延长字符串切片的借用生命周期 [英] Extending borrowed lifetime for String slice

问题描述

我有一个读取文件的函数，并为每一行将它添加到 &str 类型的 HashSet，但我不知道如何告诉借用检查器增加生命周期.

这是我目前的功能:

fn build_collection_set(reader: &mut BufReader) ->哈希集<&str>{let mut collection_set: HashSet<&str>= HashSet::new();对于 reader.lines() { 中的行让 line = line.unwrap();如果 line.len() >0 {collection_set.insert(&*line);}}返回集合集；}

如何让 Rust 知道我想让它保持更长时间?

解决方案

但我不知道如何告诉借用检查器增加生命周期.

这不可能.

<小时>

在 C、C++ 或 Rust 中，值的生命周期定义为:

• 根据它的词法范围，如果它绑定到一个自动变量
• 根据它的动态范围，如果它是在堆上分配的

您可以创建引用此值的变量，如果您的引用存在时间比该值长，那么您就有了一个悬空引用:

• 在 C 和 C++ 中，最好什么都不做
• 在 Rust 中，编译器将拒绝编译您的代码

为了验证您的程序，Rust 编译器将要求您注释引用的生命周期；您将在 &'a T 中使用诸如 'a 之类的生命周期注释，它允许命名生命周期以记录多个值的生命周期之间的关系.

这里的操作词是文档:生命周期是无形的，不能被影响，生命周期注释'a只是一个允许引用它的名称.><小时>

所以?

每当您发现自己想要延长引用的生命周期时，您应该考虑的是延长引用的生命周期……或者干脆不使用引用而是使用值.

在这种情况下，一个简单的解决方案是返回 String 而不是 &str:

fn build_collection_set(reader: &mut BufReader) ->哈希集<字符串>{让 mut collection_set = HashSet::new();对于 reader.lines() { 中的行让 line = line.unwrap();如果 line.len() >0 {collection_set.insert(行)；}}收藏集}

I have a function that reads in a file, and for each line adds it to a HashSet of type &str, but I can't work out how to tell the borrow checker to increase the lifetime.

Here's my function so far:

fn build_collection_set(reader: &mut BufReader<File>) -> HashSet<&str> {
    let mut collection_set: HashSet<&str> = HashSet::new();

    for line in reader.lines() {
        let line = line.unwrap();
        if line.len() > 0 {
            collection_set.insert(&*line);
        }
    }

    return collection_set;
}

How do I let Rust know I want to keep it around longer?

解决方案

but I can't work out how to tell the borrow checker to increase the lifetime.

It's impossible.

---

The lifetime of a value, in C, C++ or Rust, is defined either:

• by its lexical scope, if it is bound to an automatic variable
• by its dynamic scope, if it is allocated on the heap

You can create variables which reference this value, and if your reference lives longer than the value, then you have a dangling reference:

• in C and C++, you better do nothing with it
• in Rust, the compiler will refuse to compile your code

In order to validate your program, the Rust compiler will require that you annotate the lifetime of your references; you will use lifetime annotations such as 'a in &'a T which allow naming a lifetime in order to document the relationship between the lifetime of multiple values.

The operative word is document here: a lifetime is intangible and cannot be influenced, the lifetime annotation 'a is just a name to allow referring to it.

---

So?

Whenever you find yourself wanting to extend the lifetime of a reference, what you should be looking at instead is extending the lifetime of the referred... or simply not use a reference but a value instead.

In this case, a simple solution is to return String instead of &str:

fn build_collection_set(reader: &mut BufReader<File>) -> HashSet<String> {
    let mut collection_set = HashSet::new();

    for line in reader.lines() {
        let line = line.unwrap();
        if line.len() > 0 {
            collection_set.insert(line);
        }
    }

    collection_set
}

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