如何为包含字符串的类型(或任何未实现复制的类型)实现复制和克隆? [英] How do I implement Copy and Clone for a type that contains a String (or any type that doesn't implement Copy)?

问题描述

我在 Rust 中有一个枚举，它有一个带有 String 的值:

I have an enum in Rust which has one value that takes a String:

#[derive(Clone, Copy)]
enum Simple {
    Error(String),
    Okay,
    Foo([u32; 5]),
}

fn main() {
    let x = Simple::Error(String::from("blah"));
    let y = x.clone();
}

上面的枚举值 Foo 代表我使用的大约 10 个其他枚举，它们采用可复制的类型或数组.编译器似乎没有抱怨它们，只有 Error(String) 导致了这个:

The enum value Foo above represents about 10 other enums I use that take copyable types or arrays of them. The compiler doesn't seem to complain about them, only the Error(String) which causes this:

error[E0204]: the trait `Copy` may not be implemented for this type
 --> src/main.rs:1:17
  |
1 | #[derive(Clone, Copy)]
  |                 ^^^^
2 | enum Simple {
3 |     Error(String),
  |           ------ this field does not implement `Copy`
  |

出于某种原因，String 是不可复制的.我不明白这个.我如何为枚举实现 Clone 仅针对一种类型的枚举，而在其余类型使用默认 impl 时出现问题?

For some reason, String is not copyable. I don't get this. How do I implement Clone for an enum for just the one type which has a problem while using the default impl for the rest?

推荐答案

复制

复制指定类型，通过按位复制为其创建有效实例而不会使原始实例无效.

Copy

Copy designates types for which making a bitwise copy creates a valid instance without invalidating the original instance.

这不适用于 String，因为 String 包含一个指向堆上字符串数据的指针，并假定它对该数据具有唯一的所有权.当您删除 String 时，它会释放堆上的数据.如果您对 String 进行了按位复制，那么两个实例都会尝试释放相同的内存块，这是未定义行为.

This isn't true for String, because String contains a pointer to the string data on the heap and assumes it has unique ownership of that data. When you drop a String, it deallocates the data on the heap. If you had made a bitwise copy of a String, then both instances would try to deallocate the same memory block, which is undefined behaviour.

由于String没有实现Copy，你的enum也不能实现Copystrong> 因为编译器强制 Copy 类型仅由 Copy 数据成员组成.

Since String doesn't implement Copy, your enum cannot implement Copy either because the compiler enforces that Copy types are composed only of Copy data members.

克隆仅仅提供了一个标准的clone 方法，由每个实现者决定如何实现它.String 确实实现了 Clone，所以你可以把 #[derive(Clone)] 放在你的 enum 上.

Clone merely provides a standard clone method, and it's up to each implementor to decide how to implement it. String does implement Clone, so you can put #[derive(Clone)] on your enum.

这篇关于如何为包含字符串的类型(或任何未实现复制的类型)实现复制和克隆?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！