# Crate lazy_static [−] [src]

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

ⓘThis example is not tested

    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.

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