c++ forward_list 的插入

// forward_list::before_begin
#include <iostream>
#include <forward_list>

int main ()
{
  std::forward_list<int> mylist = {20, 30, 40, 50};
  mylist.insert_after ( mylist.begin(), 11 ); //mylist contains: 20 11 30 40 50
  mylist.insert_after ( mylist.before_begin(), 11 ); //mylist contains: 11 20 30 40 50
  std::cout << "mylist contains:";
  for ( int& x: mylist ) std::cout << ' ' << x;
  std::cout << '\n';
  return 0;
}

mylist.begin()
mylist.before_begin()

指向位置不一樣before_begin()，在begin() 以前