Question

请注意，此问题是指Rust 1.0之前的Rust版本。虽然语法已经改变，但概念仍然有效。

您可以使用拥有的指针轻松实现仅转发链接列表，例如：

struct Node<T> {
  next: Option<~Node<T>>,
  data: T
}

但是，想象一下，如果您想要有效地实现支持四种基本操作的队列：

push：添加到列表末尾
pop：从列表末尾删除并返回
unshift：添加到列表的前面
shift：从列表末尾删除并返回

在具有普通指针的语言中，您可以使用双向链表实现此目的，以及存储first和last指向列表中第一个和最后一个元素的指针的根对象。 / p>

我无法看到你如何在Rust中实现它。

我可以模糊地猜测你会使用一堆引用，或许类似于：

struct Node<T> {
  next: Option<&Node<T>>,
  prev: Option<&Node<T>>,
  data: T
}

...但我看不出你如何管理这些变量的生命周期范围。

有人能指出我的方向，或类似的例子涉及对象之间的引用复杂的生命周期吗？

（这种代码风格的另一个典型示例是观察者模式，其中许多对象必须将事件更新发布到单个位置，例如UINode＆lt;＆gt; ---- EventObserver＆lt ;＆gt; ---- EventCore＆lt;＆gt; ---- UINodes;复杂层次结构中的多个对象共享指针，其中事件从叶节点传播到某个核心，然后被推出到不同的叶节点）

Answer 1

我建议您查看由Rust patterns撰写的Lars Bergstrom。

这是实现双链表的代码，从@Yurume更新为Rust 1.12，（未经过全面测试）

use std::mem;
use std::ptr;

pub struct List<T> {
    list_head: Option<Box<Node<T>>>,
    list_tail: Rawlink<Node<T>>,
}

struct Rawlink<T> { p: *mut T }

impl<T> Copy for Rawlink<T> {}

impl<T> Clone for Rawlink<T> {
    fn clone(&self) -> Self { Rawlink { p: self.p } }
}

pub struct Node<T> {
    next: Option<Box<Node<T>>>,
    prev: Rawlink<Node<T>>,
    value: T,
}

impl<T> List<T> {
    pub fn is_empty(&self) -> bool {
        self.list_head.is_none()
    }

    pub fn len(&self) -> usize {
        let mut node = &self.list_head;
        let mut i = 0;
        loop {
            match *node {
                Some(ref n) => {
                    i+=1;
                    node=&n.next;
                }
                None => {
                    return i;
                }
            }
        }
    }

    /// Create an empty DList
    pub fn new() -> List<T> {
        List{list_head: None, list_tail: Rawlink::none()}
    }

    pub fn push_front(&mut self, elt: T) {
        self.push_front_node(Box::new(Node::new(elt)))
    }

    pub fn push_front_node(&mut self, mut new_head: Box<Node<T>>) {
        match self.list_head {
            None => {
                self.list_tail = Rawlink::some(&mut new_head);
                new_head.prev = Rawlink::none();
                self.list_head = Some(new_head);
            }
            Some(ref mut head) => {
                new_head.prev = Rawlink::none();
                head.prev = Rawlink::some(&mut new_head);
                mem::swap(head, &mut new_head);
                head.next = Some(new_head);
            }
        }
    }

    /// Provide a forward iterator
    #[inline]
    pub fn iter<'a>(&'a self) -> ListIterator<'a, T> {
        ListIterator{nelem: self.len(), head: &self.list_head, tail: self.list_tail}
    }
}

impl<T> Node<T> {
    fn new(v: T) -> Node<T> {
        Node{value: v, next: None, prev: Rawlink::none()}
    }
}

/// Rawlink is a type like Option<T> but for holding a raw pointer
impl<T> Rawlink<T> {
    /// Like Option::None for Rawlink
    fn none() -> Rawlink<T> {
        Rawlink{p: ptr::null_mut()}
    }

    /// Like Option::Some for Rawlink
    fn some(n: &mut T) -> Rawlink<T> {
        Rawlink{p: n as *mut T}
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve_immut<'a>(&self) -> Option<&'a T> {
        unsafe { self.p.as_ref() }
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve<'a>(&mut self) -> Option<&'a mut T> {
        unsafe { self.p.as_mut() }
    }

