81 lines
4.9 KiB
Markdown
81 lines
4.9 KiB
Markdown
# ``AnyMap``, a safe and convenient store for one value of each type
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``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.
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## Background
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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.
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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.)
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Fortunately, we can do better than these things in Rust. Our type system is quite equal to easy, robust expression of such problems.
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## Example
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```rust
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let mut data = anymap::AnyMap::new();
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assert_eq!(data.get(), None::<&i32>);
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data.insert(42i32);
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assert_eq!(data.get(), Some(&42i32));
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data.remove::<i32>();
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assert_eq!(data.get::<i32>(), None);
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#[derive(Clone, PartialEq, Debug)]
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struct Foo {
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str: String,
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}
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assert_eq!(data.get::<Foo>(), None);
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data.insert(Foo { str: format!("foo") });
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assert_eq!(data.get(), Some(&Foo { str: format!("foo") }));
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data.get_mut::<Foo>().map(|foo| foo.str.push('t'));
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assert_eq!(&*data.get::<Foo>().unwrap().str, "foot");
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```
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## Features
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- Store up to one value for each type in a bag.
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- Add `Send` or `Send + Sync` bounds.
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- 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`.)
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- no_std if you like.
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## Cargo features/dependencies/usage
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Typical Cargo.toml usage, providing `anymap::AnyMap` *et al.* backed by `std::collections::HashMap`:
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```toml
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[dependencies]
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anymap = "1.0.0-beta.2"
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```
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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](https://rust-lang.github.io/hashbrown) crate:
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```toml
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[dependencies]
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anymap = { version = "1.0.0-beta.2", default-features = false, features = ["hashbrown"] }
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```
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**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`).
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## Unsafe code in this library
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This library uses a fair bit of unsafe code for several reasons:
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- 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.
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- 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](https://github.com/rust-lang/rust/issues/51443#issuecomment-421988013).
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- 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).
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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.
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## Colophon
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**Authorship:** [Chris Morgan](https://chrismorgan.info/) is the author and maintainer of this library.
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**Licensing:** this library is distributed under the terms of the
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[Blue Oak Model License 1.0.0](https://blueoakcouncil.org/license/1.0.0), the
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[MIT License](https://opensource.org/licenses/MIT) and the
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[Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0), at your choice.
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See [COPYING](COPYING) for details.
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