A macro for declaring lazily evaluated statics in Rust.
Using this macro, it is possible to have static
s that require code to be
executed at runtime in order to be initialized.
This includes anything requiring heap allocations, like vectors or hash maps,
as well as anything that requires function calls to be computed.
lazy_static! {
static ref NAME_1: TYPE_1 = EXPR_1;
static ref NAME_2: TYPE_2 = EXPR_2;
...
static ref NAME_N: TYPE_N = EXPR_N;
}
For a given static ref NAME: TYPE = EXPR;
, the macro generates a
unique type that implements Deref<TYPE>
and stores it in a static with name NAME
.
On first deref, EXPR
gets evaluated and stored internally, such that all further derefs
can return a reference to the same object.
Like regular static mut
s, this macro only works for types that fulfill the Share
trait.
Using the macro:
#![feature(phase)]
#[phase(plugin)]
extern crate lazy_static;
use std::collections::HashMap;
lazy_static! {
static ref HASHMAP: HashMap<uint, &'static str> = {
let mut m = HashMap::new();
m.insert(0u, "foo");
m.insert(1u, "bar");
m.insert(2u, "baz");
m
};
static ref COUNT: uint = HASHMAP.len();
static ref NUMBER: uint = times_two(21);
}
fn times_two(n: uint) -> uint { n * 2 }
fn main() {
println!("The map has {} entries.", *COUNT);
println!("The entry for `0` is \"{}\".", HASHMAP.get(&0));
println!("A expensive calculation on a static results in: {}.", *NUMBER);
}