Pointers in C++ object oriented programming

1
Pointers in C++
Ahmad Baryal
Saba Institute of Higher Education
Computer Science Faculty
Nov 18, 2024

2 Table of contents
 Introduction to pointers
 How to use pointers
 References and Pointers
 Array Name as Pointers
 Pointer Expressions and Pointer Arithmetic
 Advanced Pointer Notations
 Pointers and String literals
 Pointers to pointers
 Void Pointers, Invalid Pointers and NULL Pointers
 Advantage of pointer

3 C++ Pointers
• The pointer in C++ language is a variable, it is also known as locator or indicator that
points to an address of a value.
• The symbol of an address is represented by a pointer. In addition to creating and
modifying dynamic data structures, they allow programs to emulate call-by-
reference. One of the principal applications of pointers is iterating through the
components of arrays or other data structures.
• The pointer variable that refers to the same datatype as the variable you're dealing
with, has the address of that variable set to it (such as an int or string).
Syntax
1.datatype *var_name;
2.int *ptr; // ptr can point to an address which holds int dat
a

4 How to use a pointer?
1. Establish a pointer variable.
2. employing the unary operator (&), which yields the address of the variable, to assign a
pointer to a variable's address.
3. Using the unary operator (*), which gives the variable's value at the address provided
by its argument, one can access the value stored in an address.
Since the data type knows how many bytes the information is held in, we associate it with
a reference. The size of the data type to which a pointer points is added when we
increment a pointer.

5 Usage of pointer
There are many usage of pointers in C++ language.
1) Dynamic memory allocation
In C++ language, we can dynamically allocate memory using malloc() and calloc() functions
where pointer is used.
2) Arrays, Functions and Structures
Pointers in C++ language are widely used in arrays, functions and structures. It reduces
the code and improves the performance.

7 Declaring a pointer
The pointer in C++ language can be declared using (asterisk symbol).
∗
int a; //pointer to int
∗
char c; //pointer to char
∗
Pointer Example: Let's see the simple example of using pointers printing the address and value.
#include <iostream>
using namespace std;
int main()
{
int number=30;
int p;
∗
p=&number;//stores the address of number variable
cout<<"Address of number variable is:"<<&number<<endl;
cout<<"Address of p variable is:"<<p<<endl;
cout<<"Value of p variable is:"<<*p<<endl;
return 0;
}

8 Array Name as Pointers
An array name contains the address of the first element of the array which acts like a constant
pointer. It means, the address stored in the array name can’t be changed. For example, if we
have an array named val then val and &val[0] can be used interchangeably.
void geeks()
{
// Declare an array
int val[3] = { 5, 10, 20 };
// declare pointer variable
int* ptr;
// Assign the address of val[0] to ptr
// We can use ptr=&val[0];(both are same)
ptr = val;
cout << "Elements of the array are: ";
cout << ptr[0] << " " << ptr[1] << " " << ptr[2];
}
// Driver program
int main() { geeks(); }

9 Pointer Expressions and Pointer Arithmetic
A limited set of arithmetic operations can be performed on pointers which are:
• incremented ( ++ )
• decremented ( — )
• an integer may be added to a pointer ( + or += )
• an integer may be subtracted from a pointer ( – or -= )
• difference between two pointers (p1-p2)

10 Example
void geeks()
{
int v[3] = { 10, 100, 200 };
int* ptr;
// Assign the address of v[0] to ptr
ptr = v;
for (int i = 0; i < 3; i++) {
cout << "Value at ptr = " << ptr << "n";
cout << "Value at *ptr = " << *ptr << "n";
// Increment pointer ptr by 1
ptr++;
}
}
int main() { geeks(); }
Value at ptr = 0x7ffe5a2d8060
Value at *ptr = 10
Value at *ptr = 100
Value at *ptr = 200
Output:

11 References and Pointers
There are 3 ways to pass C++ arguments to a function:
• Call-By-Value
• Call-By-Reference with a Pointer Argument
• Call-By-Reference with a Reference Argument
• In C++, function arguments can be passed in different ways, providing flexibility in how data
is accessed and modified within functions. Two common methods are using references and
pointers. This section explores three ways to pass arguments to a function: Call-By-Value,
Call-By-Reference with a Pointer Argument, and Call-By-Reference with a Reference
Argument.

