与内部可变性作斗争

Question

我有这样的数据结构：

struct R {
    hmhs: HashMap<i64, HashSet<i64>>,
}

impl R {
    fn hs_for_hmhs(&mut self) -> &mut HashSet<i64> {
        if let None = self.hmhs.get(&0) {
            self.hmhs.insert(0, HashSet::new());
        }

        self.hmhs.get_mut(&0).unwrap()
    }

    fn iter_for_hmhs<'a>(&'a mut self) -> impl Iterator<Item = &'a i64> {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&mut self, i: i64) -> bool {
        self.hs_for_hmhs().insert(i)
    }
}

这似乎可行，但所有方法都需要一个可变的 参考self，这是不幸的。我试着给 内部可变性一展身手：

struct S {
    hmhs: RefCell<HashMap<i64, HashSet<i64>>>,
}

impl S {
    fn hs_for_hmhs(&self) -> &HashSet<i64> {
        if let None = self.hmhs.borrow().get(&0) {
            self.hmhs.borrow_mut().insert(0, HashSet::new());
        }

        self.hmhs.borrow_mut().get_mut(&0).unwrap()
    }

    fn iter_for_hmhs(&mut self) -> impl Iterator<Item = &i64> {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&mut self, i: i64) -> bool {
        self.hs_for_hmhs().insert(i)
    }
}

但是，我似乎经常遇到问题。大多是How do I return a reference to something inside a RefCell without breaking encapsulation?的各种@

我在这里尝试了很多变体，但我遗漏了一些东西 在我的理解中是基本的。有没有办法实现我的目标 想要吗？

Complete Code:

use std::cell::RefCell;
use std::collections::{HashMap, HashSet};

struct R {
    hmhs: HashMap<i64, HashSet<i64>>,
}

impl R {
    fn hs_for_hmhs(&mut self) -> &mut HashSet<i64> {
        if let None = self.hmhs.get(&0) {
            self.hmhs.insert(0, HashSet::new());
        }

        self.hmhs.get_mut(&0).unwrap()
    }

    fn iter_for_hmhs<'a>(&'a mut self) -> impl Iterator<Item = &'a i64> {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&mut self, i: i64) -> bool {
        self.hs_for_hmhs().insert(i)
    }
}

struct S {
    hmhs: RefCell<HashMap<i64, HashSet<i64>>>,
}

impl S {
    fn hs_for_hmhs(&self) -> &mut HashSet<i64> {
        if let None = self.hmhs.borrow().get(&0) {
            self.hmhs.borrow_mut().insert(0, HashSet::new());
        }

        self.hmhs.borrow_mut().get_mut(&0).unwrap()
    }

    fn iter_for_hmhs(&self) -> impl Iterator<Item = &i64> {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&self, i: i64) -> bool {
        self.hs_for_hmhs().insert(i)
    }
}

fn main() {}

编译器消息：

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:36:9
   |
36 |         self.hmhs.borrow_mut().get_mut(&0).unwrap()
   |         ^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
37 |     }
   |     - temporary value only lives until here
   |
note: borrowed value must be valid for the anonymous lifetime #1 defined on the method body at 31:5...
  --> src/main.rs:31:5
   |
31 | /     fn hs_for_hmhs(&self) -> &mut HashSet<i64> {
32 | |         if let None = self.hmhs.borrow().get(&0) {
33 | |             self.hmhs.borrow_mut().insert(0, HashSet::new());
34 | |         }
35 | |
36 | |         self.hmhs.borrow_mut().get_mut(&0).unwrap()
37 | |     }
   | |_____^

Answer 1

我找到了一个解决方案——将HashMap 提取为原始指针。这反过来意味着我可以在没有包括返回迭代器在内的恶作剧的情况下访问HashSet。

我对这个作为解决方案感到非常满意。不安全代码很小且包含在内，如果我理解编译器在没有不安全的情况下抱怨的原因，则它不会出现在此代码中，因为HashMap 和HashSet 在构造后都不会被删除或替换。

这是一个很大的努力。

use std::cell::RefCell;
use std::collections::{HashMap, HashSet};

struct R {
    hmhs: HashMap<i64, HashSet<i64>>,
}

impl R {
    fn hs_for_hmhs(&mut self) -> &mut HashSet<i64> {
        if let None = self.hmhs.get(&0) {
            self.hmhs.insert(0, HashSet::new());
        }

        self.hmhs.get_mut(&0).unwrap()
    }

    fn iter_for_hmhs<'a>(&'a mut self) -> impl Iterator<Item = &'a i64> {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&mut self, i: i64) -> bool {
        self.hs_for_hmhs().insert(i)
    }
}

struct S {
    hmhs: RefCell<HashMap<i64, HashSet<i64>>>,
}

impl S {
    fn hs_as_ptr(&self) -> *mut HashMap<i64, HashSet<i64>> {
        self.hmhs.borrow_mut().entry(0).or_insert(HashSet::new());
        self.hmhs.as_ptr()
    }

    fn mut_hs_for_hmhs(&mut self) -> &mut HashSet<i64> {
        unsafe { (*self.hs_as_ptr()).get_mut(&0).unwrap() }
    }
    fn hs_for_hmhs(&self) -> &HashSet<i64> {
        unsafe { (*self.hs_as_ptr()).get(&0).unwrap() }
    }

    fn iter_for_hmhs<'a>(&'a self) -> impl Iterator<Item = &'a i64> + 'a {
        self.hs_for_hmhs().iter()
    }

    fn insert_for_hmhs(&mut self, i: i64) -> bool {
        self.mut_hs_for_hmhs().insert(i)
    }
}

fn main() {
    let mut r = R {
        hmhs: HashMap::new(),
    };
    let mut s = S {
        hmhs: RefCell::new(HashMap::new()),
    };

    r.insert_for_hmhs(10);
    s.insert_for_hmhs(20);

    println!("r next: {:?}", r.iter_for_hmhs().next());
    println!("s next: {:?}", s.iter_for_hmhs().next());
}

https://play.rust-lang.org/?gist=3ed1977bdd5f9f82d144fe128f618979&version=stable&mode=debug&edition=2015