Programming for EngineersII - Lec02

8/9/2019 Programming for EngineersII - Lec02

1/17

Programming for Engineers - IILecture 02 : Pointers

by

Mati Ullah Khan


2/17

Introduction

You have learned pass by reference

Pointers also enable pass-by-reference

Powerful, but difficult to master

Use of pointers with classes will be discussedlater on in the course


3/17

Pointer Variable Declarations and

Initialization Pointer variables Contain memory addresses as their values Normal variables contain a specific value (direct

reference)

Pointers contain the address of a variable that has aspecific value (indirect reference) Indirection

Referencing a pointer value Pointer declarations

* indicates variable is a pointerint *myPtr;

declares a pointer to an int, a pointer of type int * Multiple pointers require multiple asterisks

int *myPtr1, *myPtr2;

count

7

countPtr

count

7


4/17

Pointer Variable Declarations andInitialization Can declare pointers to any data type

Pointers initialization Initialized to 0,NULL, or an address

0 orNULL points to nothing


5/17

Pointer Operators * (indirection/dereferencing operator) Returns the value of what its operand points to *yPtr returns y (because yPtr points to y).

*can be used to assign a value to a location in

memory*yptr = 7; // changes y to 7

Dereferenced pointer (operand of*) must be anlvalue (no constants)

* and & are inverses Cancel each other out

*&myVar == myVar

and&*yPtr == yPtr


6/17

1 // Fig. 5.4: fig05_04.cpp

2 // Using the & and * operators

3 #include45 using std::cout;

6 using std::endl;

78 intmain()

9 {

10 int a; // a is an integer

11 int *aPtr; // aPtr is a pointer to an integer

1213 a = 7;

14 aPtr = &a; // aPtr set to address of a

1516 cout


7/17

Calling Functions by Reference

Call by reference with pointer arguments Pass address of argument using & operator Allows you to change actual location in memory Arrays are not passed with &because the array name is

already a pointer * operator used as alias/nickname for variable inside

of functionvoid doubleNum( int *number ){

*number = 2 * ( *number );}

*number used as nickname for the variable passed in When the function is called, must be passed an

addressdoubleNum( &myNum );


8/17

1 // Fig. 5.7: fig05_07.cpp

2 // Cube a variable using call-by-reference

3 // with a pointer argument

4 #include5

6 using std::cout;

7 using std::endl;

8

9 voidcubeByReference( int * ); // prototype

10

11 intmain()

12 {13 int number = 5;

14

15 cout


9/17

Using the Const Qualifier with Pointers

const qualifier Variable cannot be changed const used when function does not need to change a

variable Attempting to change a constvariable is a compiler error

const pointers Point to same memory location Must be initialized when declared

int *const myPtr = &x; Constant pointer to a non-constant int

const int *myPtr = &x; Non-constant pointer to a constant int

const int *const Ptr = &x; Constant pointer to a constant int


10/17

1 // Fig. 5.13: fig05_13.cpp

2 // Attempting to modify a constant pointer to

3 // non-constant data

4 #include

5

6 intmain()

7 {

8 int x, y;

9

10 int * constptr = &x; // ptr is a constant pointer to an

11 // integer. An integer can be modified

12 // through ptr, but ptr always points

13 // to the same memory location.

14 *ptr = 7;

15 ptr = &y;

16

17 return 0;

18 }

Error E2024 Fig05_13.cpp 15: Cannot modify a const object in function main()

Changing *ptr is allowed - x is not

a constant.

Changingptr is an error -ptr is

a constant pointer.


11/17

Pointer Expressions and Pointer

Arithmetic Pointer arithmetic

Increment/decrement pointer (++ or --) Add/subtract an integer to/from a pointer( + or += , -

or-=

)

Pointers may be subtracted from each other Pointer arithmetic is meaningless unless performed on

an array

5 element int array on a machine using 4 byte intsvPtr points to first elementv[ 0 ], which is at

location 3000vPtr = 3000

vPtr += 2; setsvPtr to 3008vPtr points tov[ 2 ]

pointer variablevPtr

v[0] v[1] v[2] v[4]v[3]

3000 3004 3008 3012 3016

location


12/17


Arithmetic Subtracting pointers

Returns the number of elements between twoaddresses

vPtr2 = v[ 2 ];vPtr = v[ 0 ];vPtr2 - vPtr == 2

Pointer comparison

Test which pointer points to the higher numberedarray element

Test if a pointer points to 0 (NULL)if ( vPtr == 0 )

statement


13/17


Arithmetic Pointers assignment

If not the same type, a cast operator must be used

Exception: pointer tovoid(typevoid *) Generic pointer, represents any type

No casting needed to convert a pointer tovoidpointer

voidpointers cannot be dereferenced


14/17

The Relationship Between Pointers and

Arrays Arrays and pointers closely related

Array name like constant pointer

Pointers can do array subscripting operations Having declared an arrayb[ 5 ] and a pointerbPtr

bPtr is equal tob

bptr == b

bptr is equal to the address of the first element ofb

bptr == &b[ 0 ]


15/17

The Relationship Between Pointers and

Arrays Accessing array elements with pointers Elementb[ n ] can be accessed by *( bPtr +n ) Called pointer/offset notation

Array itself can use pointer arithmetic.b[ 3 ] same as *(b + 3)

Pointers can be subscripted (pointer/subscript

notation)bPtr[ 3 ] same asb[ 3 ]


16/17

Arrays of Pointers

Arrays can contain pointers Commonly used to store an array of strings

char *suit[ 4 ] = {"Hearts", "Diamonds","Clubs", "Spades" };

Each element of suit is a pointer to a char * (a string) The strings are not in the array, only pointers to the

strings are in the array

suit array has a fixed size, but strings can be of anysize

suit[3]

suit[2]

suit[1]

suit[0] H e a r t s \0

D i a m o n d s \0

C l u b s \0

S p a d e s \0


17/17

Questions???

Documents

Programming for EngineersII - Lec02