使用构造函数初始化双链表中指向NULL的指针

Question

I am trying to initialize a new object of a class Dlist. After a new object is declared the pointers first and last are supposed to be NULL . When I declare the Dlist temp first and last however- the constructor isn't being recognized and the compiler is giving them values like 0x0 . I'm not sure why the the constructor being recognized.

// dlist.h
class Dlist {
private:
// DATA MEMBERS
struct Node
{
    char data;
    Node *back;
    Node *next;
};

Node *first;
Node *last;

// PRIVATE FUNCTION
Node* get_node( Node* back_link, const char entry, Node* for_link );


public:

// CONSTRUCTOR
Dlist(){ first = NULL; last = NULL; }  // initialization of first and last 

// DESTRUCTOR
~Dlist();

// MODIFIER FUNCTIONS
void append( char entry);
bool empty();
void remove_last();

//CONSTANT FUNCTIONS
friend ostream& operator << ( ostream& out_s, Dlist dl);

};           
#endif

// implementation file
int main()
{
Dlist temp;
char ch;

cout << "Enter a line of characters; # => delete the last character." << endl
<< "-> ";


cin.get(ch);
temp.append(ch);

cout << temp;
return 0;
}

Answer 1

0x0 is NULL. Also, initialization of class members is more efficiently done via the constructor's initialization list:

Dlist()
    : first(nullptr)
    , last(nullptr)
{ /* No assignment necessary */ }

When a class is constructed, the initialization list is applied to the memory acquired for the object before the body of the constructor is executed.