chapter 5: pointers and strings

2003 Prentice Hall, Inc. All rights reserved.

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Chapter 5: Pointers and Strings

OutlineIntroduction

Pointer Variable Declarations and Initialization Pointer Operators

Pointer of Complex StructureOther Pointer OperationsCalling Functions by ReferenceRelationship between pointer and arrayDynamic Data Storage


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Introduction

• Pointers – Powerful, but difficult to master– Simulate pass-by-reference – Close relationship with arrays and strings


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Pointer Variable Declarations and Initialization

• Pointer variables– Contain memory addresses as values – Normally, variable contains specific value (direct reference)– Pointers contain address of variable that has specific value

(indirect reference)• Indirection

– Referencing value through pointer• Pointer declarations

– * indicates variable is pointerint *countPtr;

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

int *myPtr1, *myPtr2;

count

7

countPtr

count

7


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Pointer Variable Declarations and Initialization

• Can declare pointers to any data type• Pointer initialization

– Initialized to 0, NULL, or address• 0 or NULL points to nothing


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Pointer Operators

• & (address operator)– Returns memory address of its operand– Example

int y = 5;int *yPtr;yPtr = &y; // yPtr gets address of y

– yPtr “points to” y

yPtr

y5

yptr

600004 600000

y

600000 5

address of y is value of yptr


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Pointer Operators

• * (indirection/dereferencing operator)– Returns synonym for object its pointer operand points to– *yPtr returns y (because yPtr points to y).– dereferenced pointer is lvalue

*yptr = 9; // assigns 9 to y• * and & are inverses of each other• Example 1 #include <iostream>

int main()

{ int *p;

int q=3, k=5;

p=&q;

cout << "p is " <<p<<" *p is " <<*p<<" q is "<<q<<endl;

*p=4;


*p=k;


return 0;

}


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Pointer Operators


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Pointer of complex structure

• Example 2#include <iostream>

struct Student{

int grade;

char name[20];

};

int main()

{

struct Student *p, q;

q.grade = 90;

strcpy(q.name,"Smith");

cout << "Grade " <<q.grade<< " Name " <<q.name<<endl;

p=&q;

cout << "Grade " << p->grade <<" Name "<<p->name<<endl;

return 0;

}


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Pointer of complex structure


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Other Pointer Operations

• Example 3

#include <iostream>

int main()

{

int i=3, j=3, *p1, *p2;

p1=&i, p2=&j;

if(*p1==*p2) //used to check if the content is the same.

cout <<"*p1 Equal to *p2"<<endl;

else cout<<"*p1 not Equal to *p2"<<endl;

if(p1==p2) //used to check if they are pointing to the same unit.

cout <<"p1 Equal to p2"<<endl;

else cout <<"p1 not Equal to p2"<<endl;

if(p1<=p2) //it has side-effect, do not use it.

cout <<"p1 no more than p2"<<endl;

else cout <<"p1 less than p2"<<endl;

return 0;

}


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Other Pointer Operations


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Calling Functions by Reference

• 3 ways to pass arguments to function– Pass-by-value arguments– Pass-by-reference with reference arguments– Pass-by-pointer arguments

• return can return one value from function• Example 4


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Calling Functions by Reference#include <iostream>

void f_point( int *i, int *j)

{

*i=3;

*j=4;

}

void f_reference( int &i, int &j)

{

i=3;

j=4;

}

void f_value (int i, int j)

{

i=3;

j=4;

}

int main()

{

int p=1, q=2;

f_point(&p,&q);

cout << "p is "<< p << " q is " <<q <<endl;

p=1;

q=2;

f_reference(p,q);


p=1;

q=2;

f_value(p,q);


return 0;

}


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Calling Functions by Reference


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Relationship Between Pointers and Arrays

• Arrays and pointers closely related– Array name like constant pointer– Pointers can do array subscripting operations


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• Pointer variable and name of array

c[6]

