06. Pointers

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06. Pointers

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Overview

1. Pointer Basics

2. Pointer as Function Parameter

3. Pointer and Array

4. Dynamic Memory Allocation

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6.1 Pointer Basics

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Pointers in C++ == Memory Addresses

Throuh !ointers" #e can directlyaccess$modi%y the data stored in the memory.

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Introduction

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To emulate !assin !arameters &y re%erence◦ C lanuae still uses !ointers to achie'e the e(ect

o% !ass)&y)re%erence.

Dynamic memory manaement◦ *e can decide ho# much memory s!ace #e #ant

at run-time.

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What do we use pointers for in!!"

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Continuous storae o% 1)&yte cells

ach cell has a uni,ue address ranin %rom - to2)1 in increasin order.◦ = / o% &its o% address$data the !rocessor can !rocess at a time

◦ 0n most com!uters #e use today" = 32 or 4

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omputer Memory

… 100 101 102 103 104 105 106 107 108 109 110 …

Store 1 byte or 8 bits of data

Memory Address

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ach 'aria&le is allocated an a!!ro!riateamount o% memory s!ace.

ach 'aria&le has a uni,ue memory address.◦ The address o% the rst cell &ecomes the address o%

the 'aria&le.

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#aria$%es and MemoryAddresses

char ch; // 1 byteint x; // 32-bit integer (needs 4 bytes)

… 100 101 102 103 104 105 106 107 108 109 110 …

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ach 'aria&le in the !roram is allocated ana!!ro!riate num&er o% cells in the memory%or storin data.◦ ch 1 &yte 1 cell◦ 4 &ytes 4 5continuous6 cells

The location o% each 'aria&le is determined&y the com!iler and the 07 5and thus may'ary %rom one eecution to another.6

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#aria$%es and Memory Addresses

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&" #hen a!!lied to a 'aria&le" yields the address o% the'aria&le.

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&he address-of operator '&

char ch; // Assume ch occupies 1 byte at locatio 100int x; // Assume x occupies 4 bytes starti! at locatio 104

// "rit address of ch ad x as #e$adecimal umbers%cout

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8n !arameter list◦ int #oo(int &x) $ % ◦ 8ndicates x is a re%erence !arameter

9se #ith 'aria&le in an e!ression◦ cout

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Pointer varia$%es are 'aria&les that storememory addresses

int #oo;

int ' tr; // Declarin a !ointer 'aria&le tr oo; // tr stores the address o% #oo

◦ tr is a 'aria&le that holds an address o% a

'aria&le o% ty!e int.

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Pointer #aria$%es

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int y *;int 'tr;

tr &y;

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Assi+nin+ Address of #aria$%es toPointers

51000

10002000

.

.

.

y

tr

51000

&2000

.

.

.

y

tr

tr stores t#e address of

'ariable y

or

tr poits to y

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int y *;int 'tr;

tr &y; // tr gets the address o# y

cout

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int x 1 y *;int '1 '2;

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sin+ pointers to access varia$%es

51004

&2000

.

.

.

y

1

101000 x

&2004 2

1

2

5

y

10

x

&

&

Pictorial View Memory View

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1 &x; 2 &y;

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51004

10002000

.

.

.

y

1

101000 x

10042004 2

1

2

5

y

10

x


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'1 .;'2 11;

16


111004

10002000

.

.

.

y

1

71000 x

10042004 2

1

2

11

y

7

x


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2 1; // ot the same as '2 '1

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111004

10002000

.

.

.

y

1

71000 x

10002004 2

1

2

11

y

7

x


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cout

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int x . y 11;int '1 '2;

1 &x;

2 &y;

'1 '2;

y 1;

cout

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int x . y 11;int '1 '2;

1 &x;

2 &y;'1 y;

'2 x;

cout

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int x . y 11 3 '1 '2;

2 &x; 2 &y;

'2 *;

1 2;

2 &;y ;

cout

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int 'itr; // Pointer to integer

char 'ctr; // Pointer to char

double 'dtr; // Pointer to double

ectangle 'recPtr; // Pointer to a ectangle obect(see next lecture)

Pointer 'aria&les o% di(erent ty!es all store memoryaddresses. The !ointer ty!e tells the com!uter ho# to inter!ret

the data stored at the location !ointed to &y the!ointer.

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Pointers to di2erent datatypes

ctritr Starti! #ere) treat t#e e$t

4 bytes as ite!er +reat 1 byte #ere as

c#aracter

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int 'itr; // Pointer to integer

char 'ctr; // Pointer to char

double 'dtr; // Pointer to double

ectangle 'recPtr; // Pointer to class ectangle (see next lecture)

// Pointers o# di##erent tyes are not co,atible

// unless exlicitly conerted5

itr ctr; // 6o,ile-ti,e error

ctr itr; // 6o,ile-ti,e error

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Pointers to di2erent data types

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int y;

int 'tr1 &y;

int 'tr2 // tr2 stores the address

'tr3 7899; // tr3 stores the address

7899 is a !redened constant re!resentinaddress - 5it has the 'alue -6

*e can use or 7899 to indicate that a !ointer'aria&le is not !ointin to anythin 5no datacan &e stored at location -6

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Pointer Initia%i3ation * 4u%%Pointer

Same as

int 'tr1; tr1 &y;

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int y;int 'tr &y; // Correct

tr &y; // Correct'tr 1; // Correct'tr &y; // Com!ile)time error

tr 1; // Com!ile)time error

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Pointer Initia%i3ation Pitfa%%s

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Conce!ts o% com!uter memory and ho# dataare stored in the memory.

