初始化固定长度数组的正确方法是什么？

Question

我在初始化固定长度数组时遇到问题。 My attempts so far all result in the same "use of possibly uninitialized variable: foo_array" error：

#[derive(Debug)]
struct Foo { a: u32, b: u32 }

impl Default for Foo {
    fn default() -> Foo { Foo{a:1, b:2} }
}

pub fn main() {
    let mut foo_array: [Foo; 10];

    // Do something here to in-place initialize foo_array?

    for f in foo_array.iter() {
        println!("{:?}", f);
    }
}

error[E0381]: use of possibly uninitialized variable: `foo_array`
  --> src/main.rs:13:14
   |
13 |     for f in foo_array.iter() {
   |              ^^^^^^^^^ use of possibly uninitialized `foo_array`

我实现了Default特性，但是Rust似乎并没有将其称为默认类似于C ++构造函数。

初始化固定长度数组的正确方法是什么？我想做一个有效的就地初始化而不是某种副本。

相关：Why is the Copy trait needed for default (struct valued) array initialization?

相关：Is there a way to not have to initialize arrays twice?

Answer 1

安全，但somewhat inefficient solution：

#[derive(Copy, Clone, Debug)]
struct Foo {
    a: u32,
    b: u32,
}

fn main() {
    let mut foo_array = [Foo { a: 10, b: 10 }; 10];
}

因为你特别要求a solution without copies：

use std::{mem, ptr};

#[derive(Debug)]
struct Foo {
    a: u32,
    b: u32,
}

// We're just implementing Drop to prove there are no unnecessary copies.
impl Drop for Foo {
    fn drop(&mut self) {
        println!("Destructor running for a Foo");
    }
}

pub fn main() {
    let array = unsafe {
        // Create an uninitialized array.
        let mut array: [Foo; 10] = mem::uninitialized();

        for (i, element) in array.iter_mut().enumerate() {
            let foo = Foo { a: i as u32, b: 0 };

            // Overwrite `element` without running the destructor of the old value.
            // Since Foo does not implement Copy, it is moved.
            ptr::write(element, foo)
        }

        array
    };

    for element in array.iter() {
        println!("{:?}", element);
    }
}

Answer 2

你可以使用arrayvec crate：

Cargo.toml

[package]
name = "initialize_array"
version = "0.1.0"
authors = ["author"]
edition = "2018"

[dependencies]
arrayvec = "0.4.10"

SRC / main.rs

use arrayvec::ArrayVec; 
use std::iter;

#[derive(Clone)]
struct Foo {
    a: u32,
    b: u32,
}

fn main() {
    let foo_array: [Foo; 10] = iter::repeat(Foo { a: 10, b: 10 })
        .collect::<ArrayVec<_>>()
        .into_inner()
        .unwrap_or_else(|_| unreachable!());
}