index_vec

Note: Currently in alpha, but will probably stabilize by the end of the week or so

A more type-safe version of using Vec, for when usizes are getting you down.

This library mainly provides IndexVec<I, T>, which wraps Vec so that it's accessed with I and not usize.

It also defines a macro for defining new index types (for use with IndexVec). Mostly outputting boilerplate.

Example / Overview

use index_vec::{IndexVec, index_vec};

// Define a custom index type.
index_vec::define_index_type! {
    // In this case, use a u32 instead of a usize.
    pub struct StrIdx(u32);
    // Note that this macro has a decent amount of configurability, so
    // be sure to read its documentation if you think it's doing
    // something you don't want.
}

// Create a vector which can be accessed using `StrIdx`s.
let mut strs: IndexVec<StrIdx, &'static str> = index_vec!["strs", "bar", "baz"];

// l is a `StrIdx`
let l = strs.last_idx();
assert_eq!(strs[l], "baz");

let new_i = strs.push("quux");
assert_eq!(strs[new_i], "quux");

// Indices are mostly interoperable with `usize`, and support
// a lot of what you might want to do to an index. (Note that
// it does *not* support these with other index wrappers --
// that seems too likely to lead to bugs).

// Comparison
assert_eq!(StrIdx::new(0), 0usize);
// Addition
assert_eq!(StrIdx::new(0) + 1, 1usize);

// Subtraction (Note that by default, the index will panic on overflow,
// but that can be configured in the macro)
assert_eq!(StrIdx::new(1) - 1, 0usize);

// Wrapping
assert_eq!(StrIdx::new(5) % strs.len(), 1usize);
// ...

Background

The goal is to replace the pattern of using a type FooIdx = usize to access a Vec<Foo> with something that can statically prevent using a FooIdx in a Vec<Bar>. It's most useful if you have a bunch of indices referring to different sorts of vectors.

Much of the code for this is taken from rustc's IndexVec code, however it's diverged a decent amount at this point. The largest differences are:

• No usage of unstable features.
• Different syntax for defining index types.
• Allows use of index types beyond u32 (usize, u32, u16, and u8 are all supported).
• More flexible behavior around how strictly some checks are performed.

Other crates

The indexed_vec crate predates this, and is a much closer copy of the code from rustc. Unfortunately, this means it does not compile on stable.

If you're looking for something further from a vec and closer to a map, you might find handy, slotmap, or slab to be closer what you want.

License

This is based on code from rustc's source, and retains it's status as dual-licensed under MIT (LICENSE-MIT) / Apache 2.0 (LICENSE-APACHE).