1.2.2.1 Insert iterator

• Three common iterator generator: inserter(), back_inserter(), front_inserter(); They will produces three insert_iterator: insert_iterator, back_insert_iterator and front_insert_iterator.
• When you use insert_iterator in an assignment, insert_iterator will call insert() function. back_insert_iterator will call push_back() and front_insert_iterator will call push_front(). insert_iterator++ has no any operator inside.
• Why do we need it. in some algorithm, such as copy and generator, if you read sth from a container, you can use regular iterator, but when you want to write into a container. You must keep regular iterator is valid. so a method is to use reverse before you write to a container.
  list<State> res2( src.size() ); 
  copy( src.begin(), src.end(), 
      res2.begin() ); // often works...
• Another way is to use back_insert_iterator.
  std::vector<int> foo; 
  for (int i=1; i<=5; i++) 
    { foo.push_back(i); bar.push_back(i*10); } 
    std::copy (bar.begin(),bar.end(),back_inserter(foo)); 
    //foo support push_back 
   
    std::deque<int> foo; 
    // deque or list 
    for (int i=1; i<=5; i++) 
    { foo.push_back(i); bar.push_back(i*10); } 
    std::copy (bar.begin(),bar.end(),front_inserter(foo)); 
    //foo support push_front