Stage 10: Pointers, Address-of, and Dereference Operators

Pointers: The Key to Understanding Computer Memory

Pointers are one of the most powerful and demanding elements of the C++ language. Unlike regular variables that store specific values (like numbers or characters), pointers store addresses in RAM. Understanding them is essential for working with dynamic memory allocation, arrays, and advanced data structures.

1. The Address-of Operator (&) and Pointer Declaration

Every variable in a program occupies a specific spot in memory. To find out the address where a variable is located, we use the address-of operator &.

Declaration

To declare a pointer, we use the asterisk symbol * with the data type. The pointer's type must match the type of data it points to.

cpp

int number = 100;
int *ptr; // Declaration of a pointer to an integer
ptr = &number; // Assigning the address of 'number' to the pointer

2. The Dereference Operator (*)

Once we have a pointer holding an address, we usually want to access the value stored at that address. We use the dereference operator * for this.

Note: The * symbol has two meanings:

In declaration: int *ptr; means "I am creating a pointer."
In code: *ptr = 20; means "go to the address in ptr and write 20 there."

cpp

#include <iostream>

int main() {
    int x = 10;
    int *p = &x;

    std::cout << "Address of x: " << p << std::endl;
    std::cout << "Value at address p: " << *p << std::endl; // Prints 10

    *p = 50; // Changing x's value via the pointer
    std::cout << "New value of x: " << x << std::endl; // Prints 50
    
    return 0;
}

3. Null Pointer: nullptr

A pointer that hasn't been initialized contains a random address ("garbage"). Attempting to dereference such a pointer almost always results in a Segmentation Fault. Therefore, it is best practice to initialize a pointer with nullptr.

cpp

int *p = nullptr; // Pointer points to nothing
if (p != nullptr) {
    std::cout << *p;
}

4. Pointer Arithmetic and Arrays

Arrays and pointers are closely related in C++. An array's name is actually a pointer to its first element.

tab is the address &tab[0]
*(tab + 1) is the same as tab[1]

We can move a pointer along an array by adding integers to it. C++ automatically calculates the byte offset based on the data type (e.g., for int, adding 1 moves the address by 4 bytes).

cpp

int numbers[] = {10, 20, 30};
int *ptr = numbers; // Points to numbers[0]

std::cout << *ptr << " ";     // 10
std::cout << *(ptr + 1) << " "; // 20

Exam Pitfalls

Uninitialized Pointers: Never do *p = 5; unless you have assigned p the address of an existing variable or dynamic memory.
Confusing Operators:
- & - give me the address.
- * - enter this address.
Pointers vs. References: A reference is just an alias (another name), whereas a pointer is a separate variable storing an address. A pointer can be changed during execution to point to something else—a reference cannot.

Example: Function Using a Pointer

Instead of a reference, we can use a pointer to modify a value in a function:

cpp

void setZero(int *p) {
    if (p != nullptr) {
        *p = 0;
    }
}

int main() {
    int a = 100;
    setZero(&a); // We pass the address of variable a
    // a is now 0
}

Pointers: The Key to Understanding Computer Memory

1. The Address-of Operator (&) and Pointer Declaration

Every variable in a program occupies a specific spot in memory. To find out the address where a variable is located, we use the address-of operator &.

Declaration

To declare a pointer, we use the asterisk symbol * with the data type. The pointer's type must match the type of data it points to.

cpp

int number = 100;
int *ptr; // Declaration of a pointer to an integer
ptr = &number; // Assigning the address of 'number' to the pointer

2. The Dereference Operator (*)

Once we have a pointer holding an address, we usually want to access the value stored at that address. We use the dereference operator * for this.

Note: The * symbol has two meanings:

In declaration: int *ptr; means "I am creating a pointer."
In code: *ptr = 20; means "go to the address in ptr and write 20 there."

cpp

#include <iostream>

int main() {
    int x = 10;
    int *p = &x;

    std::cout << "Address of x: " << p << std::endl;
    std::cout << "Value at address p: " << *p << std::endl; // Prints 10

    *p = 50; // Changing x's value via the pointer
    std::cout << "New value of x: " << x << std::endl; // Prints 50
    
    return 0;
}

3. Null Pointer: nullptr

cpp

int *p = nullptr; // Pointer points to nothing
if (p != nullptr) {
    std::cout << *p;
}

4. Pointer Arithmetic and Arrays

Arrays and pointers are closely related in C++. An array's name is actually a pointer to its first element.

tab is the address &tab[0]
*(tab + 1) is the same as tab[1]

We can move a pointer along an array by adding integers to it. C++ automatically calculates the byte offset based on the data type (e.g., for int, adding 1 moves the address by 4 bytes).

cpp

int numbers[] = {10, 20, 30};
int *ptr = numbers; // Points to numbers[0]

std::cout << *ptr << " ";     // 10
std::cout << *(ptr + 1) << " "; // 20

Exam Pitfalls

Uninitialized Pointers: Never do *p = 5; unless you have assigned p the address of an existing variable or dynamic memory.
Confusing Operators:
- & - give me the address.
- * - enter this address.
Pointers vs. References: A reference is just an alias (another name), whereas a pointer is a separate variable storing an address. A pointer can be changed during execution to point to something else—a reference cannot.

Example: Function Using a Pointer

Instead of a reference, we can use a pointer to modify a value in a function:

cpp

void setZero(int *p) {
    if (p != nullptr) {
        *p = 0;
    }
}

int main() {
    int a = 100;
    setZero(&a); // We pass the address of variable a
    // a is now 0
}

Stage 10: Pointers, Address-of, and Dereference Operators

Pointers: The Key to Understanding Computer Memory

1. The Address-of Operator (&) and Pointer Declaration

Declaration

2. The Dereference Operator (*)

3. Null Pointer: nullptr

4. Pointer Arithmetic and Arrays

Exam Pitfalls

Example: Function Using a Pointer

You might also like

Stage 11: Pointer Arithmetic in C++

Stage 3: Automatic One-Dimensional Arrays in C++

Stage 6: C-Style String Handling (char[])

Stage 10: Pointers, Address-of, and Dereference Operators

Pointers: The Key to Understanding Computer Memory

1. The Address-of Operator (&) and Pointer Declaration

Declaration

2. The Dereference Operator (*)

3. Null Pointer: nullptr

4. Pointer Arithmetic and Arrays

Exam Pitfalls

Example: Function Using a Pointer

You might also like

Stage 11: Pointer Arithmetic in C++

Stage 3: Automatic One-Dimensional Arrays in C++

Stage 6: C-Style String Handling (char[])