AnyMap, a safe and convenient store for one value of each type

AnyMap is a type-safe wrapper around HashMap<TypeId, Box<dyn Any>> that lets you not worry about TypeId or downcasting, but just get on with storing one each of a bag of diverse types, which is really useful for extensibility in some sorts of libraries.

Background

If you’re familiar with Go and Go web frameworks, you may have come across the common “environment” pattern for storing data related to the request. It’s typically something like map[string]interface{} and is accessed with arbitrary strings which may clash and type assertions which are a little unwieldy and must be used very carefully. (Personally I would consider that it is just asking for things to blow up in your face.) In a language like Go, lacking in generics, this is the best that can be done; such a thing cannot possibly be made safe without generics.

As another example of such an interface, JavaScript objects are exactly the same—a mapping of string keys to arbitrary values. (There it is actually more dangerous, because methods and fields/attributes/properties are on the same plane—though it’s possible to use Map these days.)

Fortunately, we can do better than these things in Rust. Our type system is quite equal to easy, robust expression of such problems.

Example

let mut data = anymap::AnyMap::new();
assert_eq!(data.get(), None::<&i32>);
data.insert(42i32);
assert_eq!(data.get(), Some(&42i32));
data.remove::<i32>();
assert_eq!(data.get::<i32>(), None);

#[derive(Clone, PartialEq, Debug)]
struct Foo {
    str: String,
}

assert_eq!(data.get::<Foo>(), None);
data.insert(Foo { str: format!("foo") });
assert_eq!(data.get(), Some(&Foo { str: format!("foo") }));
data.get_mut::<Foo>().map(|foo| foo.str.push('t'));
assert_eq!(&*data.get::<Foo>().unwrap().str, "foot");

Features

• Store up to one value for each type in a bag.
• Add Send or Send + Sync bounds.
• You can opt into making the map Clone. (In theory you could add all kinds of other functionality, but you can’t readily make this work generically, and the bones of it are simple enough that it becomes better to make your own extension of Any and reimplement AnyMap.)
• no_std if you like.

Cargo features/dependencies/usage

Typical Cargo.toml usage, providing anymap::AnyMap et al. backed by std::collections::HashMap:

[dependencies]
anymap = "1.0.0-beta.2"

No-std usage, providing anymap::hashbrown::AnyMap et al. (note the different path, required because Cargo features are additive) backed by alloc and the hashbrown crate:

[dependencies]
anymap = { version = "1.0.0-beta.2", default-features = false, features = ["hashbrown"] }

On stability: hashbrown is still pre-1.0.0 and experiencing breaking changes. Because it’s useful for a small fraction of users, I am retaining it, but with different compatibility guarantees to the typical SemVer ones. Where possible, I will just widen the range for new releases of hashbrown, but if an incompatible change occurs, I may drop support for older versions of hashbrown with a bump to the minor part of the anymap version number (e.g. 1.1.0, 1.2.0). Iff you’re using this feature, this is cause to consider using a tilde requirement like "~1.0" (or spell it out as >=1, <1.1).

Unsafe code in this library

This library uses a fair bit of unsafe code for several reasons:

• To support CloneAny, unsafe code is currently required (because the downcast methods are defined on dyn Any rather than being trait methods, and upcasting is an incomplete feature in rustc); if you wanted to ditch Clone support this unsafety could be removed.

• For dyn CloneAny + Send and dyn CloneAny + Send + Sync’s Clone implementation, an unsafe block is used to attach the auto traits where safe code used to be used, in order to avoid a spurious future-compatibility lint.

• In the interests of performance, type ID checks are skipped as unnecessary because of the invariants of the data structure (though this does come at the cost of Map::{as_raw_mut, into_raw} being marked unsafe).

It is possible to remove all unsafe code at the cost of only CloneAny functionality and a little performance. The safe branch in the Git repository contains a couple of commits demonstrating the concept. It’s quite straightforward; the core of this library is very simple and perfectly safe.

Colophon

Authorship: Chris Morgan is the author and maintainer of this library.

Licensing: this library is distributed under the terms of the Blue Oak Model License 1.0.0, the MIT License and the Apache License, Version 2.0, at your choice. See COPYING for details.