new operator in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dynamic array allocation

What is the output of this C++ code snippet?

C++

#include <iostream>
int main() {
    int* arr = new int[3]{1, 2, 3};
    std::cout << arr[1] << std::endl;
    delete[] arr;
    return 0;
}

A 3

B 1

C 2

D Compilation error

💡 Hint

Remember that array indexing starts at 0.

The array is initialized with values {1, 2, 3}. arr[1] accesses the second element, which is 2.

❓ Predict Output

intermediate

Output of new operator with class object

What will this program print?

C++

#include <iostream>
class Box {
public:
    int length;
    Box(int l) : length(l) {}
};
int main() {
    Box* b = new Box(10);
    std::cout << b->length << std::endl;
    delete b;
    return 0;
}

A Runtime error

B 0

C Compilation error

D 10

💡 Hint

Check how the constructor initializes the length.

The Box object is created with length 10, so printing b->length outputs 10.

❓ Predict Output

advanced

Output when forgetting to delete dynamic memory

What is the output of this program?

C++

#include <iostream>
int main() {
    int* p = new int(5);
    *p = 10;
    std::cout << *p << std::endl;
    // no delete called
    return 0;
}

A 10

B 5

C Compilation error

D Runtime error due to memory leak

💡 Hint

The program prints the value pointed by p before exiting.

The pointer p points to an int initialized to 5, then changed to 10. The program prints 10. Not deleting causes a memory leak but no immediate runtime error.

❓ Predict Output

advanced

Output of new operator with placement new

What will this program output?

C++

#include <iostream>
#include <new>
int main() {
    char buffer[sizeof(int)];
    int* p = new(buffer) int(42);
    std::cout << *p << std::endl;
    p->~int();
    return 0;
}

A 0

B 42

C Compilation error

D Runtime error

💡 Hint

Placement new constructs an object in pre-allocated memory.

Placement new constructs an int with value 42 in buffer. Printing *p outputs 42.

🧠 Conceptual

expert

What error occurs when deleting non-dynamically allocated memory?

Consider this code snippet: int x = 5; int* p = &x; delete p; What happens when this code runs?

A Undefined behavior, likely runtime error

B Deletes x safely, no error

C Compilation error because delete cannot be used on stack variables

D Memory leak occurs

💡 Hint

delete should only be used on memory allocated with new.

Deleting a pointer to a stack variable causes undefined behavior and often runtime errors.

Practice

1. What does the new operator do in C++?

easy

A. Allocates memory dynamically and returns a pointer to it

B. Deletes a variable from memory

C. Declares a normal variable on the stack

D. Copies data from one variable to another

Solution

1. Step 1: Understand the purpose of new

   The new operator is used to allocate memory on the heap during program execution.

2. Step 2: Identify what new returns

   It returns a pointer to the allocated memory so the program can use it.

3. Final Answer:

   Allocates memory dynamically and returns a pointer to it -> Option A

4. Quick Check:

   new = allocate memory [OK]

Hint: Remember: new creates memory and gives a pointer [OK]

Common Mistakes:

• Confusing new with delete
• Thinking new declares stack variables
• Assuming new copies data

2. Which of the following is the correct syntax to allocate an integer using new?

easy

A. int p = new int;

B. int p = new int();

C. int *p = new int[];

D. int *p = new int;

Solution

1. Step 1: Check pointer declaration

   Using new returns a pointer, so the variable must be a pointer type like int *.

2. Step 2: Verify syntax correctness

   int *p = new int; correctly allocates an int and assigns its address to p.

3. Final Answer:

   int *p = new int; -> Option D

4. Quick Check:

   Pointer = new type [OK]

Hint: new returns pointer, so use pointer variable [OK]

Common Mistakes:

• Declaring variable without pointer
• Using parentheses incorrectly
• Assigning new to non-pointer variable

3. What is the output of this code?

int *arr = new int[3]{10, 20, 30};
std::cout << arr[1] << std::endl;
delete[] arr;

medium

A. 20

B. 10

C. 30

D. Runtime error

Solution

1. Step 1: Understand array allocation with new

   The code creates an array of 3 integers with values 10, 20, 30.

2. Step 2: Access the second element

   Array indexing starts at 0, so arr[1] is the second element, which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Remember array index starts at 0 [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Forgetting to use delete[] for arrays
• Expecting runtime error without cause

4. Identify the error in this code snippet:

int *p = new int;
*p = 5;
delete p;
delete p;

medium

A. Incorrect use of new operator

B. Missing initialization of pointer

C. Double deletion of the same pointer

D. No error, code is correct

Solution

1. Step 1: Analyze pointer allocation and deletion

   Pointer p is allocated memory and deleted once correctly.

2. Step 2: Check for repeated delete

   Deleting p a second time causes undefined behavior (double deletion error).

3. Final Answer:

   Double deletion of the same pointer -> Option C

4. Quick Check:

   delete twice = error [OK]

Hint: Delete pointer only once to avoid errors [OK]

Common Mistakes:

• Deleting pointer twice
• Not deleting allocated memory
• Confusing delete and delete[]

5. You want to create a dynamic 2D array of integers with 3 rows and 2 columns using new. Which code correctly allocates and deallocates this array?

hard

A. int *arr = new int[3*2];
// use arr
delete arr;

B. int **arr = new int*[3];
for(int i=0; i<3; i++) arr[i] = new int[2];
// use arr
for(int i=0; i<3; i++) delete[] arr[i];
delete[] arr;

C. int **arr = new int[3][2];
// use arr
delete[] arr;

D. int arr[3][2];
// use arr
// no delete needed

Solution

1. Step 1: Allocate 2D array dynamically

   int **arr = new int*[3]; for(int i=0; i<3; i++) arr[i] = new int[2]; // use arr for(int i=0; i<3; i++) delete[] arr[i]; delete[] arr; correctly creates an array of pointers for rows, then allocates each row as an array of ints.

2. Step 2: Properly deallocate memory

   Each row is deleted with delete[], then the array of pointers is deleted with delete[].

3. Final Answer:

   Uses an array of pointers, allocating each row separately -> Option B

4. Quick Check:

   Nested new and delete[] for 2D arrays [OK]

Hint: Use nested new and delete[] for 2D arrays [OK]

Common Mistakes:

• Using delete instead of delete[] for arrays
• Not deleting all allocated rows
• Trying to allocate 2D array with single new incorrectly