[−][src]Crate lazy_static
A macro for declaring lazily evaluated statics.
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.
Syntax
lazy_static! { [pub] static ref NAME_1: TYPE_1 = EXPR_1; [pub] static ref NAME_2: TYPE_2 = EXPR_2; ... [pub] static ref NAME_N: TYPE_N = EXPR_N; }
Attributes (including doc comments) are supported as well:
lazy_static! { /// This is an example for using doc comment attributes static ref EXAMPLE: u8 = 42; }
Semantics
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
. (Attributes end up
attaching to this type.)
On first deref, EXPR
gets evaluated and stored internally, such that all further derefs
can return a reference to the same object. Note that this can lead to deadlocks
if you have multiple lazy statics that depend on each other in their initialization.
Apart from the lazy initialization, the resulting "static ref" variables have generally the same properties as regular "static" variables:
- Any type in them needs to fulfill the
Sync
trait. - If the type has a destructor, then it will not run when the process exits.
Example
Using the macro:
#[macro_use] extern crate lazy_static; use std::collections::HashMap; lazy_static! { static ref HASHMAP: HashMap<u32, &'static str> = { let mut m = HashMap::new(); m.insert(0, "foo"); m.insert(1, "bar"); m.insert(2, "baz"); m }; static ref COUNT: usize = HASHMAP.len(); static ref NUMBER: u32 = times_two(21); } fn times_two(n: u32) -> u32 { n * 2 } fn main() { println!("The map has {} entries.", *COUNT); println!("The entry for `0` is \"{}\".", HASHMAP.get(&0).unwrap()); println!("A expensive calculation on a static results in: {}.", *NUMBER); }
Implementation details
The Deref
implementation uses a hidden static variable that is guarded by a atomic check on each access. On stable Rust, the macro may need to allocate each static on the heap.
Macros
__lazy_static_create | |
lazy_static |
Traits
LazyStatic |
Support trait for enabling a few common operation on lazy static values. |
Functions
initialize |
Takes a shared reference to a lazy static and initializes it if it has not been already. |