为什么使用对 i32 的不可变引用 [英] Why use an immutable reference to i32

问题描述

在 Rust 书的生命周期一章中，有一个例子:

In the chapter Lifetimes of the Rust book, there's an example:

struct Foo<'a> {
    x: &'a i32,
}

fn main() {
    let y = &5; // this is the same as `let _y = 5; let y = &_y;`
    let f = Foo { x: y };

    println!("{}", f.x);
}

他们为什么使用 x: &'a i32?

我认为如果它只是 x: i32 那么他们无法证明生命周期的使用情况.然而，这背后还有其他原因吗?是否有任何生产代码使用对像 i32 这样的原始类型的不可变引用?

I think if it is just x: i32 then they cannot demonstrate the lifetime usage. However, is there any other reason behind it? Is there any production code that uses immutable reference to a primitive type like i32?

推荐答案

在这种特殊情况下，原因确实是为了展示生命周期的概念.然而，对于一般情况，我认为没有理由对原始类型进行不可变引用(当然，可变引用是另一回事)，除非它是在泛型代码中完成的:

In this particular case the reason is indeed to show the concept of lifetimes. As for the general case, however, I see no reason making an immutable reference to a primitive type (mutable references, of course, is another matter) except of when it is done in generic code:

struct Holder<'a, T> {
    r: &'a T
}

let x: i32 = 123;
let h: Holder<i32> = Holder { r: &x };

在这里，如果您有这样的结构，您别无选择，只能使用对 i32 的引用.当然，这种结构也可以用于其他非原始和不可移动的类型.

Here if you have such structure, you have no other choice as to use a reference to an i32. Naturally, this structure can also be used with other, non-primitive and non-movable types.

正如 Shepmaster 在评论中提到的，确实存在一种情况，您引用原始类型 - 它是按引用迭代器.请记住，根据约定(标准库遵循的约定)，集合上的 iter() 方法应该将引用的迭代器返回到集合中:

As Shepmaster has mentioned in comments, there is indeed a case where you have references to primitive types - it is by-reference iterators. Remember, by a convention (which the standard library follows) iter() method on a collection should return an iterator of references into the collection:

let v: Vec<i32> = vec![1, 2, 3, 4];
let i = v.iter();  // i is Iterator<Item=&i32>

然后几乎所有采用闭包的迭代器方法都将接受参数为引用的闭包:

Then almost all methods on the iterator which take a closure will accept closures whose argument is a reference:

i.map(|n| *n + 1)  // n is of type &i32

请注意，这实际上是泛型更一般情况的结果.向量和切片可能包含任意类型，包括不可移动的类型，因此它们只需要拥有允许用户借用其内容的方法.

Note that this is in fact a consequence of the more general case with generics. Vectors and slices may contain arbitrary types, including non-moveable ones, so they just have to have methods which would allow their users to borrow their contents.

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