2022-02-11 00:42:25 -06:00
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# &'static and T: 'static
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2022-04-09 06:32:01 -05:00
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`'static` 是一个 Rust 保留的生命周期名称,在之前我们可能已经见过好几次了:
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```rust
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// 引用的生命周期是 'static :
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let s: &'static str = "hello world";
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2022-02-11 00:42:25 -06:00
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2022-04-09 06:32:01 -05:00
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// 'static 也可以用于特征约束中:
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fn generic<T>(x: T) where T: 'static {}
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```
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虽然它们都是 `'static`,但是也稍有不同。
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## &'static
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作为一个引用生命周期,`&'static` 说明该引用指向的数据可以跟程序活得一样久,但是该引用的生命周期依然有可能被强转为一个更短的生命周期。
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2022-11-06 22:06:19 -06:00
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1. 🌟🌟 有好几种方法可以将一个变量标记为 `'static` 生命周期, 其中两种都是和保存在二进制文件中相关( 例如字符串字面量就是保存在二进制文件中,它的生命周期是 `'static` )。
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2022-04-09 06:32:01 -05:00
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```rust,editable
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/* 使用两种方法填空 */
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fn main() {
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__;
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need_static(v);
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println!("Success!")
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}
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fn need_static(r : &'static str) {
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assert_eq!(r, "hello");
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}
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```
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2022-11-06 22:06:19 -06:00
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2. 🌟🌟🌟🌟 使用 `Box::leak` 也可以产生 `'static` 生命周期
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2022-04-09 06:32:01 -05:00
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```rust,editable
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#[derive(Debug)]
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struct Config {
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a: String,
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b: String,
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}
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static mut config: Option<&mut Config> = None;
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/* 让代码工作,但不要修改函数的签名 */
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fn init() -> Option<&'static mut Config> {
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Some(&mut Config {
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a: "A".to_string(),
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b: "B".to_string(),
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})
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}
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fn main() {
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unsafe {
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config = init();
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println!("{:?}",config)
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}
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}
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```
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2022-11-06 22:06:19 -06:00
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3. 🌟 `&'static` 只能说明引用指向的数据是能一直存活的,但是引用本身依然受限于它的作用域
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2022-04-09 06:32:01 -05:00
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```rust,editable
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fn main() {
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{
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// 字符串字面量能跟程序活得一样久,因此 `static_string` 的生命周期是 `'static`
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let static_string = "I'm in read-only memory";
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println!("static_string: {}", static_string);
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// 当 `static_string` 超出作用域时,该引用就无法再被使用,但是引用指向的数据( 字符串字面量 ) 依然保存在二进制 binary 所占用的内存中
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}
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println!("static_string reference remains alive: {}", static_string);
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}
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```
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2022-11-06 22:06:19 -06:00
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4. `&'static` 可以被强转成一个较短的生命周期
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2022-04-09 06:32:01 -05:00
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**Example**
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```rust,editable
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// 声明一个 static 常量,它拥有 `'static` 生命周期.
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static NUM: i32 = 18;
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// 返回常量 `Num` 的引用,注意,这里的生命周期从 `'static` 强转为 `'a`
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fn coerce_static<'a>(_: &'a i32) -> &'a i32 {
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&NUM
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}
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fn main() {
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{
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let lifetime_num = 9;
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let coerced_static = coerce_static(&lifetime_num);
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println!("coerced_static: {}", coerced_static);
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}
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println!("NUM: {} stays accessible!", NUM);
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}
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```
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## T: 'static
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关于 `'static` 的特征约束详细解释,请参见 [Rust 语言圣经](https://course.rs/advance/lifetime/static.html#t-static),这里就不再赘述。
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2022-11-06 22:06:19 -06:00
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5. 🌟🌟
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2022-04-09 06:32:01 -05:00
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```rust,editable
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/* 让代码工作 */
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use std::fmt::Debug;
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fn print_it<T: Debug + 'static>( input: T) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn print_it1( input: impl Debug + 'static ) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn print_it2<T: Debug + 'static>( input: &T) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn main() {
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// i 是有所有权的数据,并没有包含任何引用,因此它是 'static
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let i = 5;
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print_it(i);
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// 但是 &i 是一个引用,生命周期受限于作用域,因此它不是 'static
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print_it(&i);
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print_it1(&i);
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// 但是下面的代码可以正常运行 !
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print_it2(&i);
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}
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```
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2022-11-06 22:06:19 -06:00
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6. 🌟🌟🌟
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2022-02-11 00:42:25 -06:00
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```rust,editable
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use std::fmt::Display;
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fn main() {
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let mut string = "First".to_owned();
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string.push_str(string.to_uppercase().as_str());
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print_a(&string);
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print_b(&string);
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print_c(&string); // Compilation error
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print_d(&string); // Compilation error
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print_e(&string);
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print_f(&string);
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print_g(&string); // Compilation error
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}
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fn print_a<T: Display + 'static>(t: &T) {
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println!("{}", t);
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}
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fn print_b<T>(t: &T)
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where
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T: Display + 'static,
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{
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println!("{}", t);
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}
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fn print_c(t: &'static dyn Display) {
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println!("{}", t)
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}
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fn print_d(t: &'static impl Display) {
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println!("{}", t)
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}
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fn print_e(t: &(dyn Display + 'static)) {
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println!("{}", t)
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}
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fn print_f(t: &(impl Display + 'static)) {
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println!("{}", t)
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}
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fn print_g(t: &'static String) {
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println!("{}", t);
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}
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2022-08-04 22:41:15 -05:00
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```
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