[완료] c++ LNK2019 에러가 떠요 ㅠㅠ.

#ifndef _TASK2_H
#define _TASK2_H
#include <iostream>
#include <string>
using namespace std;
 
template<typename T>
class task2
{
    struct node
    {
        T data;
        node* prev;
        node* next;
        node(T t, node* p, node* n) : data(t), prev(p), next(n) {}
    };
    node* head;
    node* tail;
public:
    task2() : head( NULL ), tail ( NULL ) {}
    template<int N>
    task2( T (&arr) [N]) : head( NULL ), tail ( NULL )
    {
        for( int i(0); i != N; ++i)
            push_back(arr[i]);
    }
 
    bool empty() const { return ( !head || !tail ); }
    operator bool() const { return !empty(); } 
    void push_back(T);
    void push_front(T);
    T pop_back();
    T pop_front();
 
    ~task2()
    {
        while(head)
        {
            node* temp(head);
            head=head->next;
            delete temp;
        }
    }
};
 
#endif

#include <iostream>
#include <string>
#include "task2.h"
using namespace std;
 
template<typename T>
void task2<T>::push_back(T data)
{
    tail = new node(data, tail, NULL);
    if( tail->prev )
        tail->prev->next = tail;
 
    if( empty() )
        head = tail;
}
 
template <typename T>
void task2<T>::push_front(T data)
{
    head = new node(data, NULL, head);
    if( head->next )
        head->next->prev = head;
 
    if( empty() )
        tail = head;
}
 
template<typename T>
T task2<T>::pop_back()
{
    if( empty() )
        throw("double_linked : list empty");
    node* temp(tail);
    T data( tail->data );
    tail = tail->prev ;
 
    if( tail )
        tail->next = NULL;
    else
        head = NULL ;
 
    delete temp;
    return data;
}
 
template<typename T>
T task2<T>::pop_front()
{
    if( empty() )
        throw("double_linked : list empty");
    node* temp(head);
    T data( head->data );
    head = head->next ;
 
    if( head )
        head->prev = NULL;
    else
        tail = NULL;
 
    delete temp;
    return data;
}

#include <iostream>
#include <string>
#include "task2.h"
using namespace std;
 
int main(int argc, char* argv[])
{
	int arr[] = { 4, 6, 8, 32, 19 } ;
    task2<int> dlist ( arr );
    dlist.push_back( 11 );
    dlist.push_front( 100 );
    while( dlist )
        std::cout << dlist.pop_back()  << " ";
 
	std::cout << "\n";
 
	std::string arr2[] = { "a", "bb", "cc", "ddd", "eeee" } ;
	task2<std::string> dlist1 ( arr2 );
	dlist1.push_back( "gg" );
	dlist1.push_front( "ff" );
	while( dlist1 )
		std::cout << dlist1.pop_back()  << " ";
}