キー型の変種

Any type that implements the Eq and Hash traits can be a key in HashMap. This includes:

• bool （キーになりうる値が2つしかないので実用的ではないですが…）
• int、uint、あるいは他の整数型
• Stringと&str（Tips：Stringをキーにしたハッシュマップを作製した場合、.get()メソッドの引数に&strを与えて値を取得することができます。）

f32とf64はHashを実装して いない ことに注意しましょう。おそらくこれは浮動小数点演算時に誤差が発生するため、キーとして使用すると、恐ろしいほどエラーの元となるためです。

All collection classes implement Eq and Hash if their contained type also respectively implements Eq and Hash. For example, Vec<T> will implement Hash if T implements Hash.

You can easily implement Eq and Hash for a custom type with just one line: #[derive(PartialEq, Eq, Hash)]

The compiler will do the rest. If you want more control over the details, you can implement Eq and/or Hash yourself. This guide will not cover the specifics of implementing Hash.

To play around with using a struct in HashMap, let's try making a very simple user logon system:

use std::collections::HashMap;

// Eqトレイトを使用する時は、PartialEqをderiveする必要があります。
#[derive(PartialEq, Eq, Hash)]
struct Account<'a>{
    username: &'a str,
    password: &'a str,
}

struct AccountInfo<'a>{
    name: &'a str,
    email: &'a str,
}

type Accounts<'a> = HashMap<Account<'a>, AccountInfo<'a>>;

fn try_logon<'a>(accounts: &Accounts<'a>,
        username: &'a str, password: &'a str){
    println!("Username: {}", username);
    println!("Password: {}", password);
    println!("Attempting logon...");

    let logon = Account {
        username,
        password,
    };

    match accounts.get(&logon) {
        Some(account_info) => {
            println!("Successful logon!");
            println!("Name: {}", account_info.name);
            println!("Email: {}", account_info.email);
        },
        _ => println!("Login failed!"),
    }
}

fn main(){
    let mut accounts: Accounts = HashMap::new();

    let account = Account {
        username: "j.everyman",
        password: "password123",
    };

    let account_info = AccountInfo {
        name: "John Everyman",
        email: "j.everyman@email.com",
    };

    accounts.insert(account, account_info);

    try_logon(&accounts, "j.everyman", "psasword123");

    try_logon(&accounts, "j.everyman", "password123");
}