iterators to the start/end of a container by callingmethods begin()/end() on a container instance.

I Access the element pointed to by an iterator it bydereferencing it: "*it".

I This convention is common to all containers!

12which we will encounter shortly

84


stdlibIterators: Example

1 typedef vec to r <char > : : i t e r a t o r v e c i t ;2 typedef map<s t r i n g , int > : : i t e r a t o r map i t ;3 / / g i v e n a v e c t o r < c h a r > v a r i a b l e ” v e c ”

4 f o r ( v e c i t i t = vec . beg i n ( ) ; i t != vec . end (); ++ i t )5 cout << ∗ i t ;6

7 / / g i v e n a map < s t r i n g , i n t > v a r i a b l e ” s i m ”

8 f o r ( map i t i t = sim . beg in ( ) ; i t != vec . end (); ++ i t )9 cout << (∗ i t ) . f i r s t << ”\ t ” << (∗ i t ) . second ;

I When accessing element pairs stored in std::map (and otherassociative container) by an iterator, use: "(*it).first"and "(*it).second" for getting key and value.

I The typedef keyword is used to define a new data type:Assign a convenient alias to a complicated typename.

85


stdlibPreliminary Summary: Containers

Container Features

I One can put arbitrary data types into containers, due totemplates, e.g.std::list<string>, std::vector<MyClass>,std::map<int,string>.

I All containers share a common concept of iterating theircontents, due to iterators, e.g.std::list<string>::iterator,std::vector<MyClass>::iterator,std::map<int,string>::iterator.

This gives but a hint of how powerful C++ actually is!We will see later that algorithms can be generalised as well, basedon these concepts.

86


Introduction: Pointers and References

We still need to learn some basic C++ language syntax for dealingwith variables and functions: The pointer operator "*" and thereference operator "&".

PointerA pointer is a variable that contains the physical address ofanother defined variable. It can be used for

I passing a variable to a function for modification,

I pointing into an array (the ”target element” can be selectedby adding integer values to the pointer) and

I to hold the address of a dynamically allocated variable.

87


Introduction: Pointers and References (cont.)

Reference

I A reference is a substitute name for another variable13.

I Example: When modifying a reference called Aref of a variablecalled A, then the value of A itself is changed.

I This is an important concept for passing variables to functionsand initialising/assigning pointers.

13functionally speaking

88


Referenceof something

int i1 = 3

int i2 = 10

string s="1

2

3

4"

(class )A a

attributes ...

methods ...

char c[3]=’a’

’b’

’c’

0x08bf0000

0x08bf0002

0x08bf0004

0x08bf004 + 3

I Whenever you define a variable, memory isallocated for it.

I A position in memory is described by a memoryaddress.

I You can get the address of a variable by usingthe reference operator &.

1 i n t i 1 ;2 s t d : : cout << s t d : : hex << & i 1 ;

Would print: "0x08bf0000".

Note: You can change the number base of theoutput streams by sending bin, oct, dec, hex tothem.

89


Reference: Function Parameter Passing Semantics

Opponent methods for passing variables to functions:

pass-by-value copy variable, so original variable can not bechanged by the function being called or

pass-by-reference pass reference to original variable, enablingmodification of original variable by called function.

1 void modi fy ( i n t i , i n t & j ){2 i = 1 ; j = 2 ;3 }4 / / m a i n ( ) o m i t t e d

5 i n t I = 0 ;6 i n t J = 0 ;7 modi fy ( I , J ) ; / / = > I = = 0 , J ==2

90


Pointerto something

I Pointers point to a variable (i.e. a basic type variable or aclass instance).

I A pointer is defined by "TYPE* pointerVar;".

I The pointer by itself does not own memory for a value of therespective type – unless you allocate memory for it.

I It can point to another variable definition:"int* intP = &i1;".

I For accessing the value a pointer points to one has todereference it: "*intP = 4;" (This is same syntax as with iterators!)

1 i n t i 1 = 3 ;2 i n t ∗ i n tP = & i 1 ;3 ∗ i n tP = 4 ;4 cout << ” i 1 : ” << i 1 << ”==” << ∗ i n tP ;

91


Pointer (cont.)

I When pointing to an array, one can navigate the pointer byadding integer values to it.

