20 Smart Pointer Interview Questions and Answers

Smart pointers are a type of data structure that helps manage memory in a program. They are often used in programming languages like C++ to help prevent memory leaks. When interviewing for a position that involves programming, you may be asked questions about smart pointers. Answering these questions correctly can help show that you have the necessary skills for the job. In this article, we will review some common questions about smart pointers and how to answer them.

Smart Pointer Interview Questions and Answers

Here are 20 commonly asked Smart Pointer interview questions and answers to prepare you for your interview:

1. What is a smart pointer?

A smart pointer is a type of pointer that automatically manages the memory it points to. This means that the programmer does not have to explicitly delete the object when they are finished with it, as the smart pointer will take care of that. Smart pointers can also provide other features, such as reference counting, which can help to prevent memory leaks.

2. How do you create a smart pointer in C++?

There are a few different ways to create a smart pointer in C++. The most common way is to use the std::shared_ptr class, which is part of the C++ standard library.

3. Can you explain the difference between shared_ptr, weak_ptr and unique_ptr?

Shared_ptr is a smart pointer that allows multiple objects to share ownership of a dynamically allocated object. The object is only deallocated when the last shared_ptr pointing to it is destroyed.

Weak_ptr is a smart pointer that holds a non-owning (“weak”) reference to an object that is managed by a shared_ptr. It is used to avoid circular references, which can lead to memory leaks.

Unique_ptr is a smart pointer that owns and manages a dynamically allocated object. The object is deallocated when the unique_ptr is destroyed.

4. What are some advantages of using smart pointers over raw pointers?

Smart pointers offer a number of advantages over raw pointers, including automatic memory management, object ownership tracking, and support for weak pointers. Smart pointers can help to prevent memory leaks and other problems that can occur when working with raw pointers.

5. What’s your understanding of reference counting?

Reference counting is a method of keeping track of how many references there are to an object. When an object is first created, its reference count is set to 1. Every time another object refers to that object, the reference count is incremented. When an object is no longer needed, the reference count is decremented. When the reference count reaches 0, the object is destroyed.

6. Why should we avoid cyclic references when using smart pointers?

Cyclic references can cause problems with smart pointers because the reference counting that smart pointers use can get caught in a loop. If there are two objects that reference each other, and each object has a smart pointer pointing to the other object, then the reference count for each object will never go to zero. This can cause problems because the objects will never be freed, and the memory will be leaked.

7. How can you use RAII to achieve exception safety with smart pointers?

RAII (Resource Acquisition Is Initialization) is a programming technique that helps ensure that resources are properly managed. When using smart pointers, RAII can help ensure that the pointers are properly deleted even in the event of an exception being thrown. This can help avoid memory leaks and other problems that can occur when resources are not properly managed.

8. Can you give me an example of how to delete a node from a linked list without causing memory leaks or corrupting data structures?

Assuming you have a linked list with nodes that look like this:

struct node {
int data;
node* next;
};

You can delete a node from the list by doing the following:

node* current = head;
node* previous = nullptr;

while (current != nullptr) {
if (current->data == target) {
if (previous == nullptr) {
// target is the first node in the list
head = current->next;
}
else {
// target is not the first node in the list
previous->next = current->next;
}

delete current;
break;
}

previous = current;
current = current->next;
}

9. What problems does shared ownership solve?

Shared ownership is a way of managing memory in a program so that multiple objects can have access to the same data. This can be useful in situations where you want to avoid copying data unnecessarily, or where you need to be sure that all objects have the same data. Shared ownership can help to solve problems with memory management and data consistency.

10. How do you pass a smart pointer by value if it causes it to be copied and incremented?

When you pass a smart pointer by value, it is copied and the reference count is incremented.

11. What is the advantage of using Boost Smart Pointers over std::smart_pointers?

Boost Smart Pointers offer more features and customization options than std::smart_pointers. For example, Boost Smart Pointers can be configured to use a custom deleter if necessary, whereas std::smart_pointers cannot. Boost Smart Pointers also offer more options for shared ownership, such as weak_ptr, which std::smart_pointers do not have.

12. What happens when multiple threads try to access a resource which has been wrapped in a std::shared_ptr?

If multiple threads try to access a resource which has been wrapped in a std::shared_ptr, then a race condition could occur. This is because the std::shared_ptr class is not thread-safe. If one thread tries to modify the resource while another thread is accessing it, then the results could be unpredictable.

13. Is it possible that multiple threads increment and decrement the same ref count at the same time?

Yes, it is possible for multiple threads to increment and decrement the same ref count at the same time. However, this is not generally considered to be a problem, as the ref count is designed to be an atomic operation. This means that it will be completed as a single unit of work, and thus will be thread-safe.

14. What is a garbage collector? When would you use one?

A garbage collector is a type of smart pointer that is responsible for automatically freeing memory that is no longer being used by the program. This can be helpful in situations where it is difficult to keep track of all the memory that is being allocated, such as when working with large data sets.

15. What type of locking mechanism is used to ensure thread-safety while accessing objects contained in a smart pointer object?

The type of locking mechanism used will depend on the implementation of the smart pointer. However, in general, a mutex will be used to ensure that only one thread can access the object at a time. This will prevent any race conditions from occurring.

16. Can you explain what polymorphism means in the context of smart pointers?

Polymorphism is the ability of an object to take on multiple forms. In the context of smart pointers, this means that a smart pointer can be used to point to objects of different types. This can be useful when you want to create a general-purpose pointer that can be used with different types of objects.

17. What is the main disadvantage of using smart pointers?

The main disadvantage of using smart pointers is that they can add complexity to your code. This is because smart pointers typically require more code to be written in order to manage them properly. This can make your code more difficult to read and understand, which can lead to errors.

18. How do you check for existence of elements in an associative container like std::map?

You can use the find() member function of the container to check for the existence of an element. The find() function returns an iterator to the element if it exists, and a end iterator if it does not.

19. How do you sort smart pointers?

You can sort smart pointers by using the std::sort algorithm.

20. Can you explain why sharing a mutex between several threads might not always be a good idea?

There are a few reasons why sharing a mutex between several threads might not always be a good idea. First, if the mutex is not properly managed, it can lead to a situation where one thread is constantly locking and unlocking the mutex, preventing the other threads from making any progress. Second, if the mutex is not properly released, it can lead to a deadlock situation where all threads are blocked waiting for the mutex to be unlocked. Finally, if the mutex is not properly initialized, it can lead to a race condition where two or more threads try to lock the mutex at the same time, leading to unpredictable results.