#container #data-structure #map

anymap2

A safe and convenient store for one value of each type

1 unstable release

0.13.0 Apr 10, 2021

#665 in Data structures


Used in sqlxmq

MIT/Apache

42KB
781 lines

🗺️ AnyMap

A map that stores zero or one of each type.

Crates.io CI Coverage Status

AnyMap is a wrapper around a HashMap<TypeId, Box<Any>>, exposing a typed interface which is safe and robust.

The separate CloneAny* traits mean the additional Send / Sync bounds are enforced on all types in the map, which is an ergonomic hit from the anymap crate. This is a workaround to avoid this warning: https://github.com/rust-lang/rust/issues/51443

Note: This is a fork of anymap, with the additional constraint that, if any type in the Map needs to be Send or Sync or both, then all types in the Map have that constraint. This fork should go away once the above Rust issue is fixed. I just created it so that updating Rust wouldn't inadvertently break the map functionality.

Usage

Add the following to Cargo.toml:

anymap2 = "0.13.0"

In code:

use anymap2::AnyMap; // Map<dyn Any>

let mut data = 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 {
    value: String,
}

assert_eq!(data.get::<Foo>(), None);
data.insert(Foo {
    value: format!("foo"),
});
assert_eq!(
    data.get(),
    Some(&Foo {
        value: format!("foo")
    })
);

data.get_mut::<Foo>().map(|foo| foo.value.push('t'));
assert_eq!(&*data.get::<Foo>().unwrap().value, "foot");

unsafe Code

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

  • To support Any and CloneAny, unsafe code is required (because of how the downcast methods are defined in impl Any rather than being trait methods; I think this is kind of a historical detail of the structure of std::any::Any); if you wanted to ditch Clone support this unsafety could be removed.

  • In the interests of performance, skipping various checks that are unnecessary because of the invariants of the data structure (no need to check the type ID when it’s been statically ensured by being used as the hash map key) and simplifying hashing (type IDs are already good hashes, no need to mangle them through SipHash).

It’s not possible to remove all unsafety from this library without also removing some of the functionality. Still, at the cost of the CloneAny functionality, the raw interface and maybe the concurrency support, you can definitely remove all unsafe code. Here’s how you could do it:

  • Remove the genericness of it all;
  • Merge anymap::raw into the normal interface, flattening it;
  • Change things like .map(|any| unsafe { any.downcast_unchecked() }) to .and_then(|any| any.downcast()) (performance cost: one extra superfluous type ID comparison, indirect);
  • Ditch the TypeIdHasher since transmuting a TypeId is right out (cost: SIP mangling of a u64 on every access).

Yeah, the performance costs of going safe are quite small. The more serious matters are the loss of Clone and maybe Send + Sync.

But frankly, if you wanted to do all this it’d be easier and faster to write it from scratch. The core of the library is actually really simple and perfectly safe, as can be seen in src/lib.rs in the first commit (note that that code won’t run without a few syntactic alterations; it was from well before Rust 1.0 and has things like Any:'static where now we have Any + 'static).

Author

Chris Morgan (chris-morgan) is the primary author and maintainer of AnyMap.

License

This library is distributed under similar terms to Rust: dual licensed under the MIT license and the Apache license (version 2.0).

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.

No runtime deps