#map #vec #filter-map


Reuse the memory of a Vec, Box<[T]> or Box when mapping the elements if possible

1 unstable release

Uses old Rust 2015

0.1.0 Aug 18, 2016

#90 in #vec

41 downloads per month
Used in 3 crates (via prefixopt)

Apache-2.0 / MIT

222 lines


Reuse alloations when mapping the elements of a Vec, Box<[T]> or Box<T> if possible.

To map in place the types must have identical alignment and:

  • for boxes and boxed slices the sizes must be equal,
  • for vectors the size of in must be a multiple of the out type. (so out cannot be bigger than in)

The ..._in_place() methods will panic if not possible, while the others will fall back to iterating and collecting.


extern crate map_in_place;
use map_in_place::MapVecInPlace;
fn main() {
    let v = vec![8_u32,29,14,5];
    let v = v.filter_map(|n| if n < 10 {Some( (n as u8+b'0') as char)}
                             else      {None}
                        );// happens in place
    assert_eq!(&v, &['8','5']);
    let v = v.map(|c| c as u8);// falls back to iterators
    assert_eq!(&v[..], &b"85"[..]);

Why all those restrictions?

The rust allocation interface is a bit more complex than the standard C one of malloc(size_t) and free(void*):

First, malloc and free takes the alignment of the types you want to store, and allocating with one alignment and freeing with another is undefined behaviour.

Second, rust requires the owner to know the size of the memory to free, which means one of the types' size must be a multiple of the other, since the capacity is an integer.


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