rust 的Clone

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。

#[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

#[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）

#[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 }