    /// Return the `Rawlink` and replace with `Rawlink::none()`
    fn take(&mut self) -> Rawlink<T> {
        mem::replace(self, Rawlink::none())
    }
}

pub struct ListIterator<'a, T: 'a> {
    head: &'a Option<Box<Node<T>>>,
    tail: Rawlink<Node<T>>,
    nelem: usize,
}

impl<'a, A> Iterator for ListIterator<'a, A> {
    type Item = &'a A;

    #[inline]
    fn next(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.as_ref().map(|head| {
            self.nelem -= 1;
            self.head = &head.next;
            &head.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.nelem, Some(self.nelem))
    }
}

impl<'a, A> DoubleEndedIterator for ListIterator<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        let tmp = self.tail.resolve_immut();
        tmp.as_ref().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &prev.value
        })
    }
}

fn main() {
}

Playground

Answer 2

在Rust标准库collections :: dlist中现在有一个双向链表实现。它配有一个完整的测试套件，是一个愉快的阅读和可用的工具。

Here is a link to the source.

正如评论中所建议的，这里是源本身（为简洁起见省略了测试，所以如果可以的话请阅读链接）：

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! A doubly-linked list with owned nodes.
//!
//! The `DList` allows pushing and popping elements at either end and is thus
//! efficiently usable as a double-ended queue.

// DList is constructed like a singly-linked list over the field `next`.
// including the last link being None; each Node owns its `next` field.
//
// Backlinks over DList::prev are raw pointers that form a full chain in
// the reverse direction.

#![stable(feature = "rust1", since = "1.0.0")]

use core::prelude::*;

use alloc::boxed::Box;
use core::cmp::Ordering;
use core::default::Default;
use core::fmt;
use core::hash::{Writer, Hasher, Hash};
use core::iter::{self, FromIterator};
use core::mem;
use core::ptr;

/// A doubly-linked list.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct DList<T> {
    length: uint,
    list_head: Link<T>,
    list_tail: Rawlink<Node<T>>,
}

type Link<T> = Option<Box<Node<T>>>;

struct Rawlink<T> {
    p: *mut T,
}

impl<T> Copy for Rawlink<T> {}
unsafe impl<T:'static+Send> Send for Rawlink<T> {}
unsafe impl<T:Send+Sync> Sync for Rawlink<T> {}

struct Node<T> {
    next: Link<T>,
    prev: Rawlink<Node<T>>,
    value: T,
}

/// An iterator over references to the items of a `DList`.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, T:'a> {
    head: &'a Link<T>,
    tail: Rawlink<Node<T>>,
    nelem: uint,
}

// FIXME #19839: deriving is too aggressive on the bounds (T doesn't need to be Clone).
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Clone for Iter<'a, T> {
    fn clone(&self) -> Iter<'a, T> {
        Iter {
            head: self.head.clone(),
            tail: self.tail,
            nelem: self.nelem,
        }
    }
}

/// An iterator over mutable references to the items of a `DList`.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IterMut<'a, T:'a> {
    list: &'a mut DList<T>,
    head: Rawlink<Node<T>>,
    tail: Rawlink<Node<T>>,
    nelem: uint,
}

/// An iterator over mutable references to the items of a `DList`.
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<T> {
    list: DList<T>
}

/// Rawlink is a type like Option<T> but for holding a raw pointer
impl<T> Rawlink<T> {
    /// Like Option::None for Rawlink
    fn none() -> Rawlink<T> {
        Rawlink{p: ptr::null_mut()}
    }

    /// Like Option::Some for Rawlink
    fn some(n: &mut T) -> Rawlink<T> {
        Rawlink{p: n}
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve_immut<'a>(&self) -> Option<&'a T> {
        unsafe {
            mem::transmute(self.p.as_ref())
        }
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve<'a>(&mut self) -> Option<&'a mut T> {
        if self.p.is_null() {
            None
        } else {
            Some(unsafe { mem::transmute(self.p) })
        }
    }

    /// Return the `Rawlink` and replace with `Rawlink::none()`
    fn take(&mut self) -> Rawlink<T> {
        mem::replace(self, Rawlink::none())
    }
}

impl<T> Clone for Rawlink<T> {
    #[inline]
    fn clone(&self) -> Rawlink<T> {
        Rawlink{p: self.p}
    }
}

impl<T> Node<T> {
    fn new(v: T) -> Node<T> {
        Node{value: v, next: None, prev: Rawlink::none()}
    }
}

