Custom deque(double ended queue) class.

Question

This is my first time in building a custom deque(Double Ended Queue) class in c++ .This code is giving me Multiple definition error for head and tail.What should i do? what changes should be made?

// this is the header file(dequeue.h)

        #ifndef DEQUEUE_H
#define DEQUEUE_H
#include <iostream>
#include <cstdlib>

using namespace std;
/*
 * Node Declaration
 */
struct node
{
    int info;
    node *next;
    node *prev;

}*head, *tail;

class dequeue
{
    public:
        int top1, top2;
        void insert();
        void del();
        void display();
        dequeue()
        {
            top1 = 0;
            top2 = 0;
            head = NULL;
            tail = NULL;
        }
};

#endif // DEQUEUE_H

//This is the dequeue.cpp file

#include "dequeue.h"
#include <iostream>

using namespace std;


/*
 * Insert Element in Doubly Ended Queue
 */
void dequeue::insert()
{
    struct node *temp;
    int ch, value;
    if (top1 + top2 >= 50)
    {
        cout<<"Dequeue Overflow"<<endl;
        return;
    }
    if (top1 + top2 == 0)
    {
        cout<<"Enter the value to be inserted: ";
        cin>>value;
        head = new (struct node);
        head->info = value;
        head->next = NULL;
        head->prev = NULL;
        tail = head;
        top1++;
        cout<<"Element Inserted into empty deque"<<endl;
    }
    else
    {
        while (1)
        {
            cout<<endl;
            cout<<"1.Insert Element at first"<<endl;
            cout<<"2.Insert Element at last"<<endl;
            cout<<"3.Exit"<<endl;
            cout<<endl;
            cout<<"Enter Your Choice: ";
            cin>>ch;
            cout<<endl;
            switch(ch)
            {
            case 1:
                cout<<"Enter the value to be inserted: ";
                cin>>value;
                temp = new (struct node);
                temp->info = value;
                temp->next = head;
                temp->prev = NULL;
                head->prev = temp;
                head = temp;
                top1++;
                break;
            case 2:
                cout<<"Enter the value to be inserted: ";
                cin>>value;
                temp = new (struct node);
                temp->info = value;
                temp->next = NULL;
                temp->prev = tail;
                tail->next = temp;
                tail = temp;
                top2++;
                break;
            case 3:
                return;
                break;
            default:
                cout<<"Wrong Choice"<<endl;
            }
        }
    }
}

/*
 * Delete Element in Doubly Ended Queue
 */
void dequeue::del()
{
    if (top1 + top2 <= 0)
    {
        cout<<"Deque Underflow"<<endl;
        return;
    }
    int ch;
    while (1)
    {
        cout<<endl;
        cout<<"1.Delete Element at first"<<endl;
        cout<<"2.Delete Element at last"<<endl;
        cout<<"3.Exit"<<endl;
        cout<<endl;
        cout<<"Enter Your Choice: ";
        cin>>ch;
        cout<<endl;
        switch(ch)
        {
        case 1:
            head = head->next;
            head->prev = NULL;
            top1--;
            break;
        case 2:
            tail = tail->prev;
            tail->next = NULL;
            top2--;
            break;
        case 3:
            return;
            break;
        default:
            cout<<"Wrong Choice"<<endl;
        }
    }
}

/*
 * Display Doubly Ended Queue
 */
void dequeue::display()
{
    struct node *temp;
    int ch;
    if (top1 + top2 <= 0)
    {
        cout<<"Deque Underflow"<<endl;
        return;
    }
    while (1)
    {
        cout<<endl;
        cout<<"1.Display Deque from Beginning"<<endl;
        cout<<"2.Display Deque from End"<<endl;
        cout<<"3.Exit"<<endl;
        cout<<endl;
        cout<<"Enter Your Choice: ";
        cin>>ch;
        cout<<endl;
        switch (ch)
        {
        case 1:
            temp = head;
            cout<<"Deque from Beginning:"<<endl;
            while (temp != NULL)
            {
                cout<<temp->info<<" ";
                temp = temp->next;
            }
            cout<<endl;
            break;
        case 2:
            cout<<"Deque from End:"<<endl;
            temp = tail;
            while (temp != NULL)
            {
                cout<<temp->info<<" ";
                temp = temp->prev;
            }
            temp = tail;
            cout<<endl;
            break;
        case 3:
            return;
            break;
        default:
            cout<<"Wrong Choice"<<endl;
        }
    }
}

//This the main.cpp`#include

#include "dequeue.h"

using namespace std;

int main()
{
    int choice;
    dequeue dl;
    while (1)
    {
        cout<<"\n-------------"<<endl;
        cout<<"Operations on Deque"<<endl;
        cout<<"\n-------------"<<endl;
        cout<<"1.Insert Element into the Deque"<<endl;
        cout<<"2.Delete Element from the Deque"<<endl;
        cout<<"3.Traverse the Deque"<<endl;
        cout<<"4.Quit"<<endl;
        cout<<"Enter your Choice: ";
        cin>>choice;
        cout<<endl;
        switch(choice)
        {
        case 1:
            dl.insert();
            break;
        case 2:
            dl.del();
            break;
        case 3:
            dl.display();
            break;
        case 4:
            exit(1);
            break;
        default:
            cout<<"Wrong Choice"<<endl;
        }
    }
    return 0;
}

`

Answer 1

The head and tail variables will be defined everywhere you include the dequeue.h file.

Did you mean for them to be class members of dequeue?

If you really to share them, move them to the cpp file and declare them extern in the h file.