1. Call-By-Value in C++ Function Arguments
In C++, Call-By-Value is a method of passing function arguments where the actual value of the
variable is copied into the function parameter. This example illustrates how modifications inside
the function do not affect the original value.
#include <iostream>
// Function prototype
void squareByValue(int x);
int main() {
int number = 5;
cout << "Original value: " << number << endl;
// Calling the function with a value
squareByValue(number);
cout << "Value after function call: " << number << endl;
return 0;
}
// Function definition
void squareByValue(int x) {
x = x * x;
cout << "Value inside function: " << x << endl;
}

2. Call-By-Reference with a Pointer Argument in C++ Function Arguments
Call-By-Reference with a Pointer Argument in C++ allows a function to modify the original value
by passing the address of the variable. This example demonstrates how changes made inside
the function affect the original value through a pointer.
#include <iostream>
void squareByReference(int *ptr);
int main() {
int number = 5;
// Calling the function with a pointer to the variable
squareByReference(&number);
return 0;
}
void squareByReference(int *ptr) {
*ptr = (*ptr) * (*ptr);
cout << "Value inside function: " << *ptr << endl;
}

3. Call-By-Reference with a Reference Argument in C++ Function Arguments
Call-By-Reference with a Reference Argument in C++ provides a cleaner syntax than using
pointers. This example demonstrates how changes made inside the function directly affect the
original variable.
#include <iostream>
void squareByReference(int &ref);
int main() {
int number = 5;
// Calling the function with a reference to the variable
squareByReference(number);
return 0;
}
void squareByReference(int &ref) {
ref = ref * ref;
cout << "Value inside function: " << ref << endl;
}

15 Advanced Pointer Notation
Consider pointer notation for the two-dimensional numeric arrays. consider the following declaration.
int nums[2][3] = { { 16, 18, 20 }, { 25, 26, 27 } };
In general, nums[ i ][ j ] is equivalent to *(*(nums+i)+j)
1. Using Array Notation:
nums[i][j] accesses the element at the ith row and jth column of the array.
2. Using Pointer Notation:
 *(*(nums + i) + j) is an equivalent expression using pointer notation. It can be interpreted as follows:
• nums + i: This gives the address of the ith row.
• *(nums + i): Dereferencing this gives the array at the ith row.
• *(nums + i) + j: This gives the address of the jth element in the ith row.
• *(*(nums + i) + j): Finally, this dereferences that address to get the value at the ith row and jth column.

16 Advanced Pointer Notation Example
In this C++ example, a two-dimensional array named nums is declared and initialized with integer
values. The program demonstrates two ways to access a specific element in this array:
Array Notation:
The syntax nums[i][j] is used, where i represents the row index, and j represents the column index. This
is a standard way of accessing elements in a two-dimensional array.
Pointer Notation:
The equivalent expression *(*(nums + i) + j) is used. This involves pointer arithmetic where nums + i
gives the address of the ith row, *(nums + i) dereferences that address, and *(nums + i) + j gives the
address of the jth element in the ith row. Finally, *(*(nums + i) + j) dereferences that address to
obtain the value.
Output:
Array Notation: 27
Pointer Notation: 27

17 Pointers and String literals
String literals are arrays containing null-terminated character sequences. String literals
are arrays of type character plus terminating null-character, with each of the elements
being of type const char (as characters of string can’t be modified).
const char* ptr = "geek";
This declares an array with the literal representation for “geek”, and then a pointer to its
first element is assigned to ptr. If we imagine that “geek” is stored at the memory
locations that start at address 1800, we can represent the previous declaration as:
As pointers and arrays behave in the same way in
expressions, ptr can be used to access the characters of a
string literal. For example:
char x = *(ptr+3);
char y = ptr[3];
Here, both x and y contain k stored at 1803 (1800+3).

18 Pointers to pointers
In C++, we can create a pointer to a pointer that in turn may point to data or
another pointer. The syntax simply requires the unary operator (*) for each
level of indirection while declaring the pointer.
char a;
char *b;
char **c;
a = 'g'; // Assign the character 'g' to variable a
b = &a; // b is a pointer that points to the address of a
c = &b; // c is a pointer to a pointer, and it points to the address of b
Here b points to a char that stores ‘g’ and c points to the pointer b.

19 Void Pointers
• This is a special type of pointer available in C++ which represents the
absence of type.
• Void pointers are pointers that point to a value that has no type (and thus
also an undetermined length and undetermined dereferencing
properties). This means that void pointers have great flexibility as they can
point to any data type. There is a payoff for this flexibility.
• These pointers cannot be directly dereferenced. They have to be first
transformed into some other pointer type that points to a concrete data
type before being dereferenced.

20 Invalid pointers
• A pointer should point to a valid address but not necessarily to valid
elements (like for arrays). These are called invalid pointers. Uninitialized
pointers are also invalid pointers.
int *ptr1;
int arr[10];
int *ptr2 = arr+20;
Here, ptr1 is uninitialized so it becomes an invalid pointer and ptr2 is out of
bounds of arr so it also becomes an invalid pointer. (Note: invalid pointers do
not necessarily raise compile errors)

21 NULL Pointers & Advantages of Array
A null pointer is a pointer that point nowhere and not just an invalid address.
Following are 2 methods to assign a pointer as NULL;
int *ptr1 = 0;
int *ptr2 = NULL;
Advantages of Pointers:
• Pointers reduce the code and improve performance. They are used to
retrieve strings, trees, arrays, structures, and functions.
• Pointers allow us to return multiple values from functions.
• In addition to this, pointers allow us to access a memory location in the
computer’s memory.

Pointers in C++ object oriented programming

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