Basics
好了，这就是本书最烂的部分，也是我花了 7 年时间才写完这一章的原因！是时候把我们已经做过 5 次的枯燥乏味的东西再写一遍了，但因为我们必须使用 Option<NonNull<Node
#![allow(unused)] fn main() { impl<T> LinkedList<T> { pub fn new() -> Self { Self { front: None, back: None, len: 0, _boo: PhantomData, } } } }
PhantomData 是一种奇怪的类型，没有字段，所以你只需说出它的类型名称就能创建一个。
#![allow(unused)] fn main() { pub fn push_front(&mut self, elem: T) { // SAFETY: it's a linked-list, what do you want? unsafe { let new = NonNull::new_unchecked(Box::into_raw(Box::new(Node { front: None, back: None, elem, }))); if let Some(old) = self.front { // Put the new front before the old one (*old).front = Some(new); (*new).back = Some(old); } else { // If there's no front, then we're the empty list and need // to set the back too. Also here's some integrity checks // for testing, in case we mess up. debug_assert!(self.back.is_none()); debug_assert!(self.front.is_none()); debug_assert!(self.len == 0); self.back = Some(new); } self.front = Some(new); self.len += 1; } } error[E0614]: type `NonNull<Node<T>>` cannot be dereferenced --> src\lib.rs:39:17 | 39 | (*old).front = Some(new); | ^^^^^^ }
是的，我真恨 NonNull<Node<T>>。我们需要明确地使用 as_ptr 从 NonNull 中获取原始指针，因为 DerefMut 是以 &mut 定义的，我们不想在不安全代码中随意引入安全引用！
#![allow(unused)] fn main() { (*old.as_ptr()).front = Some(new); (*new.as_ptr()).back = Some(old); Compiling linked-list v0.0.3 warning: field is never read: `elem` --> src\lib.rs:16:5 | 16 | elem: T, | ^^^^^^^ | = note: `#[warn(dead_code)]` on by default warning: `linked-list` (lib) generated 1 warning (1 duplicate) warning: `linked-list` (lib test) generated 1 warning Finished test [unoptimized + debuginfo] target(s) in 0.33s }
很好，接下来是 pop 和 len：
#![allow(unused)] fn main() { pub fn pop_front(&mut self) -> Option<T> { unsafe { // Only have to do stuff if there is a front node to pop. // Note that we don't need to mess around with `take` anymore // because everything is Copy and there are no dtors that will // run if we mess up... right? :) Riiiight? :))) self.front.map(|node| { // Bring the Box back to life so we can move out its value and // Drop it (Box continues to magically understand this for us). let boxed_node = Box::from_raw(node.as_ptr()); let result = boxed_node.elem; // Make the next node into the new front. self.front = boxed_node.back; if let Some(new) = self.front { // Cleanup its reference to the removed node (*new.as_ptr()).front = None; } else { // If the front is now null, then this list is now empty! debug_assert!(self.len == 1); self.back = None; } self.len -= 1; result // Box gets implicitly freed here, knows there is no T. }) } } pub fn len(&self) -> usize { self.len } Compiling linked-list v0.0.3 Finished dev [unoptimized + debuginfo] target(s) in 0.37s }
在我看来是合法的，是时候写一个测试了！
#![allow(unused)] fn main() { #[cfg(test)] mod test { use super::LinkedList; #[test] fn test_basic_front() { let mut list = LinkedList::new(); // Try to break an empty list assert_eq!(list.len(), 0); assert_eq!(list.pop_front(), None); assert_eq!(list.len(), 0); // Try to break a one item list list.push_front(10); assert_eq!(list.len(), 1); assert_eq!(list.pop_front(), Some(10)); assert_eq!(list.len(), 0); assert_eq!(list.pop_front(), None); assert_eq!(list.len(), 0); // Mess around list.push_front(10); assert_eq!(list.len(), 1); list.push_front(20); assert_eq!(list.len(), 2); list.push_front(30); assert_eq!(list.len(), 3); assert_eq!(list.pop_front(), Some(30)); assert_eq!(list.len(), 2); list.push_front(40); assert_eq!(list.len(), 3); assert_eq!(list.pop_front(), Some(40)); assert_eq!(list.len(), 2); assert_eq!(list.pop_front(), Some(20)); assert_eq!(list.len(), 1); assert_eq!(list.pop_front(), Some(10)); assert_eq!(list.len(), 0); assert_eq!(list.pop_front(), None); assert_eq!(list.len(), 0); assert_eq!(list.pop_front(), None); assert_eq!(list.len(), 0); } } Compiling linked-list v0.0.3 Finished test [unoptimized + debuginfo] target(s) in 0.40s Running unittests src\lib.rs running 1 test test test::test_basic_front ... ok test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s }
万幸，我们是完美的！是吗？