什么时候临时销毁? [英] When is a temporary destructed?

问题描述

直到现在，我都认为在生成它的语句的评估结束时会破坏一个临时对象.

Up until now, I assumed that a temporary was destructed at the end of the evaluation of the statement that had spawned it.

但是，使用临时对象初始化struct的字段时，似乎出现了异常.

However, it appears that there is an exception made when using a temporary to initialize the field of a struct.

PeterHall 帮助提供了一个简单的代码示例，说明了差异 在对他的回答的评论中，我已经简化了有点复杂:

PeterHall helpfully provided a simple code sample illustrating the difference in a comment to his answer, which I've simplified complicated a bit:

struct Wrapper<'a> {
    cmd: &'a Cmd<'a>,
}

struct Cmd<'a> {
    args: &'a Option<String>,
}

impl <'a> Cmd<'a> {
    fn new(args: &'a Option<String>) -> Cmd<'a> {
        Cmd {
            args: args,
        }
    }
}

pub fn main() {
    // OK
    let cmd = Cmd {
        args: &None,
    };

    // OK
    let cmd = Wrapper {
        cmd: &Cmd {
            args: &None,
        }
    };

    // Lifetime error
    let cmd = Some(Cmd {
        args: &None,
    });

    // Lifetime error
    let cmd = Cmd::new(&None);
}

那么，临时销毁的确切规则是什么?

So, what is the exact rule for when a temporary is destructed?

推荐答案

我们先看看第二个失败的行:

Let's look at the second failing line first:

let cmd = Cmd::new(&None);

&None 创建一个具有单行生命周期的临时文件.Cmd::new 返回一个具有相同生命周期的 Cmd.然后我们尝试使用 let 将该临时变量存储在一个变量中.

&None creates a temporary with a lifetime of the single line. Cmd::new returns a Cmd that has the same lifetime. Then we have attempt to store that temporary in a variable with let.

Rust 参考状态:

当一个临时右值被创建并分配给一个 let 时声明，但是，临时创建的生命周期为代替封闭块...

When a temporary rvalue is being created that is assigned into a let declaration, however, the temporary is created with the lifetime of the enclosing block instead...

它试图增加 Cmd 临时文件的生命周期，但这取决于 &None 临时文件的生命周期，并且由于该临时文件实际上并未存储在 let(关于持续到语句结束的临时规则的例外)中，它的生命周期小于 cmd 的生命周期并且你会得到一个生命周期错误.

It tries to increase the lifetime of the Cmd temporary, but that depends on the lifetime of the &None temporary, and since that temporary isn't actually stored in a let (the exception to the rule about temporaries lasting until the end of the statement), its lifetime is less than the lifetime of cmd and you get a lifetime error.

直接结构语句有效，因为 let 生命周期适用于结构及其成员.

The direct struct statements work, because the let lifetime applies to the struct and its members.

它对 Some(一个枚举)不起作用的事实对我来说似乎是一个错误(或者至少是一个缺失的功能).

The fact that it doesn't work for Some (an enum) seems like a bug to me (or at least a missing feature).

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