Moving Unique Pointers

Copying Not Allowed

A std::unique_ptr is a smart pointer that manages a dynamically allocated object and ensures its proper deletion when it goes out of scope. It has exclusive ownership of the object it manages, meaning no two std::unique_ptr can own the same object simultaneously.

main

#include <iostream>

#include <memory>

int main()

{

std::unique_ptr<int> first_unique_pointer = std::make_unique<int>(42);

// This will cause a compilation error: copying is not allowed

std::unique_ptr<int> second_unique_pointer = first_unique_pointer;

}


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#include <iostream>
#include <memory>

int main()
{
    std::unique_ptr<int> first_unique_pointer = std::make_unique<int>(42);
    // This will cause a compilation error: copying is not allowed
    std::unique_ptr<int> second_unique_pointer = first_unique_pointer;
}

Moving a Unique Pointer

Although copying is not allowed, you can transfer ownership of the object using std::move. This moves the resource from one std::unique_ptr to another, leaving the original pointer empty.

main

#include <iostream>

#include <memory>

int main()

{

std::unique_ptr<int> first_unique_pointer = std::make_unique<int>(42);

// Transferring ownership using std::move

std::unique_ptr<int> second_unique_pointer = std::move(first_unique_pointer);

if (!first_unique_pointer)

std::cout << "first_unique_pointer is now empty.\n";

std::cout << "second_unique_pointer owns the value: " << *second_unique_pointer << '\n';

}


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#include <iostream>
#include <memory>

int main() 
{
    std::unique_ptr<int> first_unique_pointer = std::make_unique<int>(42);
  
    // Transferring ownership using std::move
    std::unique_ptr<int> second_unique_pointer = std::move(first_unique_pointer);
  
    if (!first_unique_pointer)
        std::cout << "first_unique_pointer is now empty.\n";

    std::cout << "second_unique_pointer owns the value: " << *second_unique_pointer << '\n';
}

When you use std::move, ownership of the object is transferred from one unique pointer to another. After this, the original pointer no longer owns the object, and the new pointer becomes its owner.

Opgave

Swipe to start coding

Complete the following code in a way that the ownership of the dynamically allocated integer (with value 42) is safely transferred from unique_pointer_a to unique_pointer_b.