无法修改递归结构中的字段

如何解决无法修改递归结构中的字段

我有一个递归结构，其中结构的字段是对相同类型的其他结构的引用：

use std::collections::HashMap;

pub struct RecursiveStruct<'a> {
    outer: Option<Box<&'a RecursiveStruct<'a>>>,dict: HashMap<u32,String>
}

有了这个结构，我也有几个方法，比如构造函数、将 (k,v) 对添加到 calee 的字段和一个 getter 的方法：

impl<'a> RecursiveStruct<'a> {
    pub fn new(outer: Option<Box<&'a RecursiveStruct<'a>>>) -> Self {
        let dict: HashMap<u32,String> = HashMap::new();
        RecursiveStruct { outer,dict }
    }
    
    // searches for value corresponding to key in all struct layers
    pub fn get(&self,key: u32) -> Result<String,()> {
        let item = self.dict.get(&key);
        match item {
            Some(x) => Ok(x.clone()),None => {
                match &self.outer {
                    Some(x) => x.get(key),None => Err(())
                }
            }
        }
    }
    
    // adds (key,val) to "innermost" instance of struct
    pub fn add(&mut self,key:u32,val: String) {
        self.dict.insert(key,val);
    }
}

这些方法工作正常，但是当我尝试添加一个方法来尝试修改任何内层中的 dict 字段时，我收到 cannot borrow '***x' as mutable,as it is behind a '&' reference 错误。

pub fn re_assign(&mut self,key: u32,val: String) {
    if self.dict.contains_key(&key) {
        self.dict.insert(key,val);
    } else {
        match &mut self.outer {
            Some(x) => x.re_assign(key,val.clone()),None => println!("Such key Couldn't be found!"),};
    }
}

这是去游乐场的link。

解决方法

您正在使用 &，但想要一个 &mut，默认情况下，rust 引用是不可变的： 游乐场：https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=be82b8ba01dff60e106af9e59df8228e

use std::collections::HashMap;

pub struct RecursiveStruct<'a> {
    outer: Option<Box<&'a mut RecursiveStruct<'a>>>,dict: HashMap<u32,String>
}

impl<'a> RecursiveStruct<'a> {
    pub fn new(outer: Option<Box<&'a mut RecursiveStruct<'a>>>) -> Self {
        let dict: HashMap<u32,String> = HashMap::new();
        RecursiveStruct { outer,dict }
    }
    
    pub fn get(&self,key: u32) -> Result<String,()> {
        let item = self.dict.get(&key);
        match item {
            Some(x) => Ok(x.clone()),None => {
                match &self.outer {
                    Some(x) => x.get(key),None => Err(())
                }
            }
        }
    }
    
    pub fn re_assign(&mut self,key: u32,val: String) {
        if self.dict.contains_key(&key) {
            self.dict.insert(key,val);
        } else {
            match &mut self.outer {
                Some(x) => x.re_assign(key,val.clone()),None => println!("Such key couldn't be found!"),};
        }
    }
    
    pub fn add(&mut self,key:u32,val: String) {
        self.dict.insert(key,val);
    }
}

fn main() {
    // instantiate "outer" struct and set its field
    let mut S = RecursiveStruct::new(None);
    S.add(42,"Answer to the universe!".to_string());
    // instantiate "inner" struct
    let mut S1 = RecursiveStruct::new(Some(Box::new(&mut S)));
    println!("{}",S1.get(42).unwrap()); // get the field of outer struct
    // modify the field of outer struct
    S1.re_assign(42,"The answer has been changed.".to_string());
    println!("{}",S1.get(42).unwrap());
}