Linked-List unable to construct (i assumed)

[aisderu]

Hello, I am very confused of constructing linked list. The below is my code.
I have my header file and implementation file seperated. When compile, error msg as below is shown:
____________________________________________________________________________
Undefined first referenced
symbol in file
List::~List() /var/tmp//cc9ezf6S.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status

____________________________________________________________________________
May i know wat's wrong? really need help!!! having exam soon. T_T

class List
{

private:
struct Node{
int item;
Node *next;
};

int size;
Node *head;
Node *tail;


public:
List();
~List();

bool empty();
int getlength();

void add(int); //always add to the back
};

//implementation is from here onwards

List::List():size(0),head(NULL),tail(NULL)
{
}


bool List::empty(){return size==0;}

int List:: getlength(){ return size;}

void List::add(int itemadded)
{
Node *newnode = new Node;
newnode->item=itemadded;
newnode->next=NULL;

if (head == NULL)
{ head = newnode;
tail = newnode;
}
else
{ newnode=tail->next;
tail= newnode ;}

}

[jsmith]

You have declared a destructor but you have not implemented it.

[aisderu]

thx for ur help !!!!!! i m stuck for hours not knowning this problem !!!!