Common pointer mistakes in C++

Introduction

Pointers let you work directly with memory addresses. But if used wrong, they can cause bugs or crashes.

When you want to share data between functions without copying it.

When you need to work with dynamic memory (memory you create while the program runs).

When you want to build complex data structures like linked lists or trees.

When you want to optimize performance by avoiding copying large data.

When you need to interact with low-level system or hardware features.

Syntax

C++

type *pointerName;

// Example:
int *p;

A pointer stores the address of a variable of the specified type.

The asterisk (*) declares a pointer variable.

Examples

Declare a pointer to int and set it to null (no address).

C++

int *p; // pointer to int
p = nullptr; // pointer points to nothing

Pointer p stores the address of variable x.

C++

int x = 10;
int *p = &x; // pointer holds address of x

Trying to use *p without setting p first causes a crash.

C++

int *p;
*p = 5; // ERROR if p is uninitialized

Sample Program

This program shows how to use a pointer correctly and checks before dereferencing to avoid mistakes.

C++

#include <iostream>

int main() {
    int x = 42;
    int *p = &x; // p points to x

    std::cout << "Value of x: " << x << "\n";
    std::cout << "Value via pointer p: " << *p << "\n";

    p = nullptr; // p points to nothing

    // Common mistake: dereferencing nullptr causes crash
    if (p != nullptr) {
        std::cout << *p << "\n";
    } else {
        std::cout << "Pointer p is null, cannot dereference.\n";
    }

    return 0;
}

Important Notes

Never use a pointer before assigning it a valid address.

Always check if a pointer is null before dereferencing it.

Be careful with pointers to memory that has been freed or deleted.

Summary

Pointers store addresses and must be initialized before use.

Dereferencing a null or uninitialized pointer causes crashes.

Always check pointers before using them to avoid common mistakes.

Practice

1. What happens if you try to dereference a pointer that has not been initialized in C++?
int *p;
int x = *p;

easy

A. It causes undefined behavior and may crash the program.

B. It automatically initializes the pointer to zero.

C. It assigns zero to the variable x.

D. It prints the address stored in p.

Solution

1. Step 1: Understand pointer initialization

   The pointer p is declared but not initialized, so it points to an unknown memory location.

2. Step 2: Dereferencing uninitialized pointer

   Dereferencing *p tries to access memory at an undefined address, causing undefined behavior or crash.

3. Final Answer:

   It causes undefined behavior and may crash the program. -> Option A

4. Quick Check:

   Uninitialized pointer dereference = crash [OK]

Hint: Never use pointers before initializing them [OK]

Common Mistakes:

• Assuming uninitialized pointers are null
• Thinking dereferencing gives zero
• Ignoring pointer initialization

2. Which of the following is the correct way to initialize a pointer to null in C++?

easy

A. int *p = NULL;

B. int *p = nullptr;

C. int *p = '0';

D. int *p = 0x0;

Solution

1. Step 1: Understand null pointer initialization

   In modern C++, nullptr is the preferred way to initialize a pointer to null.

2. Step 2: Check other options

   NULL is old style (macro), 0x0 is a literal but less clear, and '0' is a char, not a pointer.

3. Final Answer:

   int *p = nullptr; -> Option B

4. Quick Check:

   Use nullptr for null pointers [OK]

Hint: Use nullptr for null pointers in modern C++ [OK]

Common Mistakes:

• Using character '0' instead of nullptr
• Using integer 0 instead of nullptr
• Confusing NULL macro with nullptr

3. What is the output of the following code?

int x = 10;
int *p = &x;
*p = 20;
std::cout << x << std::endl;

medium

A. 10

B. Address of x

C. Compilation error

D. 20

Solution

1. Step 1: Understand pointer assignment

   Pointer p stores the address of x. So *p refers to x.

2. Step 2: Modify value through pointer

   Assigning *p = 20; changes the value of x to 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   Pointer dereference changes original variable [OK]

Hint: Changing *p changes the original variable [OK]

Common Mistakes:

• Confusing pointer value with pointed value
• Expecting original variable unchanged
• Thinking *p prints address

4. Identify the error in the following code snippet:

int *p = nullptr;
std::cout << *p << std::endl;

medium

A. Missing semicolon after nullptr.

B. Pointer p is not declared.

C. Dereferencing a null pointer causes runtime error.

D. std::cout cannot print pointer values.

Solution

1. Step 1: Analyze pointer initialization

   Pointer p is initialized to nullptr, meaning it points to nothing.

2. Step 2: Dereferencing null pointer

   Trying to access *p causes a runtime error (crash) because it points to no valid memory.

3. Final Answer:

   Dereferencing a null pointer causes runtime error. -> Option C

4. Quick Check:

   Dereference nullptr = crash [OK]

Hint: Never dereference nullptr or uninitialized pointers [OK]

Common Mistakes:

• Assuming nullptr can be dereferenced safely
• Ignoring runtime crash possibility
• Confusing pointer declaration with usage

5. Given the code below, what is the safest way to avoid a dangling pointer?

int *p = new int(5);
delete p;
// What should be done next?

hard

A. Set p to nullptr after delete.

B. Assign p to a new integer immediately.

C. Do nothing; pointer is automatically safe.

D. Use delete[] p; instead of delete p;.

Solution

1. Step 1: Understand delete effect

   After delete p;, the pointer p still holds the old address but memory is freed (dangling pointer).

2. Step 2: Prevent dangling pointer

   Setting p = nullptr; makes it clear pointer is invalid and safe to check before use.

3. Final Answer:

   Set p to nullptr after delete. -> Option A

4. Quick Check:

   Nullify pointer after delete to avoid dangling [OK]

Hint: Nullify pointers after delete to avoid dangling [OK]

Common Mistakes:

• Using delete[] for single object
• Not setting pointer to nullptr after delete
• Assuming pointer auto-clears after delete