/// Set the .prev field on `next`, then return `Some(next)`
fn link_with_prev<T>(mut next: Box<Node<T>>, prev: Rawlink<Node<T>>)
                  -> Link<T> {
    next.prev = prev;
    Some(next)
}

// private methods
impl<T> DList<T> {
    /// Add a Node first in the list
    #[inline]
    fn push_front_node(&mut self, mut new_head: Box<Node<T>>) {
        match self.list_head {
            None => {
                self.list_tail = Rawlink::some(&mut *new_head);
                self.list_head = link_with_prev(new_head, Rawlink::none());
            }
            Some(ref mut head) => {
                new_head.prev = Rawlink::none();
                head.prev = Rawlink::some(&mut *new_head);
                mem::swap(head, &mut new_head);
                head.next = Some(new_head);
            }
        }
        self.length += 1;
    }

    /// Remove the first Node and return it, or None if the list is empty
    #[inline]
    fn pop_front_node(&mut self) -> Option<Box<Node<T>>> {
        self.list_head.take().map(|mut front_node| {
            self.length -= 1;
            match front_node.next.take() {
                Some(node) => self.list_head = link_with_prev(node, Rawlink::none()),
                None => self.list_tail = Rawlink::none()
            }
            front_node
        })
    }

    /// Add a Node last in the list
    #[inline]
    fn push_back_node(&mut self, mut new_tail: Box<Node<T>>) {
        match self.list_tail.resolve() {
            None => return self.push_front_node(new_tail),
            Some(tail) => {
                self.list_tail = Rawlink::some(&mut *new_tail);
                tail.next = link_with_prev(new_tail, Rawlink::some(tail));
            }
        }
        self.length += 1;
    }

    /// Remove the last Node and return it, or None if the list is empty
    #[inline]
    fn pop_back_node(&mut self) -> Option<Box<Node<T>>> {
        self.list_tail.resolve().map_or(None, |tail| {
            self.length -= 1;
            self.list_tail = tail.prev;
            match tail.prev.resolve() {
                None => self.list_head.take(),
                Some(tail_prev) => tail_prev.next.take()
            }
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for DList<T> {
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn default() -> DList<T> { DList::new() }
}

impl<T> DList<T> {
    /// Creates an empty `DList`.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> DList<T> {
        DList{list_head: None, list_tail: Rawlink::none(), length: 0}
    }

    /// Moves all elements from `other` to the end of the list.
    ///
    /// This reuses all the nodes from `other` and moves them into `self`. After
    /// this operation, `other` becomes empty.
    ///
    /// This operation should compute in O(1) time and O(1) memory.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut a = DList::new();
    /// let mut b = DList::new();
    /// a.push_back(1i);
    /// a.push_back(2);
    /// b.push_back(3i);
    /// b.push_back(4);
    ///
    /// a.append(&mut b);
    ///
    /// for e in a.iter() {
    ///     println!("{}", e); // prints 1, then 2, then 3, then 4
    /// }
    /// println!("{}", b.len()); // prints 0
    /// ```
    pub fn append(&mut self, other: &mut DList<T>) {
        match self.list_tail.resolve() {
            None => {
                self.length = other.length;
                self.list_head = other.list_head.take();
                self.list_tail = other.list_tail.take();
            },
            Some(tail) => {
                // Carefully empty `other`.
                let o_tail = other.list_tail.take();
                let o_length = other.length;
                match other.list_head.take() {
                    None => return,
                    Some(node) => {
                        tail.next = link_with_prev(node, self.list_tail);
                        self.list_tail = o_tail;
                        self.length += o_length;
                    }
                }
            }
        }
        other.length = 0;
    }

    /// Provides a forward iterator.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<T> {
        Iter{nelem: self.len(), head: &self.list_head, tail: self.list_tail}
    }

    /// Provides a forward iterator with mutable references.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter_mut(&mut self) -> IterMut<T> {
        let head_raw = match self.list_head {
            Some(ref mut h) => Rawlink::some(&mut **h),
            None => Rawlink::none(),
        };
        IterMut{
            nelem: self.len(),
            head: head_raw,
            tail: self.list_tail,
            list: self
        }
    }

