Common pointer mistakes in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens if you dereference a null pointer in C++?

Dereferencing a null pointer causes undefined behavior, often leading to a program crash or segmentation fault.

beginner

Why is it dangerous to use a pointer after freeing its memory?

Using a pointer after freeing its memory leads to a dangling pointer, which can cause unpredictable behavior or crashes when accessed.

beginner

What is a memory leak in the context of pointers?

A memory leak happens when allocated memory is not freed properly, causing the program to consume more memory over time.

intermediate

Why should you avoid pointer arithmetic without caution?

Pointer arithmetic can lead to accessing memory outside the intended range, causing undefined behavior or corrupting data.

beginner

What mistake occurs when you forget to initialize a pointer?

An uninitialized pointer contains a garbage address, and dereferencing it can cause crashes or unpredictable behavior.

What is the result of dereferencing a null pointer?

A Returns zero

B Undefined behavior, often a crash

C Returns a default object

D Compiles but does nothing

Dereferencing a null pointer leads to undefined behavior, usually causing a crash.

What is a dangling pointer?

A A pointer initialized to null

B A pointer that points to a constant

C A pointer that points to a valid object

D A pointer that points to freed memory

A dangling pointer points to memory that has been freed and is no longer valid.

What causes a memory leak?

A Not freeing allocated memory

B Using null pointers

C Pointer arithmetic

D Initializing pointers

Memory leaks happen when allocated memory is not freed, wasting resources.

What is the risk of pointer arithmetic without care?

A Accessing invalid memory

B Improved performance

C Automatic memory management

D Compiler errors

Pointer arithmetic can cause access outside valid memory, leading to errors.

What happens if you use an uninitialized pointer?

A It points to zero

B It points to null automatically

C It may point to random memory causing crashes

D It is safe to use

Uninitialized pointers contain garbage addresses and can cause crashes if dereferenced.

Explain common mistakes when using pointers in C++ and their consequences.

Think about what happens when pointers are misused or not properly managed.

How can you avoid common pointer mistakes in C++?

Consider good habits and modern C++ features to manage pointers safely.

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