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 thereof
String
and &str
(protip: you can have a HashMap
keyed by String
and 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");
}