连接两个C 文件(list.cc and queue.cc）困难

我必须使用我之前创建的列表文件在C 中编写一个队列，并且我遇到了艰难的时间来编译所有内容。

我目前遇到的问题是，当我编译时，我会收到错误： Queue.h：7：2：错误：'列表'不命名类型

如何正确连接队列文件和列表文件？

这是我正在使用的文件：list.h

//an item in the list
  struct ListNode {
  int _value;
  ListNode * _next;
};
class List {
public:
  //Head of list
  ListNode * _head;
  int remove_front();
  void insertSorted( int val );
  void append (int val);
  void prepend (int val);  
  int lookup( int _value );
  int remove( int val );
  void print();
  List();
  ~List();
};

list.cc

//
// Implement the List class
//
#include <stdio.h>
#include "List.h"
ListNode * _head = new ListNode();
//remove the first node in the list
int
List::remove_front(){
    int ret;
    if(_head == 0){
        return -1;
    }
    ret = _head->_value;
    ListNode *temp = new ListNode();
    temp = _head->_next;
    delete(_head);
    _head = temp;
    return ret;
}
//
// Inserts a new element with value "val" in
// ascending order.
//
void
List::insertSorted( int val ){
    ListNode* new_node = new ListNode();
    new_node->_value = val;
    ListNode* current = new ListNode();
    if(_head == 0){
        _head = new_node;
    }else{
        current = _head;
        ListNode* prev = 0;
        while(current != 0){
            if(new_node->_value > current->_value){
                prev = current;
                current = current->_next;
            }else{
                break;
            }
        }
        if(current == _head){
            new_node->_next = _head;    
            _head = new_node;
        }else{
            new_node->_next = current;
            prev->_next = new_node;
        }
    }
}
//
// Inserts a new element with value "val" at
// the end of the list.
//
void
List::append( int val ){
    //create a new node to hold the given value
    ListNode *new_node = new ListNode();        
    new_node->_value = val;
    //if the list is empty
    if(_head == 0){
        //set the new node to be the head
        _head = new_node;
        return ;
    }
    //create a node pointer to the current position (starting at the head)
    ListNode *current = new ListNode();
    current = _head;
    //Loop through the list until we find the end
    while(current->_next != NULL){
        current = current->_next;
    }
    current->_next = new_node;
}
//
// Inserts a new element with value "val" at
// the beginning of the list.
//
void
List::prepend( int val ){
    ListNode *new_node = new ListNode;
    new_node->_value = val;
    if(_head == 0){
        _head = new_node;
        return ;
    }
    ListNode *temp = new ListNode;
    temp = _head;

    _head = new_node;
    _head->_next = temp;
}
// Removes an element with value "val" from List
// Returns 0 if succeeds or -1 if it fails
int 
List:: remove( int val ){
    if(_head == 0){
        printf("List is already empty.n");
        return -1;
    }
    ListNode *current = new ListNode();
    ListNode* prev = new ListNode();
    current = _head;
    while(current != 0){
        if(current->_value == val){
            if(current == _head){
                _head = _head->_next;
                delete(current);
                return 0;
            }else{
                prev->_next = current->_next;
                delete(current);
                return 0;
            }
        }else{
            prev = current;
            current = current->_next;
        }
    }
    return -1;
}
// Prints The elements in the list. 
void
List::print(){
    ListNode* current = new ListNode();
    while(current != 0){
        printf("%dn", current->_value);
        current = current->_next;
    }
}
//
// Returns 0 if "value" is in the list or -1 otherwise.
//
int
List::lookup(int val){
    ListNode * current = new ListNode();
    current = _head;
    while(current != NULL){
        if(current->_value == val){
            return 0;
        }
        else{
            current = current->_next;
        }
    }
    return -1;
}
//
// List constructor
//
List::List(){
}
//
// List destructor: delete all list elements, if any.
//
List::~List(){
    ListNode* current = _head;
    while(current != NULL){
        ListNode* next = current->_next;
        delete current;
        current = next;
    } 
}

queue.h

#ifndef LIST_H
#define LIST_H
class Queue {
public:
    List* queue_list;
    void enqueue(int val);
    int dequeue();
    Queue();
    ~Queue();
};
#endif

queue.cc

#include <stdio.h>
#include "List.h"
#include "Queue.h"
List *queue_list = new List();
void
Queue::enqueue(int val){
    this->queue_list->prepend(val);
}
int
Queue::dequeue(){
    int value = this->queue_list->remove_front();
    return value;
}
Queue::Queue(){
    //do nothing
}
Queue::~Queue(){
}

queue_main.cc

#include <stdio.h>
#include "Queue.h"
#include "List.h"
Queue *queue;
int main(){
}

感谢您的帮助！