将字符转换为 &str [英] Converting a char to &str
问题描述
我有一个 match
语句,它返回一个 &str
:
I have a match
statement which returns a &str
:
match k {
SP_KEY_1 => "KEY_1",
SP_KEY_2 => "KEY_2",
SP_KEY_3 => "KEY_3",
SP_KEY_4 => "KEY_4",
SP_KEY_5 => "KEY_5",
SP_KEY_6 => "KEY_6",
_ => (k as char), // I want to convert this to &str
}.as_bytes()
我尝试先将 char
转换为字符串,然后再取一部分:
I have tried to convert a char
to a string first, and then taking a slice of that:
&(k as char).to_string()[..]
但这给了我终生错误:
error[E0597]: borrowed value does not live long enough
声明 (k as char).to_string()
是一个临时值,这是有道理的,因为我可以告诉 to_string()
返回一个克隆.
Stating that (k as char).to_string()
is a temporary value, which makes sense as from what I can tell to_string()
returns a clone.
我可以将 .to_string()
添加到上面的每个 &str
文字中以生成返回值 String
,但这看起来都很丑陋(很多重复的代码),而且可能效率低下,因为 to_string()
克隆了原始字符串切片.
I can add .to_string()
to every &str
literal above to make the return value String
, but that seems both ugly (a lot of repeated code), and probably inefficient, as to_string()
clones the original string slice.
具体问题是我如何将 char
变成 &str
,但更广泛的问题是是否有更好的解决方案,通常这样做情况.
The specific question is how would I make a char
into a &str
, but the broader question is that is there a better solution, what is commonly done this situation.
推荐答案
只要不需要从函数中返回&str
,就可以完全避免使用char::encode_utf8
:
So long as you don't need to return the &str
from the function, you can completely avoid heap allocation using char::encode_utf8
:
const SP_KEY_1: u8 = 0;
const SP_KEY_2: u8 = 1;
const SP_KEY_3: u8 = 2;
const SP_KEY_4: u8 = 3;
const SP_KEY_5: u8 = 4;
const SP_KEY_6: u8 = 5;
fn main() {
let k = 42u8;
let mut tmp = [0; 4];
let s = match k {
SP_KEY_1 => "KEY_1",
SP_KEY_2 => "KEY_2",
SP_KEY_3 => "KEY_3",
SP_KEY_4 => "KEY_4",
SP_KEY_5 => "KEY_5",
SP_KEY_6 => "KEY_6",
_ => (k as char).encode_utf8(&mut tmp),
};
println!("{}", s);
}
如果您需要更多控制,这可以与闭包配对:
This could be paired with a closure if you needed more control:
fn adapt<F, B>(k: u8, f: F) -> B
where
for<'a> F: FnOnce(&'a str) -> B,
{
let mut tmp = [0; 4];
let s = match k {
SP_KEY_1 => "KEY_1",
SP_KEY_2 => "KEY_2",
SP_KEY_3 => "KEY_3",
SP_KEY_4 => "KEY_4",
SP_KEY_5 => "KEY_5",
SP_KEY_6 => "KEY_6",
_ => (k as char).encode_utf8(&mut tmp),
};
f(s)
}
fn main() {
adapt(0, |s| println!("{}", s));
let owned = adapt(0, |s| s.to_owned());
}
或者存储在结构体中以提供一点抽象:
Or stored in a struct to provide a little bit of abstraction:
#[derive(Debug, Default)]
struct Foo {
tmp: [u8; 4],
}
impl Foo {
fn adapt(&mut self, k: u8) -> &str {
match k {
SP_KEY_1 => "KEY_1",
SP_KEY_2 => "KEY_2",
SP_KEY_3 => "KEY_3",
SP_KEY_4 => "KEY_4",
SP_KEY_5 => "KEY_5",
SP_KEY_6 => "KEY_6",
_ => (k as char).encode_utf8(&mut self.tmp),
}
}
}
fn main() {
let mut foo = Foo::default();
{
let s = foo.adapt(0);
}
{
let s = foo.adapt(42);
}
}
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