Question

I have a dummy question. I always read that C++ std::list container has constant time for inserting elements at the beginning, at the end and in the middle:
Which is the correct way to insert an element directly in the middle of a std::list?
Is maybe this one?

  std::list<int> l;
  l.push_back(10);
  l.push_back(20);
  l.push_back(30);
  l.push_back(40);
  l.push_back(50);
  l.push_back(60);
  l.insert( l.end()- l.begin() /2 ); //? is this 
  // inserting directly in the middle?

When we say ‘inserting in the middle’ do we really mean that we save linear time to go from the beginning of the list to the desired point ( traversing one by one through all linked elements in between)?

Answer 1

Here, “the middle” means an arbitrary point in the list, as long as you already have an iterator referring to that point. Given that, insertion is just a matter of modifying a few pointers around the insertion point.

If you don’t already have an iterator for the insertion point, and it isn’t somewhere simple like the beginning or the end, then it will take linear time to walk through the list to find it.