1 i n t i n tA [ 1 0 ] ;2 i n t ∗ i n tP = intA ; / / p o i n t e r t o 1 s t e l e m e n t

3 i n tP + 5 ;4 ∗ i n tP = 6 ; / / s e t 6 t h e l e m e n t

Beware: Invalid Pointers

Pointers that are not initialised with a valid address or that pointto an invalid address, e.g. by uncautious pointer arithmetic, willeasily result in a fatal program error (”segmentation fault”).

92


Freestore Management: new and delete

I One commonly needs to dynamically create variables atrun-time.

I This can be done by the keywords new, delete.

I new allocates, i.e. reserves, memory for a variable,

I delete frees memory for a previously allocated variable.

I For allocating arrays, use new[], delete[].

Memory Management Rule

Each new has to have a corresponding delete!All allocated variables have to be freed before exiting the program.

Memory leaks are often hard to track down and cause serioustrouble. An improved mechanism for memory management is thenotion of smart pointers.

93


Reference, Pointer, new and delete Example

1 / / m a i n ( ) o m i t t e d

2 unsigned i n t s i z e ;3 long ∗ lP ;4 count << ”Memory s i z e : ” ; c i n >> s i z e ;5 / / a l l o c a t e m e m o r y

6 lP = new long [ s i z e ] ;7 f o r ( unsigned i n t i =0; i<s i z e ; ++ i )8 lP [ i ] = s i z e− i ;9

10 unsigned i n t c e n t r e = ( unsigned i n t ) ( s i z e / 2 ) ;11 long ∗ lP2 = lP + c en t r e ;12 ∗ lP2 = 0 ;13

14 / / f r e e m e m o r y

15 de lete [ ] lP ;

94


Reference, Pointer, new and delete Example (cont.)

Calling a member function of a class instance a pointed to by aP:I (*a).testFunc() is equivalent toI a->testFunc()

1 #inc lude < c l a s sA . h>2 / / m a i n ( ) o m i t t e d

3 A a ; / / i n s t a n c e o f c l a s s A

4 A∗ aP1 ; / / p o i n t e r t o c l a s s A

5 A∗ aP2 ;6

7 aP1 = &a ;8 (∗ aP1 ) . t e s tFunc ( ) ;9

10 aP2 = new A;11 aP2−>t e s tFunc ( ) ;12 de lete aP2 ;

95


Complex Data Types

I One can define complex data types as an aggregate of other(simple) type variables.

I Either use class or, which is more appropriate, struct(ure).

I A struct is equivalent to a class with default public accessqualifier, i.e.

1 s t ruc t typeA{2 i n t x , y ;3 } ;4

5 c l a s s typeB{6 pub l i c :7 i n t x , y ;8 } ;

are equivalent declarations.

96


Complex Data Types Example

1 s t ruc t typeA{2 i n t x [ 2 ] ;3 s t r i n g name ;4 } ;5

6 i n t myfunc ( typeA& a ){7 cout << a . name << ” : ” << a . x [ 0 ] ;8 return a . x [ 1 ] ;9 }

10

11 i n t main ( ){12 typeA a ;13 a . x [ 0 ] = 1 ; a . x [ 1 ] = 3 ;14 a . name = ” s t r u c t u r e ” ;15 return myfunc ( a ) ;16 }

97


Introduction: Inheritance/Polymorphism Revisited

[1] Shape

−color :Color

+ draw ():void

[2] Circle

+ draw ():void

[3] Rectangle

+ draw ():void

1 C i r c l e c ;2 Rec tang l e r ;3 c . draw ( ) ; / / c a l l s C i r c l e : : d r a w ( )

4 r . draw ( ) ; / / c a l l s R e c t a n g l e : : d r a w ( )

I Classes 2, 3 inherit public/protected methods andmember variables of base class 1.

I Constructor of 1 is automatically invoked before ctor of 2/3.

I Destructor of 1 is automatically invoked after dtor of 2/3.

I 2,3 overload draw() method with own implementation.

I 2 and 3 are of type 1 ( Is-A relationship).

98


Introduction: Inheritance/Polymorphism (cont.)

[1] Shape

−color :Color

+ draw ():void

[2] Circle

+ draw ():void

[3] Rectangle

+ draw ():void

1 void drawShape ( Shape& shape ){2 shape . draw ( ) ; / / c a l l s S h a p e : : d r a w ( ) !

