# [−] [src] Crate heapless

`static` friendly data structures that don't require dynamic memory allocation

The core principle behind `heapless` is that its data structures are backed by a static memory allocation. For example, you can think of `heapless::Vec` as an alternative version of `std::Vec` with fixed capacity and that can't be re-allocated on the fly (e.g. via `push`).

All `heapless` data structures store their memory allocation inline and specify their capacity via their type parameter `N`. This means that you can instantiate a `heapless` data structure on the stack, in a `static` variable, or even in the heap.

    use heapless::Vec; // fixed capacity `std::Vec`
    use heapless::consts::U8; // type level integer used to specify capacity

    // on the stack
    let mut xs: Vec<u8, U8> = Vec::new(); // can hold up to 8 elements
    xs.push(42).unwrap();
    assert_eq!(xs.pop(), Some(42));

    // in a `static` variable
    // (because `const-fn` has not been fully stabilized you need to use the helper structs in
    // the `i` module, which must be wrapped in a tuple struct)
    static mut XS: Vec<u8, U8> = Vec(heapless::i::Vec::new());

    let xs = unsafe { &mut XS };

    xs.push(42);
    assert_eq!(xs.pop(), Some(42));

    // in the heap (though kind of pointless because no reallocation)
    let mut ys: Box<Vec<u8, U8>> = Box::new(Vec::new());
    ys.push(42).unwrap();
    assert_eq!(ys.pop(), Some(42));

Because they have fixed capacity `heapless` data structures don't implicitly reallocate. This means that operations like `heapless::Vec.push` are truly constant time rather than amortized constant time with potentially unbounded (depends on the allocator) worst case execution time (which is bad / unacceptable for hard real time applications).

`heapless` data structures don't use a memory allocator which means no risk of an uncatchable Out Of Memory (OOM) condition while performing operations on them. It's certainly possible to run out of capacity while growing `heapless` data structures, but the API lets you handle this possibility by returning a `Result` on operations that may exhaust the capacity of the data structure.

List of currently implemented data structures:

- `BinaryHeap` -- priority queue
- `IndexMap` -- hash table
- `IndexSet` -- hash set
- `LinearMap`
- `Pool` -- lock-free memory pool
- `String`
- `Vec`
- `mpmc::Q*` -- multiple producer multiple consumer lock-free queue
- `spsc::Queue` -- single producer single consumer lock-free queue

# Optional Features

The `heapless` crate provides the following optional Cargo features:

- `ufmt-impl`: Implement `ufmt_write::uWrite` for `String<N>` and `Vec<u8, N>`

# Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.36 and up with its default set of features. It might compile on older versions but that may change in any new patch release.

## Re-exports

`pub use binary_heap::BinaryHeap;`

## Modules

binary_heap | A priority queue implemented with a binary heap.
consts | Type aliases for many constants.
i | Unfortunate implementation detail required to construct `heapless` types in const context
mpmc | A fixed capacity Multiple-Producer Multiple-Consumer (MPMC) lock-free queue
pool | A heap-less, interrupt-safe, lock-free memory pool (*)
spsc | Fixed capacity Single Producer Single Consumer (SPSC) queue

## Structs

HistoryBuffer | A "history buffer", similar to a write-only ring buffer of fixed length.
IndexMap | Fixed capacity `IndexMap`
IndexSet | Fixed capacity `IndexSet`.
LinearMap | A fixed capacity map / dictionary that performs lookups via linear search
String | A fixed capacity `String`
Vec | A fixed capacity `Vec`

## Traits

ArrayLength | Trait making `GenericArray` work, marking types to be used as length of an array
PowerOfTwo | The marker trait for type-level numbers which are a power of two.

## Type Definitions

FnvIndexMap | A `heaples::IndexMap` using the default FNV hasher
FnvIndexSet | A `heapless::IndexSet` using the default FNV hasher. A list of all Methods and Traits available for `FnvIndexSet` can be found in the `heapless::IndexSet` documentation.