从字符串转换为具有不同生命周期的& str [英] Converting from String to &str with a different lifetime

问题描述

我有一个简单的例子:

fn make_string<'a>() -> &'a str {
    let s : &'static str = "test";
    s
}

fn make_str<'a>() -> &'a str {
    let s : String = String::from_str("test");
    s.as_slice()
}

fn main() {
    println!("{}", make_string());
    println!("{}", make_str());
}

错误消息:

<anon>:8:9: 8:10 error: `s` does not live long enough
<anon>:8         s.as_slice()
                 ^
<anon>:6:34: 9:6 note: reference must be valid for the lifetime 'a as defined on the block at 6:33...
<anon>:6     fn make_str<'a>() -> &'a str {
<anon>:7         let s : String = String::from_str("test");
<anon>:8         s.as_slice()
<anon>:9     }
<anon>:6:34: 9:6 note: ...but borrowed value is only valid for the block at 6:33
<anon>:6     fn make_str<'a>() -> &'a str {
<anon>:7         let s : String = String::from_str("test");
<anon>:8         s.as_slice()
<anon>:9     }
error: aborting due to previous error
playpen: application terminated with error code 101
Program ended.

似乎借位检查器认识到静态寿命比" a长，因此make_string的转换有效，但make_str失败.因为String是堆分配的，所以有没有办法从String创建引用并将其扩展到生存期'a?

It seems that the borrow checker recognizes that 'static is a greater lifetime than 'a so the conversion for make_string works, but make_str fails. Is there a way to create a reference from String and extend it to lifetime 'a, since String is heap allocated?

推荐答案

这里有一个基本的误解:String 未分配堆.

There is a fundamental misconception here: String is not heap allocated.

String的 content 是堆分配的，但是其生存期与String对象本身的生存期相关联，并且每当String被删除时(在此处结尾) make_str)，那么String的内容也会被删除.

The content of String is heap allocated, but its lifetime is tied to the lifetime of the String object itself, and whenever String is dropped (here, at the end of make_str), then the content of String is dropped too.

因此，编译器是正确的:结果String::as_slice的生存期小于或等于String的生存期，并且String的生存期以该函数结束...因此，您无法返回对的引用.它.

Therefore, the compiler is right: the lifetime of the result String::as_slice is less than or equal to that of String and the lifetime of String ends with the function... thus you cannot return a reference to it.

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