Using a constructor to initilize pointers to NULL in a Double linked List

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;
}

0x0 means zero, which (usually) is the address used to represent null. — Mike Seymour
– Mike Seymour, Commented Oct 6, 2014 at 3:13
What do you mean constructor isn't recognized? You can't step into it in the debugger? — Neil Kirk
– Neil Kirk, Commented Oct 6, 2014 at 3:17
You aren't initializing the pointers, you're assigning to them. — Qaz
– Qaz, Commented Oct 6, 2014 at 3:18

d3coy · Accepted Answer · 2014-10-06 03:31:35Z

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.

answered Oct 6, 2014 at 3:31

d3coy

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1 Comment

cdhowie Over a year ago

Well, technically NULL is not required to be zero by the C specification -- though it is required to compare equal to 0.

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