Alternate/custom key types
Any type that implements the Eq and Hash traits can be a key in HashMap.
This includes:
bool(though not very useful since there is only two possible keys)int,uint, and all variations thereofStringand&str(protip: you can have aHashMapkeyed byStringand call.get()with an&str)
Note that f32 and f64 do not implement Hash,
likely because floating-point precision errors
would make using them as hashmap keys horribly error-prone.
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 requires that you derive PartialEq on the type. #[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"); }