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< main >
< h1 id = "static-和-t-static" > < a class = "header" href = "#static-和-t-static" > & ’ ’ < / a > < / h1 >
< p > Rust 的难点之一就在于它有不少容易混淆的概念，例如 < code > & str< / code > 、< code > str< / code > 与 < code > String< / code > ， < / p >
< p > 与生命周期的其它章节不同， ， ， ， ’ < / p >
< p > < code > 'static< / code > 在 Rust 中是相当常见的，例如字符串字面值就具有 < code > 'static< / code > 生命周期:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mark_twain: & str = "Samuel Clemens";
print_author(mark_twain);
}
fn print_author(author: & 'static str) {
println!("{}", author);
}< / code > < / pre >
< p > 除此之外，特征对象的生命周期也是 < code > 'static< / code > ，例如< a href = "https://course.rs/compiler/fight-with-compiler/lifetime/closure-with-static.html#特征对象的生命周期" > 这里< / a > 所提到的。< / p >
< p > 除了 < code > & 'static< / code > 的用法外，我们在另外一种场景中也可以见到 < code > 'static< / code > 的使用:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Display;
fn main() {
let mark_twain = "Samuel Clemens";
print(& mark_twain);
}
fn print< T: Display + 'static> (message: & T) {
println!("{}", message);
}< / code > < / pre >
< p > 在这里，很明显 < code > 'static< / code > 是作为生命周期约束来使用了。 < strong > 那么问题来了， < code > & 'static< / code > 和 < code > T: 'static< / code > 的用法到底有何区别？< / strong > < / p >
< h2 id = "static" > < a class = "header" href = "#static" > < code > & 'static< / code > < / a > < / h2 >
< p > < code > & 'static< / code > 对于生命周期有着非常强的要求：一个引用必须要活得跟剩下的程序一样久，才能被标注为 < code > & 'static< / code > 。< / p >
< p > 对于字符串字面量来说，它直接被打包到二进制文件中，永远不会被 < code > drop< / code > ，因此它能跟程序活得一样久，自然它的生命周期是 < code > 'static< / code > 。< / p >
< p > 但是，< strong > < code > & 'static< / code > 生命周期针对的仅仅是引用，而不是持有该引用的变量，对于变量来说，还是要遵循相应的作用域规则< / strong > :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::{slice::from_raw_parts, str::from_utf8_unchecked};
fn get_memory_location() -> (usize, usize) {
// “Hello World” 是字符串字面量，因此它的生命周期是 `'static`.
// 但持有它的变量 `string` 的生命周期就不一样了，它完全取决于变量作用域，对于该例子来说，也就是当前的函数范围
let string = "Hello World!";
let pointer = string.as_ptr() as usize;
let length = string.len();
(pointer, length)
// `string` 在这里被 drop 释放
// 虽然变量被释放，无法再被访问，但是数据依然还会继续存活
}
fn get_str_at_location(pointer: usize, length: usize) -> & 'static str {
// 使用裸指针需要 `unsafe{}` 语句块
unsafe { from_utf8_unchecked(from_raw_parts(pointer as *const u8, length)) }
}
fn main() {
let (pointer, length) = get_memory_location();
let message = get_str_at_location(pointer, length);
println!(
"The {} bytes at 0x{:X} stored: {}",
length, pointer, message
);
// 如果大家想知道为何处理裸指针需要 `unsafe`，可以试着反注释以下代码
// let message = get_str_at_location(1000, 10);
}< / code > < / pre >
< p > 上面代码有两点值得注意：< / p >
< ul >
< li > < code > & 'static< / code > 的引用确实可以和程序活得一样久，因为我们通过 < code > get_str_at_location< / code > 函数直接取到了对应的字符串< / li >
< li > 持有 < code > & 'static< / code > 引用的变量，它的生命周期受到作用域的限制，大家务必不要搞混了< / li >
< / ul >
< h2 id = "t-static" > < a class = "header" href = "#t-static" > < code > T: 'static< / code > < / a > < / h2 >
< p > 相比起来，这种形式的约束就有些复杂了。< / p >
< p > 首先，在以下两种情况下，< code > T: 'static< / code > 与 < code > & 'static< / code > 有相同的约束：< code > T< / code > 必须活得和程序一样久。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Debug;
