Memory leak concept in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a memory leak in C++?

A memory leak happens when a program allocates memory but never frees it, causing the memory to stay used and unavailable for other parts of the program or system.

beginner

Why are memory leaks a problem?

Memory leaks reduce the available memory over time, which can slow down or crash programs because they run out of memory to use.

intermediate

How do you prevent memory leaks in C++?

You prevent memory leaks by always freeing memory you allocate with 'delete' or 'delete[]' and by using smart pointers that automatically manage memory.

beginner

What is an example of a memory leak in C++?

If you use 'new' to create an object but never use 'delete' to free it, the memory stays allocated and causes a leak.

intermediate

What tools can help detect memory leaks in C++?

Tools like Valgrind or AddressSanitizer can find memory leaks by tracking allocated memory and checking if it is properly freed.

What causes a memory leak in C++?

A Writing to a file

B Using variables inside functions

C Allocating memory with 'new' and not freeing it with 'delete'

D Declaring too many variables

Memory leaks happen when allocated memory is not freed, like using 'new' without 'delete'.

Which C++ feature helps automatically manage memory and avoid leaks?

A Global variables

B Smart pointers

C Macros

D Inline functions

Smart pointers automatically free memory when no longer needed, preventing leaks.

What happens if a program has a memory leak?

A It runs faster

B It reduces CPU usage

C It frees memory automatically

D It uses more memory over time and may crash

Memory leaks cause programs to use more memory and can lead to crashes.

Which tool can help find memory leaks in C++ programs?

A Valgrind

B GCC compiler

C Git

D Makefile

Valgrind detects memory leaks by monitoring memory allocation and freeing.

What must you do after using 'new' in C++ to avoid leaks?

A Use 'delete' to free the memory

B Use 'malloc' instead

C Do nothing, memory frees automatically

D Use 'cout' to print the memory

You must use 'delete' to free memory allocated with 'new' to avoid leaks.

Explain what a memory leak is and why it is important to avoid it in C++ programs.

Think about what happens when memory is allocated but never freed.

Describe how smart pointers help prevent memory leaks in C++.

Smart pointers act like helpers that clean up memory for you.

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