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c++templatespointerslinked-list

C++ Linked list using smart pointers


I have only been using raw pointers for linked list with templates. For example, the member data, Node<T>* head; and when I am inserting a node one of the lines would be head = new Node<T>(data);.

However, now I need to use a smart pointer and I am not sure how I would change it to use smart pointers. Would the member data be changed to shared_ptr<Node<T>> head; and the other line would change to
head = shared_ptr<Node<T>>( new <Node<T>>(data) );?


Solution

  • You do not "need" to use a smart pointer for a linked list, because that statement doesn't make sense. You do not use smart pointers for low-level data structures. You use smart pointers for high-level program logic.

    As far as low-level data structures are concerned, you use a standard container class from the C++ standard library, like std::list [*], which solves all your memory-management problems anyway, without using any smart pointers internally.

    If you really really need your own highly specialised/optimised custom container class because the entire C++ standard library is unfit for your requirements and you need a replacement for std::list, std::vector, std::unordered_map and other optimised, tested, documented and safe containers – which I very much doubt! –, then you have to manage memory manually anyway, because the point of such a specialised class will almost certainly be the need for techniques like memory pools, copy-on-write or even garbage collection, all of which conflict with a typical smart pointer's rather simplistic deletion logic.

    In the words of Herb Sutter:

    Never use owning raw pointers and delete, except in rare cases when implementing your own low-level data structure (and even then keep that well encapsulated inside a class boundary).

    Something along those lines is also expressed in Herb Sutter's and Bjarne Stroustrup's C++ Core Guidelines:

    This problem cannot be solved (at scale) by transforming all owning pointers to unique_ptrs and shared_ptrs, partly because we need/use owning "raw pointers" as well as simple pointers in the implementation of our fundamental resource handles. For example, common vector implementations have one owning pointer and two non-owning pointers.

    Writing a linked-list class in C++ with raw pointers can be a useful academic exercise. Writing a linked-list class in C++ with smart pointers is a pointless academic exercise. Using any of these two self-made things in production code is almost automatically wrong.


    [*] Or just std::vector, because due to cache locality that will almost always be the better choice anyway.