Reset unique_ptr in Modern C++

When working with unique_ptr in C++, it’s essential to know how to renounce the ownership of the managed object and free the associated memory when necessary. The unique_ptr class provides the reset() member function for this purpose.

The unique_ptr::reset() Function

Calling the reset() method on a unique_ptr will perform two operations:

1. It will deallocate or release the memory that the unique_ptr currently owns.
2. It will set the unique_ptr itself to point to nullptr, effectively indicating that it no longer manages any memory.

Example of reset() function of unique_ptr

Let’s explore how this works with an example using an int as the managed object for simplicity:

#include <iostream>
#include <memory>

int main()
{
    // uniquePtr now owns an int with value 11
    std::unique_ptr<int> uniquePtr(new int(11));

    // Resetting the uniquePtr
    uniquePtr.reset();

    // After reset, uniquePtr no longer owns the previous int
    if (!uniquePtr)
    {
        std::cout << "uniquePtr has been reset and is now nullptr." << std::endl;
    }

    return 0;
}

Output:

uniquePtr has been reset and is now nullptr.

Check if a unique_ptr object is NULL

After you have called reset(), it’s a good practice to check whether the unique_ptr is indeed empty (i.e., it now points to nullptr). You can do this by directly using the unique_ptr in a condition as it has an operator bool defined which checks for ownership of an object:

#include <iostream>
#include <memory>

int main()
{
    // uniquePtr now owns an int with value 11
    std::unique_ptr<int> uniquePtr(new int(11));

    // Resetting the uniquePtr
    uniquePtr.reset();

    if (uniquePtr)
    {
        // uniquePtr points to an object; it's safe to use.
    }
    else
    {
        // uniquePtr is empty (nullptr); it's not safe to use.
        std::cout << "The pointer is not pointing to anything useful." << std::endl;
    }

    return 0;
}

Output:

The pointer is not pointing to anything useful.

Why Reset a unique_ptr?

Resetting a unique_ptr can be useful in several scenarios:

• When you want to safely release the resource before the unique_ptr goes out of scope.
• If you need to reuse the unique_ptr for a new resource.
• To ensure that the unique_ptr does not maintain a dangling pointer to a resource that has been managed elsewhere.

Summary

The ability to reset a unique_ptr is an integral part of its functionality, providing you with control over when to free the managed memory. It also helps in preventing memory leaks by ensuring that memory is not inadvertently retained. WRemember, once reset, always check the unique_ptr before using it to avoid accessing nullptr.