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< h1 id = "drop-释放资源" > < a class = "header" href = "#drop-释放资源" > Drop 释放资源< / a > < / h1 >
< p > 在 Rust 中，我们之所以可以一拳打跑 GC 的同时一脚踢翻手动资源回收，主要就归功于 < code > Drop< / code > 特征，同时它也是智能指针的必备特征之一。< / p >
< h2 id = "学习目标" > < a class = "header" href = "#学习目标" > 学习目标< / a > < / h2 >
< p > 如何自动和手动释放资源及执行指定的收尾工作< / p >
< h2 id = "rust-中的资源回收" > < a class = "header" href = "#rust-中的资源回收" > Rust 中的资源回收< / a > < / h2 >
< p > 在一些无 GC 语言中，程序员在一个变量无需再被使用时，需要手动释放它占用的内存资源，如果忘记了，那么就会发生内存泄漏，最终臭名昭著的 < code > OOM< / code > 问题可能就会发生。< / p >
< p > 而在 Rust 中， ， ， ， ， ， < / p >
< p > 没错，指定这样一段收尾工作靠的就是咱这章的主角 - < code > Drop< / code > 特征。< / p >
< h2 id = "一个不那么简单的-drop-例子" > < a class = "header" href = "#一个不那么简单的-drop-例子" > 一个不那么简单的 Drop 例子< / a > < / h2 >
< pre class = "playground" > < code class = "language-rust edition2021" > struct HasDrop1;
struct HasDrop2;
impl Drop for HasDrop1 {
fn drop(& mut self) {
println!("Dropping HasDrop1!");
}
}
impl Drop for HasDrop2 {
fn drop(& mut self) {
println!("Dropping HasDrop2!");
}
}
struct HasTwoDrops {
one: HasDrop1,
two: HasDrop2,
}
impl Drop for HasTwoDrops {
fn drop(& mut self) {
println!("Dropping HasTwoDrops!");
}
}
struct Foo;
impl Drop for Foo {
fn drop(& mut self) {
println!("Dropping Foo!");
}
}
fn main() {
let _x = HasTwoDrops {
two: HasDrop2,
one: HasDrop1,
};
let _foo = Foo;
println!("Running!");
}< / code > < / pre >
< p > 上面代码虽然长，但是目的其实很单纯，就是为了观察不同情况下变量级别的、结构体内部字段的 < code > Drop< / code > ，有几点值得注意：< / p >
< ul >
< li > < code > Drop< / code > 特征中的 < code > drop< / code > 方法借用了目标的可变引用，而不是拿走了所有权，这里先设置一个悬念，后边会讲< / li >
< li > 结构体中每个字段都有自己的 < code > Drop< / code > < / li >
< / ul >
< p > 来看看输出：< / p >
< pre > < code class = "language-console" > Running!
Dropping Foo!
Dropping HasTwoDrops!
Dropping HasDrop1!
Dropping HasDrop2!
< / code > < / pre >
< p > 嗯，结果符合预期，每个资源都成功的执行了收尾工作，虽然 < code > println!< / code > 这种收尾工作毫无意义 =,=< / p >
< h4 id = "drop-的顺序" > < a class = "header" href = "#drop-的顺序" > Drop 的顺序< / a > < / h4 >
< p > 观察以上输出，我们可以得出以下关于 < code > Drop< / code > 顺序的结论< / p >
< ul >
< li > < strong > 变量级别，按照逆序的方式< / strong > ， < code > _x< / code > 在 < code > _foo< / code > 之前创建，因此 < code > _x< / code > 在 < code > _foo< / code > 之后被 < code > drop< / code > < / li >
< li > < strong > 结构体内部，按照顺序的方式< / strong > ，结构体 < code > _x< / code > 中的字段按照定义中的顺序依次 < code > drop< / code > < / li >
< / ul >
< h4 id = "没有实现-drop-的结构体" > < a class = "header" href = "#没有实现-drop-的结构体" > 没有实现 Drop 的结构体< / a > < / h4 >
< p > 实际上，就算你不为 < code > _x< / code > 结构体实现 < code > Drop< / code > 特征，它内部的两个字段依然会调用 < code > drop< / code > ，移除以下代码，并观察输出：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Drop for HasTwoDrops {
fn drop(& mut self) {
println!("Dropping HasTwoDrops!");
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 原因在于， < code > Drop< / code > 特征，因此就算你不手动为结构体实现 < code > Drop< / code > ，它依然会调用默认实现的 < code > drop< / code > 函数，同时再调用每个字段的 < code > drop< / code > 方法，最终打印出：< / p >
< pre > < code class = "language-console" > Dropping HasDrop1!
Dropping HasDrop2!
