std::next with n > std::distance(it, c.end())

Question

I do not want to use std::distance because it will calculate whole distance from my iterator to the end. But I need to be sure that I have N or more elements fr

Answer 1

The implementation of RB_Tree::iterator increment operator in libstd++ makes sure it would not return an iterator to an undefined memory location:

static _Rb_tree_node_base*
  local_Rb_tree_increment(_Rb_tree_node_base* __x) throw ()
  {
    if (__x->_M_right != 0)
      {
        __x = __x->_M_right;
        while (__x->_M_left != 0)
          __x = __x->_M_left;
      }
    else
      {
        _Rb_tree_node_base* __y = __x->_M_parent;
        while (__x == __y->_M_right)
          {
            __x = __y;
            __y = __y->_M_parent;
          }
        if (__x->_M_right != __y)
          __x = __y;
      }
    return __x;
  }

But the same is not true for std::vector:

#include 
#include 
#include 

int main() {
  std::vector vec = {1,2};
  auto it = vec.begin();
  it = std::next(it, 5);
  if (it != vec.end()) {
    std::cout << "Not end..go on" << std::endl;
  }
  return 0;
}

This will go on printing the message..

So, since the behaviour is not same across containers, you should not depend on std::next for its current correct behaviour for map based containers.