pointers - Doubly Linked List insertion infinite loop... C++ -

i implementing doubly linked list in c++. before insertion, printing node function works after insertion front, printing goes forever.

for example, have nodes of 1, 2, 3 data , insert data front 5. try print, , shows 5, 1, infinite loop without going third node 2.

here structure.

    struct dl_node     {         int               data;         struct dl_node*   prev;         struct dl_node*   next;          dl_node(dl_node* prev, dl_node* next, int data)         {             // here, prev parameter             // this->prev object             this->prev = prev;             this->next = next;             this->data = data;         }          // constructor, without pointer parameter         explicit dl_node(int data)         {             this->prev = this;             this->next = this;             this->data = data;         }     }; 

here insertion function.

    // "push-front" operation     dl_node* insert_node(dl_node* head, int data)     {         if (nullptr == head)                     return new dl_node(data);              auto insertion             = new dl_node(head->prev, head, data);             // previous node of insertion head's prev             // next node of insertion head              insertion->prev->next = insertion;             insertion->next->prev = insertion;              return insertion;     } 

here initialization.

    struct dl_node* head   = new dl_node(null);     struct dl_node* node_1 = new dl_node(null);     struct dl_node* node_2 = new dl_node(null);      head  ->data = 1;     head  ->next = node_1;     node_1->prev = head;      node_1->data = 2;     node_1->next = node_2;     node_2->prev = node_1;      node_2->data = 3;     node_2->next = nullptr; 

here insertion.

    // insert front     head = insert_node(head, 5); 

here printing loop.

struct dl_node* current_node_2 = head; while ( current_node_2 != nullptr )     {             cout << current_node_2->data << ", ";             current_node_2 = current_node_2->next;     }     // 5, 1, infinite loop here.... 

could have idea?

the problem default dl_node constructor sets both prev , next this.

when call insert_node(head, 5) end following state:

insertion->prev = head->prev;  // assigned in constructor, head->prev == head insertion->next = head; insertion->prev->next = insertion; insertion->next->prev = insertion; 

but insertion->prev == head->prev, , know head->prev == head, so

insertion->prev->next = insertion 

reduces to:

head->next = insertion; 

so end list looks this:

insertion -> head -> insertion -> ... 

you should change default constructor set both next , prev null. in insert function, should check insertion->prev , insertion->next non-null before dereferencing them.