3 }4 / / m a i n ( ) o m i t t e d

5 C i r c l e c ;6 drawShape ( c ) ;

Call of drawShape(c) is valid, but Shape::draw()implementation gets called instead of Circle::draw()!

I Normally function calls are fixed at compile-time: draw() iscalled on a variable of type Shape.

I For the previous example to work, we need late binding so thecalled method draw() is determined at run-time. This thenaccounts for methods being overloaded by subclass Circle.

I Late binding is essential to polymorphism and can beenforced by the virtual keyword.

99


OO: PolymorphismVirtual Functions

Polymorphism is a very powerful concept:

I We can define functions requiring a shape argument and passthem a circle, a rectangle or subsubclasses likeRoundedRectangle etc..

For Overloading Class Functions

. . . they need to be declared with the virtual keyword in thebase class.(being virtual in all subclasses then is implicit, hence, optional)

Rules of thumb:

I Whenever you intent to overload methods of a base class,declare them being virtual.

I You normally really want to do the previous!

I If a class has a virtual method, the destructor should also bevirtual (in case it’s defined).

100


OO: PolymorphismVirtual Functions (cont.)

1 c l a s s A{2 pub l i c :3 v i r t u a l i n t get ID ( ) { return 1 ; }4 } ;5

6 c l a s s B : pub l i c A{ / / c l a s s B I s −A c l a s s A

7 pub l i c :8 i n t get ID ( ) { return 2 ; }9 } ;

Recall:

I Class A is baseclass of class B.

I Class B Is-A class A, overloading method getID().

101


OO: PolymorphismVirtual Functions (cont.)

1 void p r i n t ID (A& a r e f ){2 s t d : : cout << ” ID i s : ” << a r e f . get ID ( ) ;3 }4

5 void main ( ){6 / / d e c l a r e / d e f i n e v a r i a b l e a , b

7 A a ;8 B b ;9 p r i n t ID ( a ) ; / / p r i n t s ” 1 ”

10 p r i n t ID ( b ) ; / / p r i n t s ” 2 ” !

11 }

102


A Note on Parameter Passing

We discussed function parameter copy and reference semantics.Be aware that

I Default semantic is to copy variable/object values totemporary objects when passing arguments, i.e. the followingwill produce a class A copy (a copy) of subclass B (my b):

1 void func (A a copy ){ cout << a copy . get ID ( ) ; }2 [ . . . ]3 B my b ;4 func (my b ) ;

a param then provides the A::getID() implementation, i.e.not subclass B::getID().

I Hence, passing by reference is needed when using subclasseswith overloaded methods.

Rule: You normally will want to to pass objects by reference!

103


Operator Overloading

I Most operators can be overloaded for your own purposes, e.g.

+, -, *, /, ++, --, +=, -=, *=, /=, ^, (), [],=, ==, !, !=, <<, >>.

I Syntax like normal function declaration but with keywordoperator.

I Operator overloading should be used with care: do notconfuse the user of your class by defining non-intuitivebehaviour of operators.

1 / / i m p l e m e n t e q u a l i t y o p e r a t o r o f c l a s s A

2 bool A : : operator==(A& a ){3 return get ID () == a . get ID ( ) ; }4

5 / / i m p l e m e n t ( ) o p e r a t o r o f c l a s s F u n c t i o n

6 double Funct i on : : operator ( ) ( double x ){7 return x∗x ; }

104


this and Returning References

I Keyword this can be used in member functions, denoting apointer to the current class instance.

1 bool A : : operator==(A& a r e f ){2 / / ” t r u e ” i f a r e f i s s a m e i n s t a n c e

3 return th i s == & a r e f ; }4 [ . . . ]5 A a , b ;6 cout << ”Equal : ” << (a==a) << ”\ t ” << (a==b ) ;

I this is commonly applied for returning a reference to thecurrent instance when overloading operators ++, --, +=,-=, *=, /=, i.e. whenever we want to return the (modified)object itself.

105


Operator OverloadingExample

1 s t ruc t I n t e g e r {2 / / d e f a u l t c t o r

3 I n t e g e r ( i n t i = 0 ) : v a l ( i ) {}4

5 / / c o p y c t o r

6 I n t e g e r ( I n t e g e r & i ) : v a l ( i . v a l ) {}7

8 / / a d d i t i o n r e t u r n i n g n e w o b j e c t

9 I n t e g e r operator+( i n t & i )10 { I n t e g e r temp ( v a l+i . v a l ) ; return temp ; }11

12 / / s e l f − a s s i g n m e n t r e t u r n i n g r e f e r e n c e

13 I n t e g e r & operator+=(I n t e g e r & i )14 { v a l+=i . v a l ; return ∗ t h i s ; }15

16 i n t v a l ; } ;

106


Operator OverloadingExample (cont.)

