Memory leak concept in C++

Introduction

A memory leak happens when a program keeps using memory but never gives it back. This can make the program slow or crash.

When you want to understand why your program is getting slower over time.

When your program uses a lot of memory and you want to find mistakes.

When you want to learn how to manage memory properly in C++.

When debugging programs that run for a long time and suddenly stop working.

When you want to write better, safer C++ code.

Syntax

C++

int* ptr = new int[10];
// use ptr
// forgot to delete[] ptr;

new allocates memory on the heap.

If you don't use delete or delete[] to free it, memory leaks happen.

Examples

This creates one integer in memory but never frees it, causing a memory leak.

C++

int* ptr = new int;
*ptr = 5;
// forgot to delete ptr;

This correctly frees the memory allocated for an array, avoiding a leak.

C++

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

Sample Program

This program creates an array of 3 integers but never frees the memory. It prints the values correctly but leaks memory.

C++

#include <iostream>

int main() {
    int* leak = new int[3];
    leak[0] = 1;
    leak[1] = 2;
    leak[2] = 3;
    std::cout << "Values: " << leak[0] << ", " << leak[1] << ", " << leak[2] << std::endl;
    // Memory leak here: no delete[] for leak
    return 0;
}

Important Notes

Always match each new with a delete and each new[] with a delete[].

Using smart pointers like std::unique_ptr can help avoid leaks automatically.

Memory leaks may not crash your program immediately but can cause problems over time.

Summary

Memory leaks happen when allocated memory is not freed.

They make programs use more memory and can cause slowdowns or crashes.

Always free memory you allocate with new or use smart pointers.

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