为什么这两种结构在受寿命影响的方式上有所不同

问题描述

这是针对以下问题的后续问题:如何解决:无法为自动强制推断合适的寿命.

This is a follow up question to: How to fix: cannot infer an appropriate lifetime for automatic coercion.

我想知道这两种结构为何受寿命影响的方式不同.

I wonder why do these both structs differ in the way they are affected by lifetimes.

示例1

use http;

pub struct Request<'a> {
    pub origin: &'a http::server::Request,
}

示例2

use http;

pub struct Response<'a, 'b> {
    pub origin: &'a mut http::server::ResponseWriter<'b>,
}

它们看起来与我非常相似，除了第二个引用具有可变的引用，而第一个引用具有不变的引用.

They look pretty much similar to me except that the second one holds a mutable reference whereas the first one holds an immutable reference.

但是，对于示例2 ，我不能仅使用生存期作为参考.我也必须给出该结构的生命期.

However, for Example 2 I can't just use a lifetime for the reference. I must give a lifetime for the struct as well.

所以，我想知道结构中是否有 导致这种行为，或者确实是因为第二个示例中的那个是可变的引用.如果是这样，为什么会导致这种情况.

So, I wonder is there something inside the struct that causes such behavior or is it really because the one in the second example is a mutable reference. And if so, why exactly does that cause that.

推荐答案

&'a T表示您引用了在整个生命周期内有效的T对象.

&'a T means that you have a reference to a T object which is valid for the lifetime'a.

T<'b>表示T对象，其中包含一个在其内部有效期为'b的对象，如struct T<'b> { t: &'b U }中一样.

T<'b> means a T object containing an object inside itself valid for the lifetime 'b, as in struct T<'b> { t: &'b U }.

&'a T<'b>是对T<'b>对象具有生存期'a的引用.

&'a T<'b>is thus a reference with lifetime 'a to a T<'b> object.

对于ResponseWriter，它包含对Request和TcpStream的引用，而Request不包含任何引用.

In the case of the ResponseWriter, it contains references to the Request and to the TcpStream, whereas the Request does not contain any references.

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