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extern crate once_cell;
use once_cell::sync::{Lazy, OnceCell};
use std::collections::HashMap;
static HASHMAP: Lazy<HashMap<u32, &'static str>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(0, "foo");
m.insert(1, "bar");
m.insert(2, "baz");
m
});
// Same, but completely without macros
fn hashmap() -> &'static HashMap<u32, &'static str> {
static INSTANCE: OnceCell<HashMap<u32, &'static str>> = OnceCell::new();
INSTANCE.get_or_init(|| {
let mut m = HashMap::new();
m.insert(0, "foo");
m.insert(1, "bar");
m.insert(2, "baz");
m
})
}
fn main() {
// First access to `HASHMAP` initializes it
println!("The entry for `0` is \"{}\".", HASHMAP.get(&0).unwrap());
// Any further access to `HASHMAP` just returns the computed value
println!("The entry for `1` is \"{}\".", HASHMAP.get(&1).unwrap());
// The same works for function-style:
assert_eq!(hashmap().get(&0), Some(&"foo"));
assert_eq!(hashmap().get(&1), Some(&"bar"));
}