C++ Pointers

Topic 12: Pointers

Pointers are a powerful feature in C++ that allow you to manipulate memory directly. A pointer is a variable that holds the memory address of another variable.

Declaring and Initializing Pointers

To declare a pointer, specify the type of the variable it points to, followed by an asterisk (*) and the pointer name. You can initialize a pointer by assigning it the address of a variable using the address-of operator (&):

Example: Declaring and Initializing a Pointer


#include 
using namespace std;

int main() {
    int var = 10;
    int *ptr = &var;

    cout << "Value of var: " << var << endl;
    cout << "Address of var: " << &var << endl;
    cout << "Value of ptr (address of var): " << ptr << endl;
    cout << "Value at the address stored in ptr: " << *ptr << endl;

    return 0;
}

Pointer Operators

Pointers use two main operators:

Address-of operator (&): Returns the memory address of a variable.
Dereference operator (*): Returns the value stored at the memory address pointed to by the pointer.

Null Pointers

A pointer that is not assigned any memory address is called a null pointer. A null pointer points to nothing and can be created by assigning the constant NULL or nullptr to the pointer:

Example: Null Pointer


#include 
using namespace std;

int main() {
    int *ptr = nullptr;

    if (ptr == nullptr) {
        cout << "Pointer is null" << endl;
    }

    return 0;
}

Pointers and Arrays

Pointers can be used to access elements of an array. The name of an array acts as a pointer to its first element. You can use pointer arithmetic to traverse the array:

Example: Pointers and Arrays


#include 
using namespace std;

int main() {
    int arr[5] = {1, 2, 3, 4, 5};
    int *ptr = arr;

    for (int i = 0; i < 5; ++i) {
        cout << "Element " << i << ": " << *(ptr + i) << endl;
    }

    return 0;
}

Pointers to Pointers

C++ allows you to create pointers to pointers, which means a pointer that stores the address of another pointer:

Example: Pointers to Pointers


#include 
using namespace std;

int main() {
    int var = 10;
    int *ptr = &var;
    int **ptr2 = &ptr;

    cout << "Value of var: " << var << endl;
    cout << "Address of var: " << &var << endl;
    cout << "Value of ptr: " << ptr << endl;
    cout << "Address of ptr: " << &ptr << endl;
    cout << "Value of ptr2: " << ptr2 << endl;
    cout << "Value at the address stored in ptr2: " << *ptr2 << endl;
    cout << "Value at the address stored in the pointer pointed to by ptr2: " << **ptr2 << endl;

    return 0;
}

Function Pointers

Pointers can also be used to point to functions. Function pointers can be useful for implementing callback functions or for passing functions as arguments to other functions:

Example: Function Pointers


#include 
using namespace std;

void displayMessage() {
    cout << "Hello, World!" << endl;
}

int main() {
    void (*funcPtr)() = displayMessage;
    funcPtr();

    return 0;
}

Practice Questions

Discussion Questions

What is the difference between the & (address-of) and * (dereference) operators in C++?
What is a NULL pointer?
What are wild pointers, and why are they dangerous?
What is pointer arithmetic in C++?
What is a void pointer? How is it different from other pointers?
How do you dynamically allocate memory for a single variable using pointers in C++, illustrate with an example?
What is the difference between new and delete in C++?
What are dangling pointers, and how can they be avoided?
What is the difference between passing by value and passing by pointer to a function in C++?
What is a pointer to a pointer? How do you declare and use it?
What is a constant pointer?
What is the difference between a constant pointer and a pointer to a constant?
What are the advantages and disadvantages of using pointers in C++?

Practical Exercise

Pointer Dereferencing: Write a C++ program to swap two numbers using pointers.
- Declare two integer variables and assign them values.
- Write a function that takes two pointers as arguments and swaps the values of the variables they point to.
- Use the function to swap the values of two variables.
Array and Pointers: Write a C++ program to input and display the elements of an array using pointers.
- Declare an array of integers.
- Use a pointer to input elements into the array.
- se the same pointer to display the elements of the array.
Pointer to Function: Write a C++ program that uses a pointer to a function. Define a function that adds two integers and then use a pointer to call this function.
- Write a function that takes two integers as arguments and returns their sum.
- Declare a pointer to the function.
- Use the pointer to call the function and display the result.
Pointer to an Array: Write a C++ program to reverse an array using pointers.
- Declare an array of integers.
- Use a pointer to iterate over the array.
- Use the pointer to reverse the elements in the array in place (without using an additional array).
Dynamic Memory Allocation: Write a C++ program to dynamically allocate memory for an integer, assign it a value, and then free the allocated memory.
- Use the new operator to dynamically allocate memory for an integer.
- Assign a value to the dynamically allocated integer.
- Display the value and then free the memory using the delete operator.
Pointer Arithmetic: Write a C++ program to demonstrate pointer arithmetic. Declare an array of integers and use a pointer to print each element of the array by incrementing the pointer.
- Declare an array of integers.
- Use a pointer to point to the first element of the array.
- Increment the pointer to print all elements of the array.
Pointer to Pointer: Write a C++ program to demonstrate the use of a pointer to a pointer. Declare an integer variable, use a pointer to point to it, and then use a pointer to the pointer to access the value of the variable.
- Declare an integer variable and assign a value to it.
- Declare a pointer to point to the integer.
- Declare a pointer to the pointer and use it to display the value of the integer.
Pointers and Strings: Write a C++ program to input a string and print it using a pointer.
- Declare a character array to store a string.
- se a pointer to input the string.
- Use the pointer to display the string.
Dynamic Array Using Pointers: Write a C++ program to dynamically allocate memory for an array of integers, input the elements, and then calculate their sum.
- Use the new operator to dynamically allocate memory for an array of integers.
- Input the elements of the array using a pointer.
- Calculate and display the sum of the array elements.
- Free the dynamically allocated memory using delete[].
Null Pointer Check: Write a C++ program that checks if a pointer is a NULL pointer before dereferencing it.
- Declare an integer pointer and initialize it to nullptr.
- Use an if condition to check if the pointer is nullptr before dereferencing it.
- If the pointer is not nullptr, dereference it to print its value; otherwise, display an error message.
Modify Variable through Pointer: Write a C++ program to modify the value of a variable through a pointer.
- Declare an integer variable and assign it a value.
- Declare a pointer to store the address of the variable.
- Use the pointer to modify the value of the variable and print the modified value.
Pass by Pointer: Write a C++ program where you pass a pointer to a function. The function should modify the value of the variable using the pointer.
- Declare an integer variable and assign a value to it.
- Write a function that takes a pointer as an argument and modifies the value of the variable.
- Call the function and display the modified value.
Memory Leak Demonstration: Write a C++ program to demonstrate a memory leak and show how to fix it.
- Dynamically allocate memory for an integer using new but forget to free the memory (this creates a memory leak).
- Modify the program to correctly free the allocated memory using delete.
Pointer to Constant: Write a C++ program to demonstrate the concept of a pointer to a constant.
- Declare a constant integer.
- Declare a pointer to a constant and use it to access the value.
- Attempt to modify the value through the pointer (which should give a compile-time error).

Solution

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