Rust Cow用法及代碼示例

本文簡要介紹rust語言中 Enum alloc::borrow::Cow 的用法。

用法

pub enum Cow<'a, B: ?Sized + 'a> where    B: ToOwned,  {
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

clone-on-write 智能指針。

類型 Cow 是一個提供 clone-on-write 函數的智能指針：它可以封裝並提供對借用數據的不可變訪問，並在需要突變或所有權時延遲克隆數據。該類型旨在通過 Borrow 特征處理一般借用數據。

Cow 實現了 Deref ，這意味著您可以直接在其包含的數據上調用非變異方法。如果需要突變，to_mut 將獲得對擁有值的可變引用，並在必要時進行克隆。

如果您需要引用計數指針，請注意 Rc::make_mut 和 Arc::make_mut 也可以提供 clone-on-write 函數。

例子

use std::borrow::Cow;

fn abs_all(input: &mut Cow<[i32]>) {
    for i in 0..input.len() {
        let v = input[i];
        if v < 0 {
            // Clones into a vector if not already owned.
            input.to_mut()[i] = -v;
        }
    }
}

// No clone occurs because `input` doesn't need to be mutated.
let slice = [0, 1, 2];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// Clone occurs because `input` needs to be mutated.
let slice = [-1, 0, 1];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// No clone occurs because `input` is already owned.
let mut input = Cow::from(vec![-1, 0, 1]);
abs_all(&mut input);

另一個顯示如何將 Cow 保留在結構中的示例：

use std::borrow::Cow;

struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
    values: Cow<'a, [X]>,
}

impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
    fn new(v: Cow<'a, [X]>) -> Self {
        Items { values: v }
    }
}

// Creates a container from borrowed values of a slice
let readonly = [1, 2];
let borrowed = Items::new((&readonly[..]).into());
match borrowed {
    Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
    _ => panic!("expect borrowed value"),
}

let mut clone_on_write = borrowed;
// Mutates the data from slice into owned vec and pushes a new value on top
clone_on_write.values.to_mut().push(3);
println!("clone_on_write = {:?}", clone_on_write.values);

// The data was mutated. Let check it out.
match clone_on_write {
    Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
    _ => panic!("expect owned data"),
}

變體

`Borrowed(&'a B)`

元組字段

0: &'a B

借來的資料。

`Owned(<B as ToOwned>::Owned)`

元組字段

0: <B as ToOwned>::Owned

擁有的數據。

相關用法

注：本文由純淨天空篩選整理自rust-lang.org大神的英文原創作品 Enum alloc::borrow::Cow。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。