< / code > < / pre >
< h2 id = "手动回收" > < a class = "header" href = "#手动回收" > 手动回收< / a > < / h2 >
< p > 当使用智能指针来管理锁的时候，你可能希望提前释放这个锁，然后让其它代码能及时获得锁，此时就需要提前去手动 < code > drop< / code > 。
但是在之前我们提到一个悬念，< code > Drop::drop< / code > 只是借用了目标值的可变引用，所以，就算你提前调用了 < code > drop< / code > ，后面的代码依然可以使用目标值，但是这就会访问一个并不存在的值，非常不安全，好在 Rust 会阻止你：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct Foo;
impl Drop for Foo {
fn drop(& mut self) {
println!("Dropping Foo!")
}
}
fn main() {
let foo = Foo;
foo.drop();
println!("Running!:{:?}", foo);
}< / code > < / pre >
< p > 报错如下：< / p >
< pre > < code class = "language-console" > error[E0040]: explicit use of destructor method
--> src/main.rs:37:9
|
37 | foo.drop();
| ----^^^^--
| | |
| | explicit destructor calls not allowed
| help: consider using `drop` function: `drop(foo)`
< / code > < / pre >
< p > 如上所示，编译器直接阻止了我们调用 < code > Drop< / code > 特征的 < code > drop< / code > 方法，原因是对于 Rust 而言，不允许显式地调用析构函数（这是一个用来清理实例的通用编程概念）。好在在报错的同时，编译器还给出了一个提示：使用 < code > drop< / code > 函数。< / p >
< p > 针对编译器提示的 < code > drop< / code > 函数，我们可以大胆推测下：它能够拿走目标值的所有权。现在来看看这个猜测正确与否，以下是 < code > std::mem::drop< / code > 函数的签名：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn drop< T> (_x: T)
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如上所示，< code > drop< / code > 函数确实拿走了目标值的所有权，来验证下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let foo = Foo;
drop(foo);
// 以下代码会报错：借用了所有权被转移的值
// println!("Running!:{:?}", foo);
}< / code > < / pre >
< p > Bingo， ， ， ！ < / p >
< p > 细心的同学可能已经注意到，这里直接调用了 < code > drop< / code > 函数，并没有引入任何模块信息，原因是该函数在< a href = "https://course.rs/appendix/prelude.html" > < code > std::prelude< / code > < / a > 里。< / p >
< blockquote >
< p > 事实上， ， ， ， ， ， < / p >
< / blockquote >
< h2 id = "drop-使用场景" > < a class = "header" href = "#drop-使用场景" > Drop 使用场景< / a > < / h2 >
< p > 对于 Drop 而言，主要有两个功能：< / p >
< ul >
< li > 回收内存资源< / li >
< li > 执行一些收尾工作< / li >
< / ul >
< p > 对于第二点，在之前我们已经详细介绍过，因此这里主要对第一点进行下简单说明。< / p >
< p > 在绝大多数情况下，我们都无需手动去 < code > drop< / code > 以回收内存资源，因为 Rust 会自动帮我们完成这些工作，它甚至会对复杂类型的每个字段都单独的调用 < code > drop< / code > 进行回收！但是确实有极少数情况，需要你自己来回收资源的，例如文件描述符、网络 socket 等，当这些值超出作用域不再使用时，就需要进行关闭以释放相关的资源，在这些情况下，就需要使用者自己来解决 < code > Drop< / code > 的问题。< / p >
< h2 id = "互斥的-copy-和-drop" > < a class = "header" href = "#互斥的-copy-和-drop" > 互斥的 Copy 和 Drop< / a > < / h2 >
< p > 我们无法为一个类型同时实现 < code > Copy< / code > 和 < code > Drop< / code > 特征。因为实现了 < code > Copy< / code > 特征的类型会被编译器隐式的复制，因此非常难以预测析构函数执行的时间和频率。因此这些实现了 < code > Copy< / code > 的类型无法拥有析构函数。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[derive(Copy)]
struct Foo;
impl Drop for Foo {
fn drop(& mut self) {
println!("Dropping Foo!")
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上代码报错如下：< / p >
< pre > < code class = "language-console" > error[E0184]: the trait `Copy` may not be implemented for this type; the type has a destructor
--> src/main.rs:24:10
|
24 | #[derive(Copy)]
| ^^^^ Copy not allowed on types with destructors
< / code > < / pre >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > < code > Drop< / code > 可以用于许多方面，来使得资源清理及收尾工作变得方便和安全，甚至可以用其创建我们自己的内存分配器！通过 < code > Drop< / code > 特征和 Rust 所有权系统， ， < / p >
< p > 我们也无需担心意外的清理掉仍在使用的值，这会造成编译器错误：所有权系统确保引用总是有效的，也会确保 < code > drop< / code > 只会在值不再被使用时被调用一次。< / p >
< / main >
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