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# `id-arena`
[](https://travis-ci.org/fitzgen/id-arena)
A simple, id-based arena.
### Id-based
Allocate objects and get an identifier for that object back, *not* a
reference to the allocated object. Given an id, you can get a shared or
exclusive reference to the allocated object from the arena. This id-based
approach is useful for constructing mutable graph data structures.
If you want allocation to return a reference, consider [the `typed-arena`
### No Deletion
This arena does not support deletion, which makes its implementation simple
and allocation fast. If you want deletion, you need a way to solve the ABA
problem. Consider using [the `generational-arena`
### Homogeneous
This crate's arenas can only contain objects of a single type `T`. If you
need an arena of objects with heterogeneous types, consider another crate.
### `#![no_std]` Support
Requires the `alloc` nightly feature. Disable the on-by-default `"std"` feature:
```toml
[dependencies.id-arena]
version = "2"
default-features = false
```
### `rayon` Support
If the `rayon` feature of this crate is activated:
```toml
[dependencies]
id-arena = { version = "2", features = ["rayon"] }
```
parallel iteration. The `Arena` type will have a `par_iter` family of
methods where appropriate.
### Example
```rust
use id_arena::{Arena, Id};
type AstNodeId = Id<AstNode>;
#[derive(Debug, Eq, PartialEq)]
pub enum AstNode {
Const(i64),
Var(String),
Add {
lhs: AstNodeId,
rhs: AstNodeId,
},
Sub {
lhs: AstNodeId,
rhs: AstNodeId,
},
Mul {
lhs: AstNodeId,
rhs: AstNodeId,
},
Div {
lhs: AstNodeId,
rhs: AstNodeId,
},
}
let mut ast_nodes = Arena::<AstNode>::new();
// Create the AST for `a * (b + 3)`.
let three = ast_nodes.alloc(AstNode::Const(3));
let b = ast_nodes.alloc(AstNode::Var("b".into()));
let b_plus_three = ast_nodes.alloc(AstNode::Add {
lhs: b,
rhs: three,
});
let a = ast_nodes.alloc(AstNode::Var("a".into()));
let a_times_b_plus_three = ast_nodes.alloc(AstNode::Mul {
lhs: a,
rhs: b_plus_three,
});
// Can use indexing to access allocated nodes.
assert_eq!(ast_nodes[three], AstNode::Const(3));
```