error: template argument required for 'struct

问题:

I'm trying to create my own template for a List class as a learning excercise. I've been having some trouble with template syntax though and i'm now getting the following error message..

main.cpp|Line 8|instantiated from here
error: template argument required for 'struct List'
In function 'int main()':
...

As far as i can tell i'm not misusing anything but this is my first time working with templates and would really appreciate someone looking through and letting me know what i'm doing wrong.

List.hpp:

#if !defined _LIST_HPP_
#define _LIST_HPP_

#include "Node.hpp"

///since we're creating a template everything must be defined in the hpp

template <typename ListType>
class List
{
   public:
      List();
      bool Empty();
      void PushFront();
      void PushBack();
      void PopBack();
      Node<ListType>& GetHead();

   private:
      int _size;
      Node<ListType>* _head;
      Node<ListType>* _tail;
};


///implement List class here
template <typename ListType>
List<ListType>::List() : _head(0), _tail(0), _size(0)
{
}
template <typename ListType>
bool List<ListType>::Empty()
{
   return _size == 0;
}
template <typename ListType>
void List<ListType>::PushFront()
{
   _head = new Node<ListType>( _head , 0 );
   if (!Empty())
      _head->_prev->_next = _head; //set previous nodes _next to new _head

   ++_size;
}
template <typename ListType>
void List<ListType>::PushBack()
{
   _tail = new Node<ListType>( 0 , _tail);
   if (!Empty())
      _tail->_next->_prev = _tail; // set old tails _prev to new tail

   ++_size;
}
template <typename ListType>
void List<ListType>::PopBack()
{

}
template <typename ListType>
Node<ListType>& List<ListType>::GetHead()
{
   return _head;
}

#endif //define

Node.hpp:

#if !defined _NODE_HPP_
#define _NODE_HPP_


template<typename NodeType>
class Node{
   public:
      Node( Node* prev = 0, Node* next = 0);
      void SetData(NodeType newData);
      void GetData();
   private:
      friend class List;

      NodeType _data;
      Node* _next;
      Node* _prev;

};


///implement Node

template <typename NodeType>
Node<NodeType>::Node(Node* prev, Node* next) : _prev(prev), _next(next)
{}
template <typename NodeType>
void Node<NodeType>::SetData(NodeType newData)
{
   _data = newData;
}
template <typename NodeType>
void Node<NodeType>::GetData()
{
   return _data;
}

#endif //define

Main.hpp

#include <iostream>
#include "List.hpp"
int main()
{
   List<int> testl;
      //test
      testl.PushFront();
      testl.GetHead().SetData(7); //Error thrown here??
      std::cout << test1.GetHead().GetData() << std::endl;

   return 0;
}

回答1:

List is a class template, so you need to declare it as such in your friend declaration

template<typename ListType>
friend class List;

If you only want List<NodeType> to be a friend, you need to tell it that template argument, so then the friend declaration becomes

friend class List<NodeType>;

For this to work, it needs to know that List exists as a class template, so you need to forward-declare it at the top of Node.hpp:

template<typename ListType>
class List;