delete operator in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to correctly delete the dynamically allocated integer.

C++

#include <iostream>
int main() {
    int* ptr = new int(10);
    [1] ptr;
    return 0;
}

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

Afree

Bdispose

Cremove

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using free() instead of delete for memory allocated with new.

Trying to use remove or dispose which are not C++ operators.

The delete operator is used to free memory allocated with new in C++.

2fill in blank

medium

Complete the code to delete a dynamically allocated array.

C++

#include <iostream>
int main() {
    int* arr = new int[5];
    [1] arr;
    return 0;
}

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

Adelete[]

Bfree

Cdelete

Ddispose

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of delete[] for arrays.

Using free() which is for C-style allocation.

When deleting arrays allocated with new[], use delete[] to properly free all elements.

3fill in blank

hard

Fix the error in the code by choosing the correct delete syntax.

C++

#include <iostream>
int main() {
    double* data = new double(3.14);
    [1] data;
    return 0;
}

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

Afree

Bdelete()

Cdelete

Ddelete[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete[] on a single object.

Trying to call delete as a function like delete().

Since data points to a single double, use delete without brackets.

4fill in blank

hard

Fill both blanks to correctly delete a dynamically allocated array and avoid memory leaks.

C++

#include <iostream>
int main() {
    char* letters = new char[10];
    [1] [2];
    return 0;
}

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

Adelete[]

Bdelete

Cletters

Dfree

Attempts:

3 left

💡 Hint

Common Mistakes

Using delete instead of delete[] for arrays.

Forgetting to specify the pointer after delete.

Use delete[] followed by the pointer variable to free the array memory.

5fill in blank

hard

Fill all three blanks to correctly delete a dynamically allocated object and set the pointer to nullptr.

C++

#include <iostream>
int main() {
    float* value = new float(5.5f);
    [1] [2];
    [3] = nullptr;
    return 0;
}

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

Adelete

Bvalue

Dptr

Attempts:

3 left

💡 Hint

Common Mistakes

Not setting the pointer to nullptr after deletion.

Using wrong pointer variable names.

Delete the object with delete, then set the pointer value to nullptr to avoid dangling pointers.

Practice

1. What is the main purpose of the delete operator in C++?

easy

A. To allocate new memory

B. To free memory allocated with new

C. To copy objects

D. To declare variables

Solution

1. Step 1: Understand memory allocation

   The new operator allocates memory dynamically on the heap.

2. Step 2: Role of delete

   The delete operator frees that allocated memory to avoid leaks.

3. Final Answer:

   To free memory allocated with new -> Option B

4. Quick Check:

   delete frees memory [OK]

Hint: Remember: new allocates, delete frees memory [OK]

Common Mistakes:

• Confusing delete with new
• Using delete to copy objects
• Thinking delete declares variables

2. Which of the following is the correct syntax to delete a dynamically allocated array arr?

easy

A. delete arr;

B. delete array arr;

C. delete arr[];

D. delete[] arr;

Solution

1. Step 1: Identify array deletion syntax

   For arrays allocated with new[], use delete[] to free all elements properly.

2. Step 2: Match correct syntax

   delete[] arr; correctly deletes the entire array.

3. Final Answer:

   delete[] arr; -> Option D

4. Quick Check:

   Use delete[] for arrays [OK]

Hint: Use delete[] for arrays, delete for single objects [OK]

Common Mistakes:

• Using delete instead of delete[] for arrays
• Placing brackets after variable name incorrectly
• Writing delete array arr (invalid syntax)

3. What will be the output of the following code?

int* p = new int(10);
std::cout << *p << std::endl;
delete p;
// std::cout << *p << std::endl; // Uncommenting this line causes what?

medium

A. 10 and then 10 again

B. Compilation error at delete

C. 10 and then undefined behavior

D. Runtime error at new

Solution

1. Step 1: Analyze output before delete

   The pointer p points to an int with value 10, so *p prints 10.

2. Step 2: Effect of delete and accessing pointer

   After delete p;, the memory is freed. Accessing *p after delete causes undefined behavior (may crash or print garbage).

3. Final Answer:

   10 and then undefined behavior -> Option C

4. Quick Check:

   Access after delete = undefined behavior [OK]

Hint: Don't use pointer after delete; it's unsafe [OK]

Common Mistakes:

• Assuming deleted pointer still holds valid data
• Expecting compilation error on delete
• Ignoring undefined behavior risks

4. Identify the error in this code snippet:

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

medium

A. Should use delete[] instead of delete

B. Should use delete without brackets

C. No error, code is correct

D. Should not delete dynamically allocated arrays

Solution

1. Step 1: Check allocation type

   arr is allocated with new int[5], which creates an array.

2. Step 2: Correct deletion method

   Arrays must be deleted with delete[] arr; to properly free all elements.

3. Final Answer:

   Should use delete[] instead of delete -> Option A

4. Quick Check:

   Array delete needs delete[] [OK]

Hint: Match new[] with delete[] always [OK]

Common Mistakes:

• Using delete for arrays causes undefined behavior
• Thinking delete works for both single and arrays
• Ignoring memory leaks from wrong delete

5. Consider this code:

int** matrix = new int*[3];
for (int i = 0; i < 3; ++i) {
    matrix[i] = new int[4];
}
// Code to delete matrix properly?

Which is the correct way to delete all allocated memory?

hard

A. for (int i = 0; i < 3; ++i) delete[] matrix[i]; delete[] matrix;

B. for (int i = 0; i < 3; ++i) delete matrix[i]; delete[] matrix;

C. delete[] matrix; delete[] matrix[0];

D. delete matrix; delete matrix[0];

Solution

1. Step 1: Understand allocation

   Each matrix[i] is an array allocated with new int[4], so must be deleted with delete[].

2. Step 2: Delete outer array

   The matrix pointer itself is an array of pointers allocated with new int*[3], so delete with delete[] matrix;.

3. Final Answer:

   for (int i = 0; i < 3; ++i) delete[] matrix[i]; delete[] matrix; -> Option A

4. Quick Check:

   Match new[] with delete[] at all levels [OK]

Hint: Delete inner arrays with delete[], then outer with delete[] [OK]

Common Mistakes:

• Using delete instead of delete[] for inner arrays
• Deleting outer array with delete instead of delete[]
• Forgetting to delete inner arrays causing leaks