    /// Consumes the list into an iterator yielding elements by value.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter{list: self}
    }

    /// Returns `true` if the `DList` is empty.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert!(dl.is_empty());
    ///
    /// dl.push_front("foo");
    /// assert!(!dl.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.list_head.is_none()
    }

    /// Returns the length of the `DList`.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.push_front(2is);
    /// assert_eq!(dl.len(), 1);
    ///
    /// dl.push_front(1);
    /// assert_eq!(dl.len(), 2);
    ///
    /// dl.push_back(3);
    /// assert_eq!(dl.len(), 3);
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> uint {
        self.length
    }

    /// Removes all elements from the `DList`.
    ///
    /// This operation should compute in O(n) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.push_front(2is);
    /// dl.push_front(1);
    /// assert_eq!(dl.len(), 2);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// dl.clear();
    /// assert_eq!(dl.len(), 0);
    /// assert_eq!(dl.front(), None);
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        *self = DList::new()
    }

    /// Provides a reference to the front element, or `None` if the list is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.front(), None);
    ///
    /// dl.push_front(1);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front(&self) -> Option<&T> {
        self.list_head.as_ref().map(|head| &head.value)
    }

    /// Provides a mutable reference to the front element, or `None` if the list
    /// is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.front(), None);
    ///
    /// dl.push_front(1);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// match dl.front_mut() {
    ///     None => {},
    ///     Some(x) => *x = 5is,
    /// }
    /// assert_eq!(dl.front(), Some(&5is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front_mut(&mut self) -> Option<&mut T> {
        self.list_head.as_mut().map(|head| &mut head.value)
    }

    /// Provides a reference to the back element, or `None` if the list is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.back(), None);
    ///
    /// dl.push_back(1);
    /// assert_eq!(dl.back(), Some(&1is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back(&self) -> Option<&T> {
        self.list_tail.resolve_immut().as_ref().map(|tail| &tail.value)
    }

    /// Provides a mutable reference to the back element, or `None` if the list
    /// is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.back(), None);
    ///
    /// dl.push_back(1);
    /// assert_eq!(dl.back(), Some(&1is));
    ///
    /// match dl.back_mut() {
    ///     None => {},
    ///     Some(x) => *x = 5is,
    /// }
    /// assert_eq!(dl.back(), Some(&5is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back_mut(&mut self) -> Option<&mut T> {
        self.list_tail.resolve().map(|tail| &mut tail.value)
    }

    /// Adds an element first in the list.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.push_front(2is);
    /// assert_eq!(dl.front().unwrap(), &2is);
    ///
    /// dl.push_front(1);
    /// assert_eq!(dl.front().unwrap(), &1);
    ///
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_front(&mut self, elt: T) {
        self.push_front_node(box Node::new(elt))
    }

    /// Removes the first element and returns it, or `None` if the list is
    /// empty.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// assert_eq!(d.pop_front(), None);
    ///
    /// d.push_front(1is);
    /// d.push_front(3);
    /// assert_eq!(d.pop_front(), Some(3));
    /// assert_eq!(d.pop_front(), Some(1));
    /// assert_eq!(d.pop_front(), None);
    ///
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_front(&mut self) -> Option<T> {
        self.pop_front_node().map(|box Node{value, ..}| value)
    }

    /// Appends an element to the back of a list
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// d.push_back(1i);
    /// d.push_back(3);
    /// assert_eq!(3, *d.back().unwrap());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_back(&mut self, elt: T) {
        self.push_back_node(box Node::new(elt))
    }

    /// Removes the last element from a list and returns it, or `None` if
    /// it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// assert_eq!(d.pop_back(), None);
    /// d.push_back(1i);
    /// d.push_back(3);
    /// assert_eq!(d.pop_back(), Some(3));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_back(&mut self) -> Option<T> {
        self.pop_back_node().map(|box Node{value, ..}| value)
    }

    /// Splits the list into two at the given index. Returns everything after the given index,
    /// including the index.
    ///
    /// This operation should compute in O(n) time.
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    ///
    /// d.push_front(1is);
    /// d.push_front(2);
    /// d.push_front(3);
    ///
    /// let mut splitted = d.split_off(2);
    ///
    /// assert_eq!(splitted.pop_front(), Some(1));
    /// assert_eq!(splitted.pop_front(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn split_off(&mut self, at: uint) -> DList<T> {
        let len = self.len();
        assert!(at < len, "Cannot split off at a nonexistent index");
        if at == 0 {
            return mem::replace(self, DList::new());
        }

