Pointers

Objectives

To describe what a pointer is To learn how to declare a pointer and assign a value to it To access elements via pointers To pass arguments by reference with pointers To understand the relationship between arrays and pointers To know how to access array elements using pointers To declare constant pointers and constant data

Introduction to Pointers

Pointer variables, simply called pointers, are designed to hold memory addresses as their values

Normally, a variable contains a specific value, e.g., an integer, a floating-point value, and a character

However, a pointer contains the memory address of a variable that in turn contains a specific value

Introducing Pointers

When we declare a variable, some memory is allocated for it.

Thus, we have two properties for any variable:

Its Address and

Its Data value

For Example, int count = 5;

1245064

5

count

Introducing Pointers

How to get the memory-address of a variable?

Address of a variable can be accessed through the referencing operator “&”

Example:

&count will return memory location where the data value for “count” is stored.

A pointer variable is one that stores an address.

1245064

5

&count

count

Pointers Declaration

We can declare pointers as follows:

Type* varName;

Example:

int* pCount;

creates a pointer variable named “pCount”, that will store address (memory location) of some int type variable.

Location of pCount

1245064 1245064

5

&count

count pCount

Pointers Initialization

int var1 = 11;int *ptr = &var1;

int var2 = 22;int* ptr; ptr = &var2;

int var2;int* ptr = &var2;cin>>*ptr;

The Address Of Operator (&)

The & operator can be used to determine the address of a variable, which can be assigned to a pointer variable

Example:

int val = 100;

int* ptr;

ptr = &val;

Here, ptr points to the memory location, where val is stored.

val is an integer variable

ptr is an integer pointer

The Dereferencing Operator *

C++ uses the * operator in yet another way with pointers

To access variable values pointed to by a pointer variable

Example:

int val = 100;

int* ptr = &val;

cout<<“ P points to the value: “<<*p;

Here, the * is the dereferencing operator and p is said to be dereferenced

Example using Pointer, Address Of, and Derefrenecing Operator

void main(){

int count = 1000;int *pCount = &count;

cout<<"The address of count is = "<<&count;cout<<"The address of count is = "<<pCount;cout<<"The value of count is = "<<count;cout<<"The value of count is = "<<*pCount;

}

Examples using Pointer, Address Of, and Derefrenecing Operator

void main(){

int val = 0;int* ptr = &val;*ptr = 500;cout << val << endl;cout << *ptr << endl;

}

void main(){

int var1 = 11; int var2 = 22;cout << &var1<<endl;cout<< &var2<<endl;int* ptr; ptr = &var1;cout << ptr << endl; ptr = &var2; cout << ptr << endl;

}

Pointer Assignment

Assignment operator ( = ) is used to assign value of one pointer to another

Pointer stores addresses so p1=p2 copies an address value into another pointer

Example:void main(){

int a = 10;int* pA = &a;int* pB;pB = pA;cout << *pA << endl;cout << *pB << endl;

}

Pointer Assignment

Caution:

ptr1 = ptr2;

// changes the location that p1 "points" to

Is different from

*ptr1 = *ptr2;

// changes the value at location that p1 "points" to

Pointer Assignment - Example

void main(){

int x = 40;int y = 20;int* ptr1, *ptr2;

ptr1 = &x;ptr2 = &y;

*ptr1 = *ptr2;cout << x << " " << y << endl;cout<<*ptr1<< " " <<*ptr2<<endl;

}

Pointer Assignment - Example

void main(){ int x = 40; int y = 20; int* ptr1, *ptr2;

ptr1 = &x; ptr2 = &y;

ptr1 = ptr2;

cout << x << " " << y << endl; cout << *ptr1 << " " << *ptr2 << endl;}

Pointer Assignment - Example

void main(){

int x = 40;int y = 20;int* ptr;ptr = &x; // ptr points to location of x*ptr = 50; // contents of x are updatedptr = &y; // ptr points to location of y*ptr = 30; // contents of y are updatedcout << x << " " << y << endl;

}

Pointer Type

A pointer variable is declared with a type such as int, double, etc.

You have to assign the address of the variable of the same type.

It is a syntax error if the type of the variable does not match the type of the pointer

Example:int area = 1;double *pArea = &area; // Wrong

Pointer Type

All memory addresses are of the same length. Why is it important to declare the type of the variable that a pointer points to?

Reason:

To make sense for operation of the type “p++” where “p” is a pointer variable , the compiler needs to know the data type of the variable “p” points to.

