C++ - symbol(s) not found for architecture x86_64

I have researched this answer on other StackOverflow threads, and I have implemented the advice given in those responses - despite this, the code I have written is still giving me the error of "Symbol(s) not found for architecture x86_64". Any help would be wonderful.

File: DoubleEndedQueue.h

#include <iostream>
#include <cmath>
#include <vector>
using namespace std;

class DoubleEndedQueue
{
public:

    DoubleEndedQueue();
    ~DoubleEndedQueue();

    void push_back(int element);
    int pop_back();
    int back();
    void push_front(int element);
    int pop_front();
    int front();

private:
    std::vector<int> queue;

};

File: DoubleEndedQueue.cpp

#include <iostream>
#include "DoubleEndedQueue.h"
using namespace std;

DoubleEndedQueue::DoubleEndedQueue() {
    //Constructor
    vector<int> queue;
}

DoubleEndedQueue::~DoubleEndedQueue() {
    //Destructor
}

void DoubleEndedQueue::push_back(int element) {
    //Insert element at the back
    queue.push_back(element);
}

int DoubleEndedQueue::pop_back() {
    //Return and remove element at the back
    int elem = queue[queue.size()-1];
    queue.erase(queue.begin() + queue.size()-1);
    return elem;
}

int DoubleEndedQueue::back() {
    //Peek at the element at the back of the queue
    return queue[queue.size()-1];
}

void DoubleEndedQueue::push_front(int element) {
    //Insert element at the front
    vector<int>::iterator it = queue.begin();
    queue.insert(it, element);
}

int DoubleEndedQueue::pop_front() {
    //Return and remove element at the front
    int elem = queue[0];
    queue.erase(queue.begin());
    return elem;
}

int DoubleEndedQueue::front() {
    //Peek at the element at the front of the queue
    return queue[0];
}

Thanks so much!