        // Below, we iterate towards the `i-1`th node, either from the start or the end,
        // depending on which would be faster.
        let mut split_node = if at - 1 <= len - 1 - (at - 1) {
            let mut iter = self.iter_mut();
            // instead of skipping using .skip() (which creates a new struct),
            // we skip manually so we can access the head field without
            // depending on implementation details of Skip
            for _ in 0..at - 1 {
                iter.next();
            }
            iter.head
        }  else {
            // better off starting from the end
            let mut iter = self.iter_mut();
            for _ in 0..len - 1 - (at - 1) {
                iter.next_back();
            }
            iter.tail
        };

        let mut splitted_list = DList {
            list_head: None,
            list_tail: self.list_tail,
            length: len - at
        };

        mem::swap(&mut split_node.resolve().unwrap().next, &mut splitted_list.list_head);
        self.list_tail = split_node;
        self.length = at;

        splitted_list
    }
}

#[unsafe_destructor]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Drop for DList<T> {
    fn drop(&mut self) {
        // Dissolve the dlist in backwards direction
        // Just dropping the list_head can lead to stack exhaustion
        // when length is >> 1_000_000
        let mut tail = self.list_tail;
        loop {
            match tail.resolve() {
                None => break,
                Some(prev) => {
                    prev.next.take(); // release Box<Node<T>>
                    tail = prev.prev;
                }
            }
        }
        self.length = 0;
        self.list_head = None;
        self.list_tail = Rawlink::none();
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> Iterator for Iter<'a, A> {
    type Item = &'a A;

    #[inline]
    fn next(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.as_ref().map(|head| {
            self.nelem -= 1;
            self.head = &head.next;
            &head.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.nelem, Some(self.nelem))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.tail.resolve_immut().as_ref().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &prev.value
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> ExactSizeIterator for Iter<'a, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> Iterator for IterMut<'a, A> {
    type Item = &'a mut A;
    #[inline]
    fn next(&mut self) -> Option<&'a mut A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.resolve().map(|next| {
            self.nelem -= 1;
            self.head = match next.next {
                Some(ref mut node) => Rawlink::some(&mut **node),
                None => Rawlink::none(),
            };
            &mut next.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.nelem, Some(self.nelem))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut A> {
        if self.nelem == 0 {
            return None;
        }
        self.tail.resolve().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &mut prev.value
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}

// private methods for IterMut
impl<'a, A> IterMut<'a, A> {
    fn insert_next_node(&mut self, mut ins_node: Box<Node<A>>) {
        // Insert before `self.head` so that it is between the
        // previously yielded element and self.head.
        //
        // The inserted node will not appear in further iteration.
        match self.head.resolve() {
            None => { self.list.push_back_node(ins_node); }
            Some(node) => {
                let prev_node = match node.prev.resolve() {
                    None => return self.list.push_front_node(ins_node),
                    Some(prev) => prev,
                };
                let node_own = prev_node.next.take().unwrap();
                ins_node.next = link_with_prev(node_own, Rawlink::some(&mut *ins_node));
                prev_node.next = link_with_prev(ins_node, Rawlink::some(prev_node));
                self.list.length += 1;
            }
        }
    }
}

impl<'a, A> IterMut<'a, A> {
    /// Inserts `elt` just after the element most recently returned by `.next()`.
    /// The inserted element does not appear in the iteration.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut list: DList<int> = vec![1, 3, 4].into_iter().collect();
    ///
    /// {
    ///     let mut it = list.iter_mut();
    ///     assert_eq!(it.next().unwrap(), &1);
    ///     // insert `2` after `1`
    ///     it.insert_next(2);
    /// }
    /// {
    ///     let vec: Vec<int> = list.into_iter().collect();
    ///     assert_eq!(vec, vec![1i, 2, 3, 4]);
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "collections",
              reason = "this is probably better handled by a cursor type -- we'll see")]
    pub fn insert_next(&mut self, elt: A) {
        self.insert_next_node(box Node::new(elt))
    }

