std::forward_list::before_begin, cbefore_begin

From cppreference.com
iterator before_begin();
(since C++11)
const_iterator before_begin() const;
(since C++11)
const_iterator cbefore_begin() const;
(since C++11)

Returns an iterator to the element before the first element of the container. This element acts as a placeholder, attempting to access it results in undefined behavior. The only usage cases are in functions insert_after(), emplace_after(), erase_after(), splice_after() and the increment operator: incrementing the before-begin iterator gives exactly the same iterator as obtained from begin()/cbegin().

Parameters

(none)

Return value

Iterator to the element before the first element.

Exceptions

noexcept specification:  
noexcept
  

Complexity

Constant.

See also

returns an iterator to the beginning
(public member function)
returns an iterator to the end
(public member function)