【RUST】Deref

Deret

首先先看一段代码

fn main() {
    let x = 5;
    let y = &x;

    assert_eq!(5, x);
    assert_eq!(5, y)
}

编译的时候报错:

error[E0277]: can't compare `{integer}` with `&{integer}`
 --> src/main.rs:6:5
  |
6 |     assert_eq!(5, y)
  |     ^^^^^^^^^^^^^^^^ no implementation for `{integer} == &{integer}`
  |
  = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`
  = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)

具体原因是因为不能拿一个数值和一个引用做比较，可以像下面这样写不报错

fn main() {
    let x = 5;
    let y = &x;

    assert_eq!(5, x);
    assert_eq!(5, *y)
}

也可以这样写

fn main() {
    let x = 5;
    let y = Box::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

其实，在RUST中，使用*就是调用对应结构体的deref()方法，所以我们也可以自己实现一个Box像下面这样:

use std::ops::Deref;

struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}
fn main() {
    let x = 5;
    let y = MyBox::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);
}

强制隐式转换

use std::ops::Deref;
struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.0
    }
}
fn hello(name: &str) {
    println!("Hello, {}!", name);
}

fn main() {
    let m = MyBox::new(String::from("Rust"));
    hello(&m);
}

• 注意hello的参数是&str
• 我们传入的参数是&MyBox类型
• MyBox实现了Deref trait，Rust就可以通过调用MyBox的deref把&MyBox转变成&String
• 由于标准库对String又有一个Deref的实现将string转换为string slice，所以这里又把&String转换&str