放置“mut"和“mut"有什么区别?在变量名之前和“:"之后? [英] What's the difference between placing "mut" before a variable name and after the ":"?

问题描述

这是我在 Rust 文档中看到的两个函数签名:

Here are two function signatures I saw in the Rust documentation:

fn modify_foo(mut foo: Box<i32>) { *foo += 1; *foo }
fn modify_foo(foo: &mut i32) { *foo += 1; *foo }

为什么 mut 的位置不同?

Why the different placement of mut?

似乎第一个函数也可以声明为

It seems that the first function could also be declared as

fn modify_foo(foo: mut Box<i32>) { /* ... */ }

推荐答案

mut foo: T 表示你有一个名为 foo 的变量，它是一个 T.您可以更改变量引用的内容:

mut foo: T means you have a variable called foo that is a T. You are allowed to change what the variable refers to:

let mut val1 = 2;
val1 = 3; // OK

let val2 = 2;
val2 = 3; // error: re-assignment of immutable variable

这还允许您修改您拥有的结构的字段:

This also lets you modify fields of a struct that you own:

struct Monster { health: u8 }

let mut orc = Monster { health: 93 };
orc.health -= 54;

let goblin = Monster { health: 28 };
goblin.health += 10; // error: cannot assign to immutable field

foo: &mut T 意味着你有一个引用 (&) 值的变量，你可以改变 (mut)code>) 引用值(包括字段，如果是结构体):

foo: &mut T means you have a variable that refers to (&) a value and you are allowed to change (mut) the referred value (including fields, if it is a struct):

let val1 = &mut 2;
*val1 = 3; // OK

let val2 = &2;
*val2 = 3; // error: cannot assign to immutable borrowed content

请注意，&mut 仅对引用有意义 - foo: mut T 不是有效语法.如果有意义，您还可以组合使用两个限定符(let mut a: &mut T).

Note that &mut only makes sense with a reference - foo: mut T is not valid syntax. You can also combine the two qualifiers (let mut a: &mut T), when it makes sense.

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