fn print_it< T: Debug + 'static> ( input: T) {
println!( "'static value passed in is: {:?}", input );
}
fn print_it1( input: impl Debug + 'static ) {
println!( "'static value passed in is: {:?}", input );
}
fn main() {
let i = 5;
print_it(& i);
print_it1(& i);
}< / code > < / pre >
< p > 以上代码会报错，原因很简单: < code > & i< / code > 的生命周期无法满足 < code > 'static< / code > 的约束，如果大家将 < code > i< / code > 修改为常量，那自然一切 OK。< / p >
< p > 见证奇迹的时候，请不要眨眼，现在我们来稍微修改下 < code > print_it< / code > 函数:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Debug;
fn print_it< T: Debug + 'static> ( input: & T) {
println!( "'static value passed in is: {:?}", input );
}
fn main() {
let i = 5;
print_it(& i);
}< / code > < / pre >
< p > 这段代码竟然不报错了！原因在于我们约束的是 < code > T< / code > ，但是使用的却是它的引用 < code > & T< / code > ，换而言之，我们根本没有直接使用 < code > T< / code > ，因此编译器就没有去检查 < code > T< / code > 的生命周期约束！它只要确保 < code > & T< / code > 的生命周期符合规则即可，在上面代码中，它自然是符合的。< / p >
< p > 再来看一个例子:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Display;
fn main() {
let r1;
let r2;
{
static STATIC_EXAMPLE: i32 = 42;
r1 = & STATIC_EXAMPLE;
let x = "& 'static str";
r2 = x;
// r1 和 r2 持有的数据都是 'static 的，因此在花括号结束后，并不会被释放
}
println!("& 'static i32: {}", r1); // -> 42
println!("& 'static str: {}", r2); // -> & 'static str
let r3: & str;
{
let s1 = "String".to_string();
// s1 虽然没有 'static 生命周期，但是它依然可以满足 T: 'static 的约束
// 充分说明这个约束是多么的弱。。
static_bound(& s1);
// s1 是 String 类型，没有 'static 的生命周期，因此下面代码会报错
r3 = & s1;
// s1 在这里被 drop
}
println!("{}", r3);
}
fn static_bound< T: Display + 'static> (t: & T) {
println!("{}", t);
}< / code > < / pre >
< h2 id = "static-到底针对谁" > < a class = "header" href = "#static-到底针对谁" > static 到底针对谁？< / a > < / h2 >
< p > 大家有没有想过，到底是 < code > & 'static< / code > 这个引用还是该引用指向的数据活得跟程序一样久呢？< / p >
< p > < strong > 答案是引用指向的数据< / strong > ，而引用本身是要遵循其作用域范围的，我们来简单验证下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
{
let static_string = "I'm in read-only memory";
println!("static_string: {}", static_string);
// 当 `static_string` 超出作用域时，该引用不能再被使用，但是数据依然会存在于 binary 所占用的内存中
}
println!("static_string reference remains alive: {}", static_string);
}< / code > < / pre >
< p > 以上代码不出所料会报错，原因在于虽然字符串字面量 “I’ < code > 'static< / code > ，但是持有它的引用并不是，它的作用域在内部花括号 < code > }< / code > 处就结束了。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/lifetime/static.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的习题解答。（本节暂无习题解答）< / p >
< / blockquote >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 总之， < code > & 'static< / code > 和 < code > T: 'static< / code > 大体上相似，相比起来，后者的使用形式会更加复杂一些。< / p >
< p > 至此，相信大家对于 < code > 'static< / code > 和 < code > T: 'static< / code > 也有了清晰的理解，那么我们应该如何使用它们呢？< / p >
< p > 作为经验之谈，可以这么来:< / p >
< ul >
< li > 如果你需要添加 < code > & 'static< / code > 来让代码工作，那很可能是设计上出问题了< / li >
< li > 如果你希望满足和取悦编译器，那就使用 < code > T: 'static< / code > ，很多时候它都能解决问题< / li >
< / ul >
< blockquote >
< p > 一个小知识，在 Rust 标准库中，有 48 处用到了 & ’ ， < code > T: 'static< / code > ，看来取悦编译器不仅仅是菜鸟需要的，高手也经常用到 :)< / p >
< / blockquote >
< / main >
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