What smart pointers do you know in C++?

std::unique_ptr - a unique owner of an object, cannot be copied but can be moved.

#include <memory>

int main() {
    std::unique_ptr<int> ptr(new int(10)); // Creation
    // std::unique_ptr<int> ptr2 = ptr; // Compilation error - no copying
    std::unique_ptr<int> ptr3 = std::move(ptr); // Move ownership
    return 0;
}

std::shared_ptr - a shared owner, counts references to the object. The object is deleted when the reference count reaches zero.

#include <memory>
#include <iostream>

int main() {
    std::shared_ptr<int> ptr(new int(20));
    std::cout << "Refs: " << ptr.use_count() << std::endl; // 1

    std::shared_ptr<int> ptr2 = ptr; // Copying owner
    std::cout << "Refs: " << ptr.use_count() << std::endl; // 2

    ptr.reset(); // Decrease reference count
    std::cout << "Refs: " << ptr2.use_count() << std::endl; // 1

    return 0;
}

std::weak_ptr - a non-owning observer, used to prevent cyclic references between shared_ptr. Does not increase the reference count. To access the object, convert it to shared_ptr using the lock() method.

#include <memory>
#include <iostream>

int main() {
    std::shared_ptr<int> shared_ptr(new int(30));
    std::weak_ptr<int> weak_ptr = shared_ptr; // Create weak_ptr

    if (auto locked_ptr = weak_ptr.lock()) {
        std::cout << "Value: " << *locked_ptr << std::endl; // Access to object
    } else {
        std::cout << "Object expired." << std::endl;
    }

    shared_ptr.reset(); // Object deleted

    if (auto locked_ptr = weak_ptr.lock()) {
        std::cout << "Value: " << *locked_ptr << std::endl;
    } else {
        std::cout << "Object expired." << std::endl; // Object deleted
    }

    return 0;
}

std::auto_ptr - deprecated (removed in C++17), a smart pointer with move semantics on copy, which could lead to errors. Use std::unique_ptr instead.

Pointer	Ownership semantics	Copying	Moving	Cyclic references
`unique_ptr`	Unique	No	Yes	Not applicable
`shared_ptr`	Shared	Yes	Yes	Can cause
`weak_ptr`	Does not own	Yes	Yes	Prevents
`auto_ptr`	Moving (deprecated)	No (except)	Implicit	Not applicable