如何将双向链接列表中的头部初始化为NULL？

Question

I have a homework to implement a doubly-linked list in C++ with two structs. One of them should hold the next and previous pointers , as well as the data. The other struct should hold two pointers, one for the head and one for the tail

Struct DlistElem{
    int data;
    DlistElem* next;
    DlistElem* prev;
  }

Struct Dlist{
 Dlist * head;
 Dlist * tail;
}

//some functions

the problem I'm having is I don't know how to initialize both the head and tail to be null in order to use them. Any help would be greatly appreciated!

Answer 1

Initialize the pointers with NULL when you create a DlistElem and Dlist.

struct DlistElem{
    DlistElem* prev;
    DlistElem* next;
    int data;
    DlistElem() :
        prev(NULL),
        next(NULL),
        data()
    {}
}

struct Dlist{
    Dlist* head;
    Dlist* tail;
    Dlist() :
        head(NULL),
        tail(NULL)
    {}
}

Answer 2

I don't know how to initialize both the head and tail to be null in order to use them.

struct DlistElem {
    int data;
    DlistElem* next;
    DlistElem* prev;

    DlistElem() : data{}, next{}, prev{}
    {}
};

or

struct DlistElem {
    int data;
    DlistElem* next{};
    DlistElem* prev{};
};

Answer 3

Another way this can be done in c++11 is:

struct DlistElem{
    int data;
    DlistElem* next = nullptr;
    DlistElem* prev = nullptr;
}

struct Dlist{
    Dlist * head = nullptr;
    Dlist * tail = nullptr;
}