    /// Provides a reference to the next element, without changing the iterator.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut list: DList<int> = vec![1, 2, 3].into_iter().collect();
    ///
    /// let mut it = list.iter_mut();
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert_eq!(it.peek_next().unwrap(), &2);
    /// // We just peeked at 2, so it was not consumed from the iterator.
    /// assert_eq!(it.next().unwrap(), &2);
    /// ```
    #[inline]
    #[unstable(feature = "collections",
              reason = "this is probably better handled by a cursor type -- we'll see")]
    pub fn peek_next(&mut self) -> Option<&mut A> {
        if self.nelem == 0 {
            return None
        }
        self.head.resolve().map(|head| &mut head.value)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Iterator for IntoIter<A> {
    type Item = A;

    #[inline]
    fn next(&mut self) -> Option<A> { self.list.pop_front() }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.list.length, Some(self.list.length))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> DoubleEndedIterator for IntoIter<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> { self.list.pop_back() }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> FromIterator<A> for DList<A> {
    fn from_iter<T: Iterator<Item=A>>(iterator: T) -> DList<A> {
        let mut ret = DList::new();
        ret.extend(iterator);
        ret
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Extend<A> for DList<A> {
    fn extend<T: Iterator<Item=A>>(&mut self, mut iterator: T) {
        for elt in iterator { self.push_back(elt); }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: PartialEq> PartialEq for DList<A> {
    fn eq(&self, other: &DList<A>) -> bool {
        self.len() == other.len() &&
            iter::order::eq(self.iter(), other.iter())
    }

    fn ne(&self, other: &DList<A>) -> bool {
        self.len() != other.len() ||
            iter::order::ne(self.iter(), other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Eq> Eq for DList<A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: PartialOrd> PartialOrd for DList<A> {
    fn partial_cmp(&self, other: &DList<A>) -> Option<Ordering> {
        iter::order::partial_cmp(self.iter(), other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Ord> Ord for DList<A> {
    #[inline]
    fn cmp(&self, other: &DList<A>) -> Ordering {
        iter::order::cmp(self.iter(), other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: Clone> Clone for DList<A> {
    fn clone(&self) -> DList<A> {
        self.iter().map(|x| x.clone()).collect()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A: fmt::Debug> fmt::Debug for DList<A> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        try!(write!(f, "DList ["));

        for (i, e) in self.iter().enumerate() {
            if i != 0 { try!(write!(f, ", ")); }
            try!(write!(f, "{:?}", *e));
        }

        write!(f, "]")
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<S: Writer + Hasher, A: Hash<S>> Hash<S> for DList<A> {
    fn hash(&self, state: &mut S) {
        self.len().hash(state);
        for elt in self.iter() {
            elt.hash(state);
        }
    }
}

免责声明：我不是此代码的作者，它是Rust语言标准库的一部分。我没有信任（或责备）。

Answer 3

另外，对于发现此问题的其他人来说，对我的问题最清楚，最简单的回答是“使用*”。

*是'第四种没有人喜欢谈论的指针'。

请参阅：http://static.rust-lang.org/doc/0.9/rust.html#pointer-types

也就是说，一个普通的指针可以有NULL值，而且使用的是1）不安全，2）对其他对象的生命周期没有影响。

两个方向的链接列表将实现为：

struct ListNode<T> {
  _next: Option<~ListNode<T>>,
  _prev: Option<*ListNode<T>>,
  _data: Option<T>
}

使用以下函数设置_prev值：

// Set the previous pointer
fn set_prev(&mut self, mut prev: &ListNode<T>) {
  unsafe {
    self._prev = Some(prev as *ListNode<T>);
  }
}

// Reset the previous pointer
fn reset_prev(&mut self) {
  self._prev = None::<*ListNode<T>>;
}

// Create a new next node and return it
fn extend_end(&mut self, value:T) {
  let mut next = ~ListNode::<T>::new();
  next.set_data(value);
  next.set_prev(self);
  self._next = Some(next);
}

显然这是非常混乱的，但它确实似乎是做涉及指针的事情的推荐方法。

Lars Bergstrom谈话（在接受的答案中）的关键是，做这样的不安全操作是好的，只要它们被仔细地写成并安全地包装到继续暴露“安全”的类型的内部实现中公众api。

你会如何在Rust中实现双向链表？

3 个答案: