如何创建包含大型数组的结构数组？ [英] How to create an array of structs containing large arrays?

问题描述

如何创建包含固定大小的大数组的结构数组？我想使用数组而不是向量。

How could I create an array of structs containing big arrays with a fixed size? I want to use arrays and not vectors.

此代码是一个示例，但不编译

This code is an example but doesn't compile

struct _Tmove {
    data1: usize,
    data2: u64,
    data3: bool,
}

struct _TmoveP {
    data4: Box<[_Tmove]>,
    data5: isize,
}

fn main() {
    let mut gen_list = Box::new([
        _TmoveP {
            data5: 1,
            data4: Box::new([_Tmove { data1: 5, data2: 1, data3: true }; 300]),
        }
        ; 100000]);

    assert!(gen_list[0].data4[0].data1==5);
}

error[E0277]: the trait bound `_Tmove: std::marker::Copy` is not satisfied
--> src/main.rs:16:29
       |
    16 |             data4: Box::new([_Tmove { data1: 5, data2: 1, data3: true }; 300]),
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the `Copy` trait is required because the repeated element will be copied

error[E0277]: the trait bound `_TmoveP: std::marker::Copy` is not     satisfied
--> src/main.rs:13:33
    |
13  |     let mut gen_list = Box::new([
    |                                 ^
    |
    = note: the `Copy` trait is required because the repeated element will be copied

我正在使用Rust 1.12。

I am using Rust 1.12.

推荐答案

为了受益于初始化语法： [expr; N] ，需要将 expr 的结果复制（因为需要制作副本）。

In order to benefit from the initialization syntax: [expr; N], the result of expr need to be Copy (as copies need be made).

#[derive(Copy, Clone)]
struct _Tmove {
    data1: usize,
    data2: u64,
    data3: bool,
}

#[derive(Copy, Clone)]
struct _TmoveP {
    data4: Box<[_Tmove]>,
    data5: isize,
}

但是，在这种情况下， _TmoveP 不能 Copy ，因为它包含一个 Box ，而不是 Copy 。

However, in this case, _TmoveP cannot be Copy because it contains a Box which is not Copy.

好的，让我们摆脱 Box ：

#[derive(Copy, Clone)]
struct _TmoveP {
    data4: [_Tmove; 300],
    data5: isize,
}

听起来很棒吗？

但不幸的是， [_ Tmove; 300] 也不是 Clone 之一：（很遗憾，我们遇到了Rust编译器的限制（它的大小小于32）。

But unfortunately, [_Tmove; 300] is not Clone either :( We are unfortunately hitting a limitation of the Rust compiler (it works for size less than 32).

复制很容易...但是首先我们必须实现 Clone 天真的方法很有趣，但是很简单：

The Copy is easy enough... but first we have to implement Clone manually. The naive way is no fun, but it's easy enough:

impl Clone for _TmoveP {
    fn clone(&self) -> _TmoveP {
        unsafe {
            let mut res = _TmoveP {
                data4: std::mem::uninitialized(),
                data5: self.data5,
            };

            std::ptr::copy_nonoverlapping(
                &self.data4 as *const _Tmove,
                std::mem::transmute(&mut res.data4),
                300
            );

            res
        }
    }
}

注意：出于某些原因，& mut res.data4为* mut _ 无法编译...无论：x

Note: for some reason &mut res.data4 as *mut _ would not compile... whatever :x

但是， @FrancisGagné在评论中提醒我， Copy 类型有一个怪异的窍门：

However, @Francis Gagné reminded me in the comments, there is a weird trick with Copy types:

impl Clone for _TmoveP {
    fn clone(&self) -> _TmoveP { *self }
}

此方法由于某些原因很方便

This works, for some reason, and is handy in those situations.

最后，这可行...哦，等等， main 中有问题！

And, finally, this works... oh wait, there's an issue in main!

fn main() {
    let gen_list = Box::new([
        _TmoveP {
            data5: 1,
            data4: [_Tmove { data1: 5, data2: 1, data3: true }; 300],
        }
        ; 100000]);

    assert!(gen_list[0].data4[0].data1==5);
}

好的，我们到这里。

仅对小于32的大小真正起作用的数组有什么处理？

What's the deal with arrays only really working for sizes smaller than 32?

简单地说：Rust还没有（现在？）对non的支持。类型的通用参数。

Simply put: Rust does not have (yet?) support for non-type generic parameters.

数组在某种程度上是特殊情况，但需要独立地为每个大小实现特征...因此标准库为数组实现了其特征到32号，因为这似乎是一个不错的权衡。

Arrays are special-cased, to an extent, but require implementing traits for each size independently... so the standard library implements its traits for arrays up to size 32, because it seemed a good trade-off.

这篇关于如何创建包含大型数组的结构数组？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！