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rust - 在 Rust 中一次从全局哈希图中访问两个可变引用

问题描述

假设我们有一个使用lazy_static :制作的特征对象的全局可访问哈希图MY_ANIMALS: Mutex<HashMap<i32, AnimalBox>>，其中type AnimalBox = Box<dyn AnimalExt+Send>

现在，我们希望这个全局哈希图中的动物能够相互交互。例如，一个AnimalBox可以AnimalExt::eat(&mut self, prey: &mut AnimalBox)另一个。

问题是我们的eat()函数既需要对 self 的可变引用，也需要对祈祷的可变引用（因为我们希望AnimalExt::perish(&mut self)当它被吃掉时，祈祷会指向它。

但是，获得两个对我们的 hashmap 的可变引用会导致WouldBlock错误：

use lazy_static::lazy_static;
use std::sync::Mutex;
use std::collections::HashMap;

//type alias for our boxed animals
type AnimalBox = Box<dyn AnimalExt+Send>;

//globally accessible hashmap for keeping track of our animals throughout the scope of our application
lazy_static! {
    static ref MY_ANIMALS: Mutex<HashMap<i32, AnimalBox>> = Mutex::new(HashMap::new());
}

//simple trait for our animals
trait AnimalExt{
    //eat() function requires a mutable reference to another AnimalBox
    fn eat(&mut self, pray: &mut AnimalBox);
    fn perish(&mut self);
    fn energy(&self)->i32;
    fn id(&self)->i32;
}

struct Wolf{
    id: i32,
    energy: i32,
    alive: bool,
}
impl AnimalExt for Wolf{
    fn id(&self)->i32{
        self.id
    }
    fn eat(&mut self, pray: &mut AnimalBox) {
        pray.perish();
        self.energy+= pray.energy()
    }
    fn energy(&self) ->i32 {
        self.energy
    }
    fn perish(&mut self){
        self.alive = false; 
    }
}
impl Wolf{
    pub fn new(id: i32)->Self{
        Wolf{
            id: id,
            energy: 50,
            alive: true,
        }
    }
}
struct Cow{
    id: i32,
    energy: i32,
    alive: bool,
}
impl Cow{
    pub fn new(id: i32)->Self{
        Cow{
            id: id,
            energy: 100,
            alive: true,
        }
    }
}
impl AnimalExt for Cow{
    fn id(&self)->i32{
        self.id
    }
    fn eat(&mut self, pray: &mut AnimalBox) {
        pray.perish();
        self.energy+= pray.energy()
    }
    fn energy(&self) ->i32 {
        self.energy
    }
    fn perish(&mut self){
        self.alive = false; 
    }
}
fn main() {
    println!("Hello, world!");
    //define our animals
    let cow1 = Box::new(Cow::new(1)) as AnimalBox;
    let cow2 = Box::new(Cow::new(2)) as AnimalBox;
    let wolf1 = Box::new(Wolf::new(3)) as AnimalBox;
    let wolf2 = Box::new(Wolf::new(4)) as AnimalBox;

    //insert them into the global hashmap
    MY_ANIMALS.lock().unwrap().insert(cow1.id(), cow1);
    MY_ANIMALS.lock().unwrap().insert(cow2.id(), cow2);
    MY_ANIMALS.lock().unwrap().insert(wolf1.id(), wolf1);
    MY_ANIMALS.lock().unwrap().insert(wolf2.id(), wolf2);

    //getting one animal to eat() another causes a WouldBlock error
    match (MY_ANIMALS.try_lock().unwrap().get_mut(&0), MY_ANIMALS.try_lock().unwrap().get_mut(&1)){
        (Some(a1), Some(a2))=>{
            a1.eat(a2);
        }
        _=>()
    }
}

有什么好的解决方法吗？或者没有安全的方法可以使用哈希图来做到这一点？我已经看到了类似问题的答案，但建议的答案建议使用RefCell，这与lazy_static 的Send特征要求不兼容。

标签： rusthashmaprefcell

解决方案

我最终使用了muti_mut crate，它提供了几种方法来获得对 HashMap 或 BTreeMap 的多个可变引用。

use lazy_static::lazy_static;
use std::sync::Mutex;
use std::collections::HashMap;
use multi_mut::{HashMapMultiMut, HashMapMutWrapper};

//type alias for our boxed animals
type AnimalBox = Box<dyn AnimalExt+Send>;

//globally accessible Hashmap for keeping track of our animals throughout the scope of our application
lazy_static! {
    static ref MY_ANIMALS: Mutex<HashMap<i32, AnimalBox>> = Mutex::new(HashMap::new());
}

//simple trait
trait AnimalExt{
    //eat() function requires a mutable reference to another AnimalBox
    fn eat(&mut self, pray: &mut AnimalBox);
    fn perish(&mut self);
    fn energy(&self)->i32;
    fn id(&self)->i32;
}

struct Wolf{
    id: i32,
    energy: i32,
    alive: bool,
}
impl AnimalExt for Wolf{
    fn id(&self)->i32{
        self.id
    }
    fn eat(&mut self, pray: &mut AnimalBox) {
        pray.perish();
        self.energy+= pray.energy()
    }
    fn energy(&self) ->i32 {
        self.energy
    }
    fn perish(&mut self){
        self.alive = false; 
    }
}
impl Wolf{
    pub fn new(id: i32)->Self{
        Wolf{
            id: id,
            energy: 50,
            alive: true,
        }
    }
}
struct Cow{
    id: i32,
    energy: i32,
    alive: bool,
}
impl Cow{
    pub fn new(id: i32)->Self{
        Cow{
            id: id,
            energy: 100,
            alive: true,
        }
    }
}
impl AnimalExt for Cow{
    fn id(&self)->i32{
        self.id
    }
    fn eat(&mut self, pray: &mut AnimalBox) {
        pray.perish();
        self.energy+= pray.energy()
    }
    fn energy(&self) ->i32 {
        self.energy
    }
    fn perish(&mut self){
        self.alive = false; 
    }
}
fn main() {
    println!("Hello, world!");
    //define our animals
    let cow1 = Box::new(Cow::new(1)) as AnimalBox;
    let wolf1 = Box::new(Wolf::new(2)) as AnimalBox;
    let before_eating_cow = wolf1.energy();
    //insert them into the global hashmap
    MY_ANIMALS.lock().unwrap().insert(cow1.id(), cow1);
    MY_ANIMALS.lock().unwrap().insert(wolf1.id(), wolf1);


    //use get_pair_mut method from the multi_mut crate
    match MY_ANIMALS.try_lock().unwrap().get_pair_mut(&1, &2){
        Some((hunter, prey))=>{
            dbg!("hunter eats prey");
            hunter.eat(prey);
        }
        None=>()
    }
    let after_eating_cow = MY_ANIMALS.lock().unwrap().get(&1).unwrap().energy();
    assert_ne!(before_eating_cow, after_eating_cow);
}

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