struct_index.rs

use std::ops::{Index, IndexMut};

pub struct User {
    id: u64,
    name: String,
}

impl User {
    pub fn new(id_var: u64, name_var: String) -> Self {
        User {
            id: id_var,
            name: name_var,
        }
    }
}

// Userへのインデクシング専用の型を定義する
#[allow(non_camel_case_types)]
pub struct id;
#[allow(non_camel_case_types)]
struct name;

// それぞれのインデックスとフィールドを対応づけてIndexとIndexMutを実装する
impl Index<id> for User {
    type Output = u64;
    fn index(&self, _: id) -> &Self::Output {
        &self.id
    }
}

impl IndexMut<id> for User {
    fn index_mut(&mut self, _: id) -> &mut Self::Output {
        &mut self.id
    }
}

impl Index<name> for User {
    type Output = String;
    fn index(&self, _: name) -> &Self::Output {
        &self.name
    }
}

impl IndexMut<name> for User {
    fn index_mut(&mut self, _: name) -> &mut Self::Output {
        &mut self.name
    }
}

fn main() {
    let mut user = User::new(1, "user".into());

    assert_eq!(user[name], "user");

    user[id] += 1;

    assert_eq!(user[id], 2);
}