Getting started with linked-list – Design using Sentry Node

When designing a linked list, you can avoid all the special-cases (empty list, first node,
last node, etc) by using a sentry node. Let’s see how that is done:

struct Node
    Node* next;
    Node* prev;
    T data;

// helper function to link 2 nodes
void Link(Node* n1, Node* n2)
    n1->next = n2;
    n2->prev = n1;

// this inserts new data before 'here'
Node* Insert(Node* here, const T& data)
    Node* item = new Node{0,0,data};  // create new item. use T's copy-constructor
    Link(here->prev, item);           // link in new node. item comes before here,
    Link(item, here);                 // so in-between `here->prev´ and `here´
    size += 1;                        // update size
    return item;

// erase one item
Node* Erase(Node* here)
    Node* nxt = here->next;           // save next item for return value
    Link(here->prev, here->next);     // unlink item. no special cases needed when using sentry
    delete here;                      // delete item. this will call T's destructor
    size -= 1;                        // update size
    return nxt;

This looks like it would fail for en empty list for example, but with a sentry node
the list is never truly empty, it always contain the sentry node, that link to
itself if there is no data-nodes. The sentry node also double as the one past last

Node* sentry;
void Init()
    sentry = (Node*)your_preferred_allocator();
    Link(sentry, sentry);
    size = 0;

A more comprehensive tutorial can be found at

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Getting started with linked-list – Design using Sentry Node - CodeDay