-45

6

0

72

1543

-89

0

62

-3

1

6453

78

Name of array (Note that all elements of this array starts from here, c)

c[0]

c[1]

c[2]

c[3]

c[11]

c[10]

c[9]

c[8]

c[7]

c[5]

c[4]

Position number of the element within array c

Pointer pointing to the address of c[0]



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c[6]

-45

6

0

72

1543

-89

0

62

-3

1

6453

78

Name of array (Note that all elements of this array starts from here, c)

c[0]

c[1]

c[2]

c[3]

c[11]

c[10]

c[9]

c[8]

c[7]

c[5]

c[4]

Last element

*(pointer+0), or *p

*(pointer+7)



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• Accessing array elements with pointers– Element b[ n ] can be accessed by *( bPtr + n )

• Called pointer/offset notation– Addresses

• &b[ 3 ] same as bPtr + 3– Array name can be treated as pointer

• b[ 3 ] same as *( b + 3 )– Pointers can be subscripted (pointer/subscript notation)

• bPtr[ 3 ] same as b[ 3 ]


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#include <iostream>

void f_point( char * i, char * j)

{

*(i+2)='E';

*(j+3)='E';

}

void f_array( char i[], char j[])

{

i[2]='E';

j[3]='E';

}

int main()

{

char p[6]="hello", q[10]="howareyou";

f_point(p,q);


strcpy(p,"hello");

strcpy(q,"howareyou");

f_array(p,q);


return 0;

}


•Example 5


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Dynamic Data Storage

• Dynamic memory management– Control allocation and deallocation of memory– Operators new and delete

• new operator– Create memory for object– Calls default constructor for object– Returns pointer of specified type– Format

• Providing initializersdouble *ptr = new double( 3.14159 );Time *timePtr = new time( 12, 0, 0 );• Allocating arraysint *gradesArray = new int[ 10 ];

• Example 6


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• delete– Destroy dynamically allocated object and free space– Operator delete

• Calls destructor for object• Deallocates memory associated with object

– Memory can be reused to allocate other objects– Deallocating arrays

• delete [] gradesArray;– First calls destructor for each object in array– Then deallocates memory

• delete time;


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• Example 6 #include <iostream>

#include <string>

int main()

{

char * p;

int index;

cout << "Input how many characters:" ;

cin >>index;

p = new char [index+1];

cin >> p;

cout <<"p is: " <<p;

delete [] p;

p=NULL;

return 0;

}


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• Example 7: Multiple-phase development– Step 1: 7_1.cc– Step 2: 7_2.cc– Step 3: 7_3.cc


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• 7_3.cc#include <iostream>

class MyArray{

public:

MyArray();

MyArray(int);

~MyArray();

void printall();

void array_copy(MyArray&);

private:

int size;

int *ptr;

};

MyArray::MyArray()

{

size=0;

ptr=NULL;

}

MyArray::MyArray(int t_size)

{

if(t_size>0){

size=t_size;

ptr=new int[t_size+1];

cout << "Please input "<<t_size<< " intergers:";

for(int i=0;i<t_size;i++)

cin >> ptr[i];

}

else{

size=0;

ptr=NULL;

}

}

MyArray::~MyArray()

{

size=0;

delete [] ptr;

}


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Dynamic Data Storage• 7_3.cc (continue)void MyArray::printall()

{

if(size==0) {

cout <<"NULL"<<endl;

return;

}

for(int i=0;i<size;i++)

cout<<ptr[i]<<" ";

cout <<endl;

}

void MyArray::array_copy(MyArray & b)

{

size=b.size;

delete [] ptr;

ptr=new int [size+1];

for(int i=0; i<size;i++)

ptr[i]=b.ptr[i];

ptr[size]=0;

}

int main()

{

MyArray a, b(4);

cout << "Before copying"<<endl;

a.printall();

b.printall();

a.array_copy(b);

cout << "After copying"<<endl;

a.printall();

b.printall();

return 0;

}


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chapter 5: pointers and strings

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