Memory allocation and deallocation in C++ - Mini Project: Build & Apply

Memory allocation and deallocation

📖 Scenario: Imagine you are managing a small library system where you need to store the number of books available in different sections. You want to learn how to create space in memory to store this data and then clean up the space when you are done.

🎯 Goal: You will write a simple C++ program that allocates memory dynamically for storing the number of books in three sections, assigns values to them, and then frees the memory.

📋 What You'll Learn

Use new to allocate memory for an array of integers

Assign values to the allocated memory

Use delete[] to free the allocated memory

Print the values stored in the allocated memory

💡 Why This Matters

🌍 Real World

Dynamic memory allocation is used in programs where the amount of data is not known before running the program, like managing lists of items or user inputs.

💼 Career

Understanding memory allocation and deallocation is essential for software developers to write efficient and safe C++ programs, especially in systems programming and game development.

1

Create a pointer and allocate memory

Create an integer pointer called books and allocate memory for 3 integers using new.

C++

// Your code here

Hint

Use int* books = new int[3]; to allocate memory for 3 integers.

2

Assign values to the allocated memory

Assign the values 10, 20, and 30 to the first, second, and third elements of the books array respectively.

C++

int* books = new int[3];
// Your code here

Hint

Use books[0] = 10; and similarly for the other elements.

3

Print the values stored in the allocated memory

Use a for loop with variable i from 0 to 2 to print each value in the books array separated by spaces.

C++

int* books = new int[3];
books[0] = 10;
books[1] = 20;
books[2] = 30;

// Your code here

Hint

Use a for loop and std::cout to print each element.

4

Deallocate the memory

Use delete[] books; to free the memory allocated for the books array. Then print a newline character.

C++

int* books = new int[3];
books[0] = 10;
books[1] = 20;
books[2] = 30;

for (int i = 0; i < 3; i++) {
    std::cout << books[i] << " ";
}

// Your code here

Hint

Use delete[] books; to free the memory and then print a newline.

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