Common pointer mistakes in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer to an integer.

C++

int [1] ptr;

Drag options to blanks, or click blank then click option'

A*

B#

C&&

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of * to declare a pointer.

The asterisk (*) is used to declare a pointer variable in C++.

2fill in blank

medium

Complete the code to assign the address of variable x to pointer ptr.

C++

int x = 10;
int* ptr;
ptr = [1]x;

Drag options to blanks, or click blank then click option'

A&

B*

C&&

D#

Attempts:

3 left

💡 Hint

Common Mistakes

Using *x instead of &x when assigning to a pointer.

The ampersand (&) operator gives the address of a variable.

3fill in blank

hard

Fix the error in dereferencing the pointer to get the value of x.

C++

int x = 5;
int* ptr = &x;
int value = [1]ptr;

Drag options to blanks, or click blank then click option'

A&

Bptr

C->

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using &ptr instead of *ptr to get the value.

The * operator dereferences the pointer to access the value it points to.

4fill in blank

hard

Fill both blanks to correctly allocate and assign memory for an integer pointer.

C++

int* ptr = [1] int;
*ptr = [2];

Drag options to blanks, or click blank then click option'

Anew

B5

Cmalloc

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using malloc in C++ without casting or forgetting to assign value.

Use new int to allocate memory and then assign a value to the dereferenced pointer.

5fill in blank

hard

Fill all three blanks to safely delete a dynamically allocated pointer and avoid dangling pointer.

C++

int* ptr = new int(20);
delete [1];
[2] = [3];

Drag options to blanks, or click blank then click option'

Aptr

Cnullptr

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to delete allocated memory or not setting pointer to nullptr after deletion.

Use delete ptr; to free memory and then set ptr to nullptr to avoid dangling pointer.

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