为什么不鼓励接受对String的引用（

Question

我写了一些带

fn awesome_greeting(name: &String) {
    println!("Wow, you are awesome, {}!", name);
}

我还编写了代码，其中包含对Vec或框的引用：

fn total_price(prices: &Vec<i32>) -> i32 {
    prices.iter().sum()
}

fn is_even(value: &Box<i32>) -> bool {
    **value % 2 == 0
}

然而，我收到了一些反馈，认为这样做不是一个好主意。为什么不呢？




Answer 1

除了Shepmaster的回答之外，接受

如果您有：

fn awesome_greeting(name: &String) {
    println!("Wow, you are awesome, {}!", name);
}

但是你需要用奶牛来称呼它

let c: Cow<str> = Cow::from("hello");
// Allocate an owned String from a str reference and then makes a reference to it anyway!
awesome_greeting(&c.to_string());

当您将参数类型更改为时

let c: Cow<str> = Cow::from("hello");
// Just pass the same reference along
awesome_greeting(&c);

let c: Cow<str> = Cow::from(String::from("hello"));
// Pass a reference to the owned string that you already have
awesome_greeting(&c);

接受

Answer 2

TL； DR：可以改为使用

>

接受

awesome_greeting(&String::from("Anna"));

total_price(&vec![42, 13, 1337])

is_even(&Box::new(42))

另一个性能考虑因素是

相反，您应该接受字符串片段（

fn awesome_greeting(name: &str) {
    println!("Wow, you are awesome, {}!", name);
}

fn total_price(prices: &[i32]) -> i32 {
    prices.iter().sum()
}

fn is_even(value: &i32) -> bool {
    *value % 2 == 0
}

现在，您可以使用更广泛的类型集调用这些方法。例如，可以使用字符串文本（“Anna”）或分配的字符串来调用awesome\u greeting<代码>总价可以通过对数组（

如果要添加或删除字符串或Vec中的项目

fn add_greeting_target(greeting: &mut String) {
    greeting.push_str("world!");
}

fn add_candy_prices(prices: &mut Vec<i32>) {
    prices.push(5);
    prices.push(25);
}

特别是对于切片，您也可以接受

fn reset_first_price(prices: &mut [i32]) {
    prices[0] = 0;
}

fn lowercase_first_ascii_character(s: &mut str) {
    if let Some(f) = s.get_mut(0..1) {
        f.make_ascii_lowercase();
    }
}