rust 的Clone - 技术栈

Clone 是 Rust 编程语言中一个核心特质（trait），定义了类型如何安全、明确地创建其值的深拷贝（deep copy）。

下面用实例来演示Clone的作用，先看一下如下的代码，注意此代码编译不过。

rust 复制代码

#[derive(Debug)]
struct Item{
    value: i32,
}

fn main() {
    let a = Item{value:7};
    let b = a;
    println!("value a {:?}, value b {:?}", a, b);
}

编译报错：

cargo run

error $E0382$ : borrow of moved value: `a`

--> src/main.rs:10:44

|

8 | let a = Item{value:7};

| - move occurs because `a` has type `Item`, which does not implement the `Copy` trait

9 | let b = a;

| - value moved here

10 | println!("value a {:?}, value b {:?}", a, b);

| ^ value borrowed here after move

|

note: if `Item` implemented `Clone`, you could clone the value

--> src/main.rs:3:1

|

3 | struct Item{

| ^^^^^^^^^^^ consider implementing `Clone` for this type

...

9 | let b = a;

| - you could clone this value

= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0382`.

报错的意思是变量a的所有权已经被移动到b，所以println！无法再使用a。

如果要想Item类的变量赋值后所有权继续有效，就需要Item类实现clone()。

方案一：手动为Item类实现trait Clone

rust 复制代码

#[derive(Debug)]
struct Item{
    value: i32,
}

impl Clone for Item{
    fn clone(&self) -> Self{
        Item { value: self.value, }
    }
}

fn main() {
    let a = Item{value:7};
    let b = a.clone();
    println!("value a {:?}, value b {:?}", a, b);
}

编译运行：

Running `target\debug\greeting.exe`

value a Item { value: 7 }, value b Item { value: 7 }

方案二：

使用属性（Attribute）

rust 复制代码

#[derive(Debug, Clone)]
struct Item{
    value: i32,
}


fn main() {
    let a = Item{value:7};
    let b = a.clone();
    println!("value a {:?}, value b {:?}", a, b);
}

编译运行

Finished `dev` profile $unoptimized + debuginfo$ target(s) in 0.01s

Running `target\debug\greeting.exe`

value a Item { value: 7 }, value b Item { value: 7 }