Bad Vector Code

 void  duplicate(vector<int>  &v)  {
         vector<int>::const_iterator
                                            b  =  v.begin(),
                                            e  =  v.end();
         for(;  b  !=  e;  ++b)
                 v.push_back(*b);
 }
Not safe
Reallocation may invalidate  b  and  e
Easily fixed in this case