19   List() : front_(nullptr), back_(nullptr) {}  in List()
21                                    back_(std::exchange(other.back_, nullptr)) {}  in List()
24     back_ = std::exchange(other.back_, nullptr);
30     other->back_ = back_;  in ShallowCopyTo()
36     if (back_) {  in PushBack()
38       InsertAfter(element, back_);  in PushBack()
48       DCHECK(back_);  in PushFront()
57     if (back_ == element) {  in Remove()
58       back_ = element->list_node().prev();  in Remove()
80   bool Empty() const { return !front_ && !back_; }  in Empty()
83   T* back() { return back_; }  in back()
86   const T* back() const { return back_; }  in back()
90     DCHECK(!back_);  in AddFirstElement()
97     back_ = element;  in AddFirstElement()
108       back_ = element;  in InsertAfter()
124   T* back_;  variable