Memory leak concept in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate memory for an integer pointer.

C++

int* ptr = new [1];

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

Adouble

Bfloat

Cchar

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different type than the pointer declaration.

Forgetting to use new for dynamic allocation.

The new operator allocates memory for the specified type. Here, we want an integer pointer, so int is correct.

2fill in blank

medium

Complete the code to properly deallocate the dynamically allocated memory.

C++

delete [1];

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

Aptr

Bptr[]

C*ptr

D&ptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete[] for single object allocation.

Deleting the dereferenced pointer instead of the pointer.

To free memory allocated with new, use delete followed by the pointer variable name.

3fill in blank

hard

Fix the error in the code that causes a memory leak by missing deallocation.

C++

void leak() {
    int* data = new int[10];
    // Missing [1]
}

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

Afree(data)

Bdelete data

Cdelete[] data

Ddelete *data

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of delete[] for arrays.

Not deleting allocated memory at all.

When memory is allocated with new[], it must be deallocated with delete[] to avoid memory leaks.

4fill in blank

hard

Fill both blanks to create a loop that allocates and then deallocates memory correctly.

C++

for (int i = 0; i < 5; i++) {
    int* ptr = new [1];
    // use ptr
    [2] ptr;
}

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

Aint

Bdelete

Cdelete[]

Dfloat

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete[] for single object allocation.

Forgetting to delete the allocated memory inside the loop.

Each iteration allocates an int and then deallocates it with delete to avoid memory leaks.

5fill in blank

hard

Fill all three blanks to create a dictionary-like structure using std::map that stores dynamically allocated integers and properly deletes them.

C++

#include <map>
#include <string>

void cleanup(std::map<std::string, int*>& data) {
    for (auto& [1] : data) {
        [2] [3];
    }
    data.clear();
}

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

Apair

Bdelete

Cpair.second

Dauto

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect loop variable type.

Forgetting to delete the pointers before clearing the map.

The loop iterates over each pair in the map. The dynamically allocated integer is pointed to by pair.second, which must be deleted to avoid memory leaks.

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