Memory leak concept in C++ - Mini Project: Build & Apply

Memory Leak Concept

📖 Scenario: Imagine you are managing a small library system where you keep track of books using pointers. You want to learn how to properly allocate and free memory to avoid memory leaks, which happen when memory is not released after use.

🎯 Goal: Build a simple C++ program that creates a dynamic array of book titles, then properly frees the memory to prevent memory leaks.

📋 What You'll Learn

Create a pointer to a dynamic array of strings for book titles

Set a variable for the number of books

Use a loop to assign book titles to the dynamic array

Properly free the allocated memory using delete[]

Print the book titles before freeing memory

💡 Why This Matters

🌍 Real World

Managing memory properly is important in programs that use dynamic data, like games, databases, or any software that handles large or changing data sets.

💼 Career

Understanding memory leaks and how to prevent them is a key skill for software developers, especially those working with C++ or other low-level languages.

1

Create a dynamic array for book titles

Create an integer variable called numBooks and set it to 3. Then create a pointer called books that points to a dynamic array of std::string with size numBooks.

C++

#include 
#include 

int main() {
    int numBooks = 3;
    // Your code here
    return 0;
}

Hint

Use new std::string[numBooks] to create the dynamic array.

2

Assign book titles to the dynamic array

Use a for loop with variable i from 0 to numBooks - 1 to assign these exact titles to books[i]: "C++ Basics", "Data Structures", "Algorithms".

C++

#include 
#include 

int main() {
    int numBooks = 3;
    std::string* books = new std::string[numBooks];
    // Your code here
    return 0;
}

Hint

Use if or else if inside the loop to assign the correct title for each index.

3

Print the book titles

Use a for loop with variable i from 0 to numBooks - 1 to print each book title on its own line using std::cout.

C++

#include 
#include 

int main() {
    int numBooks = 3;
    std::string* books = new std::string[numBooks];
    for (int i = 0; i < numBooks; i++) {
        if (i == 0) books[i] = "C++ Basics";
        else if (i == 1) books[i] = "Data Structures";
        else if (i == 2) books[i] = "Algorithms";
    }
    // Your code here
    return 0;
}

Hint

Use std::cout inside the loop to print each book title.

4

Free the allocated memory

Add the line delete[] books; before the return 0; statement to properly free the dynamic memory and avoid memory leaks.

C++

#include 
#include 

int main() {
    int numBooks = 3;
    std::string* books = new std::string[numBooks];
    for (int i = 0; i < numBooks; i++) {
        if (i == 0) books[i] = "C++ Basics";
        else if (i == 1) books[i] = "Data Structures";
        else if (i == 2) books[i] = "Algorithms";
    }
    for (int i = 0; i < numBooks; i++) {
        std::cout << books[i] << std::endl;
    }
    // Your code here
    return 0;
}

Hint

Use delete[] to free the memory allocated with new[].

Practice

1.

What is a memory leak in C++?

easy

A. When variables are declared but not used

B. When a program runs too fast

C. When allocated memory is not released back to the system

D. When the program uses too many functions

Solution

1. Step 1: Understand memory allocation

   Memory allocated with new must be freed with delete.

2. Step 2: Define memory leak

   If memory is allocated but not freed, it stays reserved and causes a leak.

3. Final Answer:

   When allocated memory is not released back to the system -> Option C

4. Quick Check:

   Memory leak = not freeing allocated memory [OK]

Hint: Memory leak means forgetting to free allocated memory [OK]

Common Mistakes:

• Confusing memory leak with slow program
• Thinking unused variables cause leaks
• Mixing memory leak with logical errors

2.

Which of the following C++ code snippets correctly frees allocated memory?

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

easy

A. free(ptr);

B. delete ptr;

C. ptr = nullptr;

D. ptr = new int(10);

Solution

1. Step 1: Identify allocation method

   Memory is allocated with new, so it must be freed with delete.

2. Step 2: Match correct freeing syntax

   free() is for malloc(), not new. Setting pointer to nullptr does not free memory.

3. Final Answer:

   delete ptr; -> Option B

4. Quick Check:

   Use delete for new, free for malloc [OK]

Hint: Use delete for new, not free [OK]

Common Mistakes:

• Using free() with new
• Just setting pointer to nullptr without delete
• Allocating new memory without deleting old

3.

What is the output of this C++ code?

int* ptr = new int(10);
ptr = new int(20);
std::cout << *ptr << std::endl;

medium

A. 10

B. Compilation error

C. Undefined behavior

D. 20

Solution

1. Step 1: Analyze pointer reassignment

   First, ptr points to 10. Then ptr is reassigned to new int(20) without deleting old memory.

2. Step 2: Determine output

   Pointer now points to 20, so output is 20. The old memory is leaked but does not affect output.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   Pointer points to last new value = 20 [OK]

Hint: Pointer shows last assigned value, old memory leaks [OK]

Common Mistakes:

• Expecting output 10 after reassignment
• Thinking code causes compile error
• Confusing memory leak with runtime error

4.

Find the memory leak in this code and fix it:

void func() {
  int* arr = new int[5];
  arr = new int[10];
  delete[] arr;
}

medium

A. Delete first array before reassigning arr

B. Use delete instead of delete[]

C. No leak, code is correct

D. Change new int[10] to new int(10)

Solution

1. Step 1: Identify memory allocations

   First, arr points to array of 5 ints, then reassigned to array of 10 ints without deleting first.

2. Step 2: Fix memory leak

   Delete the first array before reassigning arr to avoid leak. Use delete[] for arrays.

3. Final Answer:

   Delete first array before reassigning arr -> Option A

4. Quick Check:

   Delete old array before new assignment [OK]

Hint: Delete old array before assigning new to pointer [OK]

Common Mistakes:

• Not deleting first allocated array
• Using delete instead of delete[] for arrays
• Ignoring memory leak warnings

5.

You have a class that allocates memory in its constructor using new. Which approach best prevents memory leaks?

hard

A. Use smart pointers like std::unique_ptr to manage memory automatically

B. Manually call delete in destructor and follow Rule of Three

C. Use malloc and free instead of new and delete

D. Never use new, only stack variables

Solution

1. Step 1: Understand manual memory management risks

   Manually calling delete and following Rule of Three is error-prone and can cause leaks if forgotten.

2. Step 2: Use smart pointers for safety

   Smart pointers automatically free memory when no longer used, preventing leaks safely and easily.

3. Final Answer:

   Use smart pointers like std::unique_ptr to manage memory automatically -> Option A

4. Quick Check:

   Smart pointers prevent leaks automatically [OK]

Hint: Use smart pointers to avoid manual delete [OK]

Common Mistakes:

• Relying only on manual delete
• Avoiding new but not managing memory
• Mixing malloc/free with new/delete