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Pointers and Strings

Chapter 5

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What You Will Learn . . .

How to use pointers

Passing arguments to functions with pointers See relationship of

pointers to strings and arrays

Pointers can point to functions

Declaring & using arrays of strings

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Introduction

Pointer variables contain memory addresses as their values

Contrast normal variables which contain a specific value

x holds a value xptr holds memory address of some int location

int *xptr, x;

x 5

xptr

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Pointer Variable Declarations and Initializations Must declare the type of variable to which

pointer variable will point

Must use the leading * to specify this is a pointer variable

Good idea to include ptr in the name of the pointer variable

float amt, *fptr;

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Pointer Initialization

When declared, variables should NOT be assumed to have any certain value

As with all variables, pointer variables should be explicitly initialized– Initialized with address of a specific

variable– Initialized to 0 or NULL (points to nothing)

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Pointer Operators

Address operator &– unary operator– returns address of operand

int *xptr, x;

x = 5;

xptr = &x;

x 5

xptr

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Pointer Operators

Address operator &– unary operator– returns address of operand

int *xptr, x;

x = 5;

xptr = &x;x 5

xptr 1234

1234

6500

Address

Values actually stored

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Pointer Operators

The indirection or dereferencing operator– Use the * symbol

*xptr will yield the value stored at the location to which xptr points

int x, *xptr;x = 50;xptr = &x;cout << *xptr;// what gets printed?

50

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Pointer Operators

The precedence of – & the address operator– * the dereferencing operator

Higher precedence than multiplication and division

Lower precedence than parentheses

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Calling Functions by Reference

Three ways to pass arguments to a function– call by value– call by reference with reference

parameters– call by reference with pointer parameters

void doit (int amt);void whatever ( float &value);void fribble (char *ch);

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Calling Functions by Reference

Call by value– value gets passed one way (to the function) only– can use constant, expression, or variable in call

Reference parameter– value passed both ways– must use variable in call– parameter automatically dereferenced locally

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Calling Functions by Reference

Must use address in call – use address operator

fribble (&c1); Must explicitly dereference locally within

function– use * operator

void fribble (char *ch){ *ch = …. ; }

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Using the const Qualifier with Pointers Recal that const informs the compiler

that the value of a "variable" should not be modified

This can also be used on function parameters so that actual parameters are NOT affected by changes in formal parameters

void try_a_change (const char *s_ptr)

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const Used on Parameters

When used, do not try to alter contents of where pointer points to

void double_int (const int *num_ptr){ *num_ptr = 2 * *num_ptr;}

compiler error

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const Used on Parameters

Consider the advantage of passing large data objects (structures, arrays) using points to constant data– saves memory

• function does not need to create duplicate data object

– saves time• program need not copy large number of bytes

to the local object

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Bubble Sort Using Call-by-reference Refer to “Cyber Classroom” CD for

author’s description of the program Run the program

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Bubble Sort Using Call-by-reference -- Note ... array declared as int *array

not int array [ ] Parameter size declared as const to

enforce sorting function not altering size– when passing an array to function, send size

also -- don’t have it built in Prototype for swap included inside

bubbleSort -- it is the only function that calls swap

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Pointer Expressions & Pointer Arithmetic Pointer values are valid operands in

expressions– arithmetic– assignment– comparison

Not all such operators are valid with pointer variables

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Pointer Expressions & Pointer Arithmetic Valid operations on pointers

++ - - + += - - =

int v[5], *vPtr;vPtr = v; // same as vPtr = &v[0];

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Pointer Expressions & Pointer Arithmetic Consider:

vPtr += 2; // same as vPtr = vPtr + 2;

/* But, beware of vPtr -= 4; Why?

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Pointer Expressions & Pointer Arithmetic -- warnings !! Beware use of pointer arithmetic on pointers

which do not reference an array Consider results of subtracting or comparing two

pointers which do not reference the same array C++ has no range checking -- if you run off the

end of an array, you can be in trouble Pointers can be assigned to other pointers only

if they are pointers to the same type– although typecasting can be used (carefully)

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Pointers to void ( void * )

This is a generic pointer– can represent any pointer type

All pointer types can be assigned a pointer to void without casting

A pointer to void cannot be be assigned a pointer of another type (without casting)

Void pointer cannot be dereferenced(why not??)

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Relationship Between Pointers and Arrays

// Givenint list [5], *intPtr;intPtr = list; // name of array is a pointer constant, an address

Then we can use eitherlist[2] or *(intPtr + 2)

to reference the second element of the array The former is clearer but takes longer to

compile

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Relationship Between Pointers and Arrays Note how pointer is used to traverse

array:

int b[] = { 10, 20, 30, 40 };int *bPtr = b; // set bPtr to point to array b

for ( offset = 0; offset < 4; offset++ ) cout << "*(bPtr + " << offset << ") = " << *( bPtr + offset ) << '\n';

for (bPtr = b; bPtr < b + 4; bPtr++) cout << *bPtr << '\n'; How are these

two different?How are these two different?

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Relationship Between Pointers and Arrays What is wrong with this use of the name

of the array?

for ( bPtr = b ; b < bPtr + 4; b++ ) cout << *b << '\n';

The name of the array is a pointer constant

The name of the array is a pointer constant

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Arrays of Pointers

Common use is for an array of strings (character arrays)char *suit [4] = {"Hearts","Diamonds", "Clubs", "Spades"};

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Arrays of Pointers

Character values stored in memory, one byte longer than length of string

Array suit actually holds pointers to these locations– point to first character of each string

Note that memory not wasted for unneeded characters of shorter strings

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

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Function Pointers

Pointer to a function contains the address of the function in memory

A function name is a pointer constant to the starting address in memory of the code of the function

Pointers to functions can be …– passed as parameters (both directions)– stored in arrays– assigned to other function pointers

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Function Pointers Consider the following code:

Function sort receives a pointer to a function– function ascending

– function descending

void bubble( int [], const int, int (*)( int, int ) );int ascending( int, int );int descending( int, int ); . . . void bubble( int work[], const int size, int (*compare)( int, int ) )

depending onpointer passed in call

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Function Pointers

Sending a function name as the actual parameter sends the address of that function to another function

if ( order == 1 ) { bubble( a, arraySize, ascending ); cout << "\nData items in ascending order\n"; } else { bubble( a, arraySize, descending ); cout << "\nData items in descending order\n"; }

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Function Pointers

Consider an array of functions

void (*f [3] ) ( int ) = { f1, f2, f3 }; Assumptions

– f1, f2, and f3 have been previously declared– each has a single int parameter– they are called with an array number

f [ choice ] (x_int);

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Characters and Strings

Character constant– an integer value represented as a

character in single quotes 'x' or '\t' String

– series of characters treated as a single unit String constants (string literals)

– enclosed in double quotes "Hi Mom" "2/14/1999"

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Strings

An array of characters ending in the null character '\0'

Accessed via pointer to first character in the string

Value of a string– the constant address of its first character

Assigned in declaration as array or char pointer char name[ ] = "Snuffy Snail"; char *addr = "123 Frogpond";

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Strings

Make sure to allocate enough characters in the array to have room for the '\0'

If you create a "string", make sure the '\0' gets tagged on the end

Passing a character as a parameter when a string is expected can cause run time problems

Vice-versa is a syntax error

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String Manipulation Functions

Recall that strings must be handled in a special manner char name[30]; name = "Osgood Smart";

Use functions provided– See page 325

– make sure to

#include <string.h>#include <string.h>

Don't do it!! Why?