CODEWITHSUNDEEP
rust
TSK
TSK

Reputation: 751

Use `for` loop on rust array and vec

Use for loop on a rust array works correctly:

fn main() {
    let v = [1, 2, 3, 4, 5];
    for _ in v.into_iter() {}
    for _ in v.into_iter() {}
}

But substituting a vec doesn't compile:

fn main() {
    let v = vec![1, 2, 3, 4, 5];
    for _ in v.into_iter() {}
    for _ in v.into_iter() {}
}

The error:

use of moved value: `v`

I understand why this program does not work with vec. But why does it work with array? I was expecting a similar error in the array example, but it gives no error.

Upvotes: 3

Views: 899

Answers (1)

spdrman
spdrman

Reputation: 1503

Added detail:

  • Arrays are stored on the stack in Rust (like C/C++), therefore, they can be moved -- Copy() in rust -- which makes a deep copy of the data, but it's ok because stack data is supposed to be light-weight.

  • Vectors are stored on the heap in Rust (like C/C++), therefore, they usually CANNOT be moved, and must usually be deep copied.

Here is a great explanation on the difference between Copy() and Clone()

Original Response:

As another commenter mentioned, Arrays in Rust implement the Copy trait, and can therefore be passed-by-value multiple times, whereas Vector types must be explicitly clone()d to achieve the same behavior.

In Rust, when a function's parameter is pass-by-value, the compiler defaults to doing a move of a copy on all calls except the last one, on the last call it will transfer ownership of the original, instead of a copy/clone. The compiler will not automatically run clone(), if Copy isn't implemented.

Here is the doc for Copy trait: https://doc.rust-lang.org/std/marker/trait.Copy.html

Array's impl of Copy documentation can be found here: https://doc.rust-lang.org/std/primitive.array.html#impl-Copy-for-%5BT%3B%20N%5D

Here is a great article with more details: https://colinsblog.net/2021-04-16-rust-ownership-comparisons/

Upvotes: 4

Related Questions