cap_vec

A heap-allocated, fixed-capacity, variable-size array, no_std compatible.

CapVec<T, N> provides a middle ground between stack-allocated arrays [T; N] and dynamically growing vectors Vec<T>. It allocates a heap-backed buffer of fixed capacity N, but allows the logical length to grow or shrink dynamically, up to that capacity.

✨ Features

• ✅ Fixed capacity — set at compile time.
• ✅ Heap allocation — no stack overflow even for large N.
• ✅ no_std compatible — uses only alloc.
• ✅ Supports most common collection operations:
  • push, pop, insert, remove
  • clear, extend, drain
  • iter, iter_mut, and into_iter
• ✅ Safe, ergonomic API — mirrors Vec<T> where possible.
• ✅ Zero-cost iteration (implements Iterator, DoubleEndedIterator, FusedIterator).
• ✅ No hidden allocations after initialization.

🚀 Example

use cap_vec::CapVec;

fn main() {
    // Create a CapVec with capacity for 4 elements
    let mut cv = CapVec::<i32, 4>::new();

    // Push values
    cv.push(10).unwrap();
    cv.push(20).unwrap();
    cv.push(30).unwrap();

    assert_eq!(cv.len(), 3);
    assert_eq!(cv.capacity(), 4);
    assert_eq!(cv.as_slice(), &[10, 20, 30]);

    // Insert in the middle
    cv.insert(1, 15).unwrap();
    assert_eq!(cv.as_slice(), &[10, 15, 20, 30]);

    // Remove one element
    let removed = cv.remove(2);
    assert_eq!(removed, Some(20));
    assert_eq!(cv.as_slice(), &[10, 15, 30]);

    // Iterate immutably
    for x in cv.iter() {
        println!("{x}");
    }

    // Iterate mutably
    for x in cv.iter_mut() {
        *x *= 2;
    }

    assert_eq!(cv.as_slice(), &[20, 30, 60]);

    // Consume into an iterator
    let v: Vec<_> = cv.into_iter().collect();
    assert_eq!(v, vec![20, 30, 60]);
}

🔨 Installation

Add cap_vec to your Cargo.toml:

cargo add cap_vec

or edit your Cargo.toml manually by adding:

[dependencies]
cap_vec = "0.2"

Safety and Coverage

This crate contains a small portion of unsafe code.
All tests run under miri and the tests cover about 80% of the code.
You can generate the coverage report using tarpaulin.

Contributions

Contributions are always welcome! Feel free to open an issue or submit a pull request.

License

This crate is licensed under the MIT License. See LICENSE for more details.