1 / / a d d i 2 t o i 1 , m o d i f y i n g i 1 b y t h i s

2 void add ( I n t e g e r & i1 , I n t e g e r & i 2 ){3 / / a d d i 2 t o i 1 r e f e r e n c e

4 i 1 += i 2 ;5 }6 / / m a i n ( ) o m i t t e d

7 I n t e g e r i 1 ( 1 ) , i 2 ( 2 ) ; / / i 1 = = 1 , i 2 ==2

8 add ( i 1 += 3 , i 2 ) ; / / i 1 ==5

This only works as we return a reference of this with"Integer& operator+=(Integer&)".

107


Smart Pointers

I Conventional C pointers like ”int* pInt” are sometimescalled dumb pointers.

I They often cause major problems during development, esp.when using dynamic memory management.

Use the smart alternative when a pointer is inevitable:A smart pointer

I manages ownership of a dynamically created object and

I provides automatic memory deallocation.

Pointer Rules

Avoid pointers whenever possible. (It’s for your own good.)Allocated memory should be managed by a smart pointer.

108


Smart Pointers (cont.)

I Smart pointers mimic dumb pointer by providing operators"*" and "->" for dereferencing it.

I Their main purpose is to provide automatic lifetime control.

I Create a new object with new and pass the pointer to a smartpointer.

I You don’t have to take care of destruction (i.e. delete) anymore!

STL’s only smart pointer14 is the std::auto ptr. Betterimplementations can be found in the boost library.

14this will hopefuly change in the next standard

109


STL’s std::auto ptr

Usage:

1 s t d : : au t o p t r <int> i n tP (new in t ) ;2 s t d : : au t o p t r <A> a (new A) , b ;3 / / m a i n ( ) o m i t t e d

4 ∗ i n tP = 3 ;5 a−>get ID ( ) ;

std::auto ptr caveat:

I ownership transfer semantic for copy operation.

1 b=a ; / / a s s i g n a n d t r a n s f e r o w n e r s h i p

2 b−>get ID ( ) ; / / v a l i d

3 a−>get ID ( ) ; / / i n v a l i d !

Strictly single ownership of the allocated object!

110


STL’s std::auto ptr (cont.)

STL containers may internally rearrange their elements, therebyde- and reallocating objects.Hence, due to single ownership policy:

Note

std::auto ptr is not appropriate for STL containers! Useboost::shared ptr instead.

May nevertheless be useful for source-sink models:

1 void s i n k ( au to p t r <A> aP ) ;2 / / m a i n ( ) o m i t t e d

3 au to p t r <A> s ou r c e (new A) ;4 s i n k ( s ou r c e ) ; / / e a t s u p s o u r c e

111


boost’s Smart Pointers

The boost library is a collection of free extensions to the C++standard library. We will use smart pointers and the linear algebrapackage µBLAS later.boost offers smart pointers for

shared ownership boost::shared ptr, boost::shared array:multiple pointers can share the same reference.The last living pointer will destroy the reference.

strictly single ownership boost::scoped ptr,boost::scoped array: Single ownership, notcopyable, cannot be used in STL containers.

weak reference boost::weak ptr: pointer is owned by aboost::shared ptr, boost::shared array andis not affected by the weak reference. Termination ishandled by owning pointer(s).

112


boost’s Smart Pointers (cont.)

You will need to include boost’s declarations by:

1 #inc lude <boos t / s h a r e d p t r . hpp>2 #inc lude <boos t / s h a r e d a r r a y . hpp>

Usage is similar to std::auto ptr:

1 boos t : : s h a r e d p t r <A> a ;2

3 { / / d e f i n e l o c a l s c o p e

4 boos t : : s h a r e d p t r <A> b (new A) ;5 a = b ;6 b−>get ID ( ) ; / / a a n d b v a l i d

7 } / / b o u t o f s c o p e − > d t o r

8

9 a−>get ID ( ) ; / / s t i l l v a l i d !

113

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