;no# ho# to declare !ointer 'aria&les

;no# ho# to use the address)o% o!erator 5&6and the dere%erence o!erator 5'6

9nderstand the di(erence &et#een co!yin

!ointers and co!yin the data !ointed &y the!ointers.◦ 1 2; 's '1 '2;

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(ummary

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6.5 Pointer as

unction Parameter

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Pointers are !assed &y 'alue◦ The address store in one !ointer 'aria&le 5the actual

!arameter6 is co!ied to the another !ointer 'aria&le5the %ormal !arameter6.

Passin !ointers allo# us to emulate thee(ect o%

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oid #oo(int ') $ ' ;%

// :n ,ain()int x 3;int 'tr &x;#oo(tr);cout

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oid #oo(int ') $ ' ;%

// :n ,ain()int x 3;int 'tr &x;#oo(tr);cout

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6.7 Pointer and Array

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int #oo1; // =ssu,e int is 4 byte

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8ow are 1, arrays stored inmemory"

800 801 802 803 804 805 806 807 808 809 810 811 812 813

Suppose array #oo starts at locatio 800

#oo #oo1 #oo2

Address of a array Address of its first elemet

-also o* as t#e base address.

+#e address of foo0 800

+#e address of foo 800

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800 801 802 803 804 805 806 807 808 809 810 811 812 813

#oo #oo1 #oo2

Address of #ooi base address i 4

(e assume eac# 'alue of type int is 4 bytes i sie Array sie plays o role i determii! t#e address of a

array elemet%

+#e address of #oo2 800 24 808

+#e address of #oo> 800 94 836 +#e address of #oo1 800 1004 1200

8ow are 1, arrays stored in memory"

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datatye array+:?@;

8% #e no# the &ase address o% array and the ty!e o% itselement" then #e can calculate the address o% any o% itselements.

For eam!le" address o% arrayindex is&ase address o% array + index > sieo#5datatye6

sieo#(tyeAna,e)◦ yields the num&er o% &ytes used to re!resent the 'alue o% ty!e

tyeAna,e.◦ e.." sieo#(int) yields 4

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!! 9epresentation of 1, Arrays

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An array in C++ is re!resented usin its &aseaddress.

e..

int array=1 arrayB1;double array62;

cout

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An array is re!resented &y a &ase address

A !ointer 'aria&le stores address

A !ointer 'aria&le can &e used as i% it is anarray◦ *e can treat the address stored in a !ointer 'aria&le

as the &ase address o% an array

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Pointer #aria$%es andArrays

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int 'tr #oo* $11 22 33 44 **;int bar3 $1 > !;

tr #oo; // tr !ets t#e base address of array #oo

// tr ca o* be used as a array// +#e 'alue stored i tr is used as t#e base addresscout

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int 'tr #oo* $11 22 33 44 **;

tr oo1; // tr !ets t#e address of #oo1

// ,irtually) tr is treated as a array i *#ic# its base// address is t#e address of #oo1%

// +#e 'alue stored i tr is used as t#e base addresscout

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oid rint=rray(int tr int 7) $ #or (int i ; i < 7; iCC) cout

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6.: ,ynamic Memory

A%%ocation

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,ynamic Memory

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Determine the si?e o% an array at com!ile timeint a*;

Can #e use the %ollo#in code determine the si?e o% an arrayat runtime@

int n;cin DD n

int an;

o. Placin n inside an #ill cause com!ile)time error

*e ha'e to use !ointers %or dynamic memory manaement◦ *hile your !roram is runnin" you can e!licitly allocate or reser'e

memory s!ace to store data

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,ynamic MemoryMana+ement

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int n;cin DD n;

int '#oo; // store the address o% the dynamic array

#oo neE intn;

// allocate a continuous memory s!ace %or n inteers

//assin the &ase address o% the array to

#oo

// #oo can &e used as an array

#or(int i ; i < n; iCC)

#ooi i;

// %ree u! A the s!ace occu!ied &y #oo

delete #oo;

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sin+ neE with Arrays,ynamic Array A%%ocation

800 801 802 803 804 805 806 807 808 809 810 811 812 813

#oo #oo1 #oo2

Memory a%%ocation and

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0!erator neE For allocatin memory s!ace to store data

◦ 8t reser'es a continuous chun o% memory s!ace

◦ 9!on success%ul allocation" it returns the memory

address o% the allocated s!ace◦ :ou should al#ays use a !ointer 'aria&le to sa'e

the memory address returned &y neE

0!erator delete For %reein u! the !re'iously allocated

memory s!ace

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Memory a%%ocation anddea%%ocation Operator neEand

delete

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int ''tEoF=rray; // 2D arrayint nu,#oE 4; // / o% ro#s o% 2D array

int nu,#6ol *; // / o% columns o% 2D array

// allocate sace #or tEoF=rray

tEoF=rray neE int'nu,#oE;

// allocate sace #or each tEoF=rrayi#or(int i ; i < nu,#oE; iCC) $

tEoF=rrayi neE intnu,#6ol;

555 555

// #ree u the sace occuied by tEoF=rray a#ter using it#or(int i ; i < nu,#oE; iCC) $

delete tEoF=rrayi;

delete tEoF=rray;

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A%%ocate ,ynamic 5, ArrayOptiona%

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Passin+ ,ynamic 5, Array to

unction Optiona%/' a #unction to rint the 2F array '/oid rint2F=rray(int ''tr int n int ,) $ #or (int i ; i < n; iCC) $ #or (int ; < ,; CC) $ cout

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9nderstand the synta o% !assin !ointers to%unctions that acce!ts !ointers as !arameters

9nderstand ho# !assin !ointers to %unctionscan emulate the e(ect o%

9nderstand ho# 1D arrays are re!resented inC++ and its relationshi! #ith !ointers

;no# ho# to use neE to dynamically allocatememory s!ace

(ummary

06. Pointers

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