Error in storing outer class object in inner class C++ - c++11

Question

I was implementing the ring buffer and have encountered an error. What does it mean to store a reference of outer class(class ring) object(m_ring) in inner class(class iterator) and when I remove the reference(&) the program compiles correctly but crashes. Please explain what is happening.(See the comment in Ring.h) Sorry for bad English.
// Ring.h
#ifndef RING.H
#define RING.H
#include <iostream>
using namespace std;
template<class T>
class ring {
unsigned int m_size;
int m_pos;
T *m_values;
public:
class iterator;
public:
ring(unsigned int size) : m_size(size), m_pos(0)
{
m_values = new T[m_size];
}
~ring()
{
delete[] m_values;
}
void add(const T &val)
{
m_values[m_pos] = val;
m_pos++;
m_pos %= m_size;
}
T& get(int pos)
{
return m_values[pos];
}
iterator begin()
{
return iterator(0, *this);
}
iterator end()
{
return iterator(m_size, *this);
}
};
template<class T>
class ring<T>::iterator {
int m_pos;
ring &m_ring; // Removing & gives garbage output.
public:
iterator(int pos, ring& aRing) : m_pos(pos), m_ring(aRing){}
bool operator!=(const iterator &other) const
{
return other.m_pos != m_pos;
}
iterator &operator++(int)
{
m_pos++;
return *this;
}
iterator &operator++()
{
m_pos++;
return *this;
}
T &operator*()
{
// return m_ring.m_values[m_pos];
return m_ring.get(m_pos);
}
};
#endif // RING
Driver program :
// Ring_Buffer_Class.cpp
#include <iostream>
#include "ring.h"
using namespace std;
int main()
{
ring<string> textring(3);
textring.add("one");
textring.add("two");
textring.add("three");
textring.add("four");
// C++ 98
for(ring<string>::iterator it = textring.begin(); it != textring.end(); it++)
{
cout << *it << endl;
}
cout << endl;
// C++11
for(string value : textring)
{
cout << value << endl;
}
return 0;
}
I also observed that removing ~ring() (Destructor) results into correct output.
Expected output :
four
two
three
four
two
three