Trouble Connecting Two C++ Files (List.cc and Queue.cc) - c++

Question

I have to write a Queue in C++ using a List file that I created earlier and I'm having a rough time getting everything to compile.
The issue I am currently having is that when I compile I get the error:
Queue.h:7:2: error: 'List' does not name a type
How do I go about properly connecting my Queue file and my List file?
Here are the files I am using:
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("%d\n", 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(){
}
Thanks for the help!

Answer 1

The compiler tells you what's wrong:
Queue.h:7:2: error: 'List' does not name a type
While reading Queue.h, the compiler cannot possibly know what List is, as there is nothing in this file that defines it.
You simply have to add a forward declaration:
#ifndef LIST_H
#define LIST_H
class List; // this is a forward declaration.
class Queue {
public:
List* queue_list;
void enqueue(int val);
int dequeue();
Queue();
~Queue();
};
#endif
Alternatively (but not necessary here), you could simply #include List.h. The rule of thumb is: Use forward declaration if possible. If the compiler complains about it, replace it by the corresponding include. The include is only necessary if the compiler must know the size of the class / struct.