Common pointer mistakes in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pointer arithmetic

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

C++

#include <iostream>
int main() {
    int arr[3] = {10, 20, 30};
    int* p = arr;
    std::cout << *(p + 1) << std::endl;
    return 0;
}

A Compilation error

B 10

C 30

D 20

💡 Hint

Pointer arithmetic moves the pointer by the size of the pointed type.

The pointer p points to arr[0]. Adding 1 moves it to arr[1], which is 20.

❓ Predict Output

intermediate

Dereferencing a null pointer

What happens when this C++ code runs?

C++

#include <iostream>
int main() {
    int* p = nullptr;
    std::cout << *p << std::endl;
    return 0;
}

A Runtime error (segmentation fault)

B Prints 0

C Compilation error

D Prints garbage value

💡 Hint

Dereferencing a null pointer is unsafe.

Dereferencing nullptr causes undefined behavior, usually a crash (segmentation fault).

❓ Predict Output

advanced

Dangling pointer after delete

What is the output of this code?

C++

#include <iostream>
int main() {
    int* p = new int(42);
    delete p;
    std::cout << *p << std::endl;
    return 0;
}

A Undefined behavior (likely garbage or crash)

B Compilation error

C 42

D 0

💡 Hint

Accessing memory after delete is unsafe.

After delete, pointer p is dangling. Dereferencing it causes undefined behavior.

❓ Predict Output

advanced

Pointer type mismatch

What error or output does this code produce?

C++

#include <iostream>
int main() {
    int x = 5;
    double* p = (double*)&x;
    std::cout << *p << std::endl;
    return 0;
}

A Prints 5.0

B Undefined behavior (prints garbage or crashes)

C Compilation error

D Prints 0

💡 Hint

Casting pointer types incorrectly can cause problems.

Casting int* to double* and dereferencing causes undefined behavior because the memory layout differs.

🧠 Conceptual

expert

Common cause of memory leak with pointers

Which option best describes a common cause of memory leaks when using pointers in C++?

A Casting pointers to void*

B Using nullptr instead of a valid pointer

C Forgetting to delete memory allocated with new

D Dereferencing a null pointer

💡 Hint

Memory leaks happen when allocated memory is not freed.

If you allocate memory with new but never call delete, the memory stays allocated and leaks.