Challenge accepted :) 
             I've looked at what Amir and Saeid did, nice job, but I was wondering how you guys checked it if your code not even complied without doing this function modifiable with & sign.
             I decided to go a bit further and I did my operator[] method able to add new Nodes to the end of the Queue if we have index > size.
 Here is the code
int& Queue::operator[](unsigned int index){
  Node* temp = _head;
  int i;
  if (index >= _size) {
     while(index>=_size){
         int data = (_tail->_data/10)+(_head->_data/10);
         Node* newnode = new Node(data);
         _tail->_next = newnode;
         _tail = newnode;
         _size++;
     }
  }
  for (i = 0; i < index % _size; i++){
    temp = temp-> _next;
  }
  return temp-> _data;
}
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So then in the tester instead of:
for(i=0;Q.size();i++){
   cout<<Q[i]<<endl;
   Q[i] = Q[i] * 10;
  }

we have:
for(i=0;i<10;i++){
   cout<<Q[i]<<endl;
   Q[i] = Q[i] * 10;
  }
And it will continue add Nodes with valid data.
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Comments are welcome...