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