Examples:

If “p” is a character-pointer then “p++” will increment “p” by one byte

if “p” is an integer-pointer its value on “p++” would be incremented by 4 bytes

Pointer Type – void pointervoid* is a pointer to no type at all. Any pointer type may be assigned to void *

int iVar = 5;float fVar = 5.5;char cVar = ‘A’; int* ptr; ptr = &iVar; // Allowed ptr = &fvar; // Not Allowed ptr = &cVar; // Not Allowed

int iVar = 5;float fVar = 5.5;char cVar = ‘A’; float* ptr; ptr = &iVar; // Not Allowed ptr = &fvar; // Allowed ptr = &cVar; // Not Allowed

int iVar = 5;float fVar = 5.5;char cVar = ‘A’; char* ptr; ptr = &iVar; // Not Allowed ptr = &fvar; // Not Allowed ptr = &cVar; // Allowed

int iVar = 5;float fVar = 5.5;char cVar = ‘A’; void* ptr; ptr = &iVar; // Allowed ptr = &fvar; // Allowed ptr = &cVar; // Allowed

Arrays and Pointer Arrays and pointers are closely related

Array name is the starting address of the array int arr[3] = {10,20,30}; cout<<arr; // Prints the address of the first element of array arr[0]

int ara[25];int *ptr;

int i = 1, j = 3;

ptr = ara;

It means that ptr points to ara[0]

ptr + i points to ara[i] &ara[j] == ptr + j*(ptr+j) is the same as ara[j]

Arrays and Pointer – Example

void main(){ char * str = “hello”; for(int j=0; j<5; j++) cout << *(str+j) << endl;}

Arrays and Pointer – Example

void main(){ int intarray[5] = { 31, 54, 77,52,83}; for(int j=0; j<5; j++) cout << intarray[j] << endl;}

void main(){ int intarray[5] = { 31, 54, 77,52,83}; for(int j=0; j<5; j++) cout << *(intarray + j) << endl;}

Arrays and Pointer – Example

int arr[] = { 31, 54, 77, 52, 93 };

int* ptr;

ptr = arr;

for(int j=0; j<5; j++) cout << *(ptr++) << endl;

Function and Pointers

There are three ways to pass arguments to a function in C++

Pass-by-value

Pass-by-reference

Pass-by-reference with pointer (Passing Address)

Pass by Value – Example

void func(int num){ cout<<"num = "<<num<<endl; num = 10; cout<<"num = "<<num<<endl;}

void main(){ int n = 5; cout<<"Before function call: n = "<<n<<endl; func(n); cout<<"After function call: n = "<<n<<endl;}

Pass by Reference – Example

void func(int &num){ cout<<"num = "<<num<<endl; num = 10; cout<<"num = "<<num<<endl;}

void main(){ int n = 5; cout<<"Before function call: n = "<<n<<endl; func(n); cout<<"After function call: n = "<<n<<endl;}

Pass by Reference with Pointers– Example

void func(int *num){ cout<<"num = "<<*num<<endl; *num = 10; cout<<"num = "<<*num<<endl;}

void main(){ int n = 5; cout<<"Before function call: n = "<<n<<endl; func(&n); cout<<"After function call: n = "<<n<<endl;}

Using const with Pointers A constant cannot be changed once it is declared. You can declare a constant pointer. For example,

double radius = 5;const double * const pValue = &radius;

const double * const pValue = &radius;

Constant data Constant pointer

Using const with pointers

Nonconstant pointer to constant dataconst int* Ptr;

Constant pointer to noncosntant dataint* const Ptr;

Constant pointer to constant dataconst int* const Ptr;

Exampledouble var1 = 10;double var2 = 20;const double * ptr = &var1;//*ptr = 100; Not Possible because value is constantptr = &var2; // ok because value is not constant

= = = = = = = = = = double var1 = 10;double var2 = 20;double * const ptr = &var1;*ptr = 100; // ok because value is not constant// ptr = &var2; Not Possible because ptr is constant

= = = = = = = = = = =double var1 = 10;double var2 = 20;const double * const ptr = &var1;//*ptr = 100; Not Possible because value is

constant//ptr = &var2; Not Possible because ptr is constant

Pointers to Pointers

C++ allows the use of pointers that point to pointers, that these, in its turn, point to data (or even to other pointers).

char a;char *b;char **c;a = 'z';b = &a;c = &b;cout<<**c;

Pointer to string constant

Syntax: char * string Literalvoid main() {

char str1[] = “Defined as an array”;char* str2 = “Defined as a pointer”;cout<< str1 <<endl;cout<< str2 <<endl;

//str1++; // can’t do this; str1 is a constantstr2++; // this is ok, str2 is a pointer

cout<< str2 <<endl; // efined as a pointer }

Pointer to string constant// Copying string using Pointersvoid main(){char* str1 = “Self-conquest is the greatest victory.”;char str2[80]; //empty stringchar* src = str1;char* dest = str2;

while( *src ) //until null character, *dest++ = *src++; //copy chars from src to dest

*dest = ‘\0’; //terminate dest

cout << str2 << endl; //display str2}