[][src]Crate arc_interner

An interner that deallocates unused values.

This crate is a fork of David Roundy's internment crate. It provides an alternative implementation of the internment::ArcIntern type. It inherits David's high-level design and API; however it is built completely on Rust's standard Arc and the dashmap crate and does not contain any unsafe code.

Interning reduces the memory footprint of an application by storing a unique copy of each distinct value. It speeds up equality comparison and hashing operations, as only pointers rather than actual values need to be compared. On the flip side, object creation is slower, as it involves lookup in the interned object pool.

Interning is most commonly applied to strings; however it can also be useful for other object types. This library supports interning of arbitrary objects.

There exist several interning libraries for Rust, each with its own set of tradeoffs. This library makes the following design choices:

  • Interned objects are reference counted. When the last reference to an interned object is dropped, the object is deallocated. This prevents unbounded growth of the interned object pool in applications where the set of interned values changes dynamically at the cost of some CPU and memory overhead (due to storing and maintaining an atomic counter).
  • Multithreading. A single pool of interned objects is shared by all threads in the program. Inside DashMap this pool is protected by mutexes that are acquired every time an object is being interned or a reference to an interned object is being dropped. Although Rust mutexes are fairly cheap when there is no contention, you may see a significant drop in performance under contention.
  • Not just strings: this library allows interning any data type that satisfies the Eq + Hash + Send + Sync trait bound.
  • Safe: this library is built on Arc type from the Rust standard library and the dashmap crate and does not contain any unsafe code (although std and dashmap do of course)


use arc_interner::ArcIntern;
let x = ArcIntern::new("hello");
let y = ArcIntern::new("world");
assert_ne!(x, y);
assert_eq!(x, ArcIntern::new("hello"));
assert_eq!(*x, "hello"); // dereference an ArcIntern like a pointer



A pointer to a reference-counted interned object.