vector - Automatic casting Vec to array -

while trying different ways send vec<u8> function expects &[u8], made "mistake" works. wrote sample code show this:

fn sum(v: &[u8]) -> u8 {     let mut s = 0u8;     x in v.iter() {         s = s + x;     }     return s; }  fn main() {     let myarr = [1u8, 2u8, 3u8];     let myvec = vec![1u8, 2u8, 3u8];      println!("{}", sum(&myarr));     println!("{}", sum(&myvec)); } 

i have few questions in relation this:

• why , how work?
• is there automatic casting between these 2 types?
• does incur penalty or taking memory location of underlying array of vector?
• does mean type of usage (read operations on array of numbers) better use array , not vector api?

let's 1 thing out of way: there no arrays in sum function. rust has 3 related types, , can search here on stack overflow or the rust programming language more information them:

1. slices &[t].
2. vectors vec<t>
3. arrays [t; n]

the v argument slice, pointer chunk of data , number of elements.

why , how work? there automatic casting between these 2 types?

the deref trait coming play here. there's implementation of trait looks like:

impl<t> deref vec<t> {     type target = [t];     fn deref(&self) -> &[t]; } 

this means reference vec<t> can act reference [t] , gains of methods target type. deref understood compiler, type able implement it, it's sort of special.

does incur penalty or taking memory location of underlying array of vector?

this zero-cost transformation, why compiler you. it's unusual have compiler expensive.

does mean type of usage better use array , not vector api?

absolutely! 100% of time should accept &[t] instead of &vec<t>. more things vec can provide &[t], , arrays example of that.