Memory allocation and deallocation in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate memory for an integer using new.

C++

int* ptr = [1] int;

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

Amalloc

Bnew

Calloc

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using malloc instead of new in C++.

Forgetting to use new keyword.

In C++, new is used to allocate memory dynamically.

2fill in blank

medium

Complete the code to deallocate memory allocated for an integer pointer.

C++

delete [1];

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

A*ptr

Bptr()

Cptr

D&ptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ptr instead of ptr in delete.

Using ptr() which is invalid syntax.

Use delete followed by the pointer variable to free memory.

3fill in blank

hard

Fix the error in the code to allocate an array of 5 integers.

C++

int* arr = [1] int[5];

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

Anew

Bmalloc

Calloc

Dnew[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using malloc instead of new.

Writing new[] without the type.

Use new with square brackets to allocate arrays: new int[5].

4fill in blank

hard

Fill both blanks to correctly deallocate the array allocated with new.

C++

delete[1] [2];

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

A[]

B()

Cptr

Dptr[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete without brackets for arrays.

Using parentheses instead of brackets.

Use delete[] followed by the pointer to free arrays allocated with new[].

5fill in blank

hard

Fill all three blanks to allocate memory for a double, assign a value, and then deallocate it.

C++

double* [1] = [2] double;
*[1] = 3.14;
delete [3];
[1] = nullptr;

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

Aptr

Bnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Forgetting to delete the allocated memory.

First declare pointer ptr, allocate with new, assign value, then delete ptr.

Practice

1. What does the new operator do in C++?

easy

A. Allocates memory on the heap for a variable or object

B. Deletes a variable from memory

C. Allocates memory on the stack

D. Copies data from one variable to another

Solution

1. Step 1: Understand the role of new

   The new operator allocates memory dynamically on the heap during program execution.

2. Step 2: Differentiate from other memory actions

   It does not delete memory or allocate on the stack; it only reserves space on the heap.

3. Final Answer:

   Allocates memory on the heap for a variable or object -> Option A

4. Quick Check:

   new = allocate heap memory [OK]

Hint: Remember: new = allocate memory on heap [OK]

Common Mistakes:

• Confusing new with delete
• Thinking new allocates on stack
• Assuming new copies data

2. Which of the following is the correct syntax to deallocate memory allocated with new for a single integer variable?

easy

A. delete[] ptr;

B. free(ptr);

C. delete ptr;

D. ptr = nullptr;

Solution

1. Step 1: Identify correct deallocation for single object

   Memory allocated with new for a single variable must be freed with delete, not delete[].

2. Step 2: Exclude incorrect options

   free() is for C-style malloc, and setting pointer to nullptr does not free memory.

3. Final Answer:

   delete ptr; -> Option C

4. Quick Check:

   Single object deallocation = delete [OK]

Hint: Use delete for single object, delete[] for arrays [OK]

Common Mistakes:

• Using delete[] for single variable
• Using free() instead of delete
• Only setting pointer to nullptr without delete

3. What is the output of the following code?

int* ptr = new int(10);
std::cout << *ptr << std::endl;
delete ptr;

medium

A. 0

B. 10

C. Garbage value

D. Compilation error

Solution

1. Step 1: Analyze memory allocation and initialization

   The pointer ptr points to a new integer initialized with 10.

2. Step 2: Understand output before deletion

   Dereferencing *ptr prints 10 before memory is deleted.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Dereference new int(10) = 10 [OK]

Hint: Dereference pointer before delete to see value [OK]

Common Mistakes:

• Assuming output is 0 or garbage
• Thinking delete affects output before cout
• Confusing syntax causing compile error

4. Identify the error in the following code snippet:

int* arr = new int[5];
delete arr;

medium

A. Pointer arr is not initialized

B. Memory is not allocated properly

C. No error, code is correct

D. Should use delete[] instead of delete

Solution

1. Step 1: Check allocation type

   Memory allocated with new[] must be deallocated with delete[].

2. Step 2: Identify mismatch in deallocation

   Using delete instead of delete[] causes undefined behavior.

3. Final Answer:

   Should use delete[] instead of delete -> Option D

4. Quick Check:

   Array deallocation = delete[] [OK]

Hint: Match new[] with delete[] always [OK]

Common Mistakes:

• Using delete for arrays
• Ignoring pointer initialization
• Assuming no error in mismatched delete

5. You want to allocate memory for an array of 10 integers and initialize all elements to zero. Which code snippet correctly does this and safely deallocates the memory?

hard

A. int* arr = new int[10](); delete[] arr;

B. int* arr = new int[10]; delete arr;

C. int* arr = (int*)malloc(10 * sizeof(int)); delete[] arr;

D. int* arr = new int(10); delete[] arr;

Solution

1. Step 1: Allocate and initialize array to zero

   Using new int[10]() allocates an array of 10 integers and initializes all to zero.

2. Step 2: Properly deallocate array memory

   Memory allocated with new[] must be freed with delete[] to avoid leaks or undefined behavior.

3. Final Answer:

   int* arr = new int[10](); delete[] arr; -> Option A

4. Quick Check:

   new[] with () + delete[] = safe zero-init array [OK]

Hint: Use new int[size]() and delete[] for arrays [OK]

Common Mistakes:

• Using delete instead of delete[] for arrays
• Mixing malloc/free with new/delete
• Allocating single int instead of array