如何将 Entry API 与仅在 Entry 空置时构建的昂贵密钥一起使用? [英] How do I use the Entry API with an expensive key that is only constructed if the Entry is Vacant?

问题描述

是否可以使用 Entry API 通过 AsRef<str> 获取值，但使用 Into 插入它?

Is it possible to use the Entry API to get a value by a AsRef<str>, but inserting it with Into<String>?

这是工作示例:

use std::collections::hash_map::{Entry, HashMap};

struct Foo;

#[derive(Default)]
struct Map {
    map: HashMap<String, Foo>,
}

impl Map {
    fn get(&self, key: impl AsRef<str>) -> &Foo {
        self.map.get(key.as_ref()).unwrap()
    }

    fn create(&mut self, key: impl Into<String>) -> &mut Foo {
        match self.map.entry(key.into()) {
            Entry::Vacant(entry) => entry.insert(Foo {}),
            _ => panic!(),
        }
    }

    fn get_or_create(&mut self, key: impl Into<String>) -> &mut Foo {
        match self.map.entry(key.into()) {
            Entry::Vacant(entry) => entry.insert(Foo {}),
            Entry::Occupied(entry) => entry.into_mut(),
        }
    }
}

fn main() {
    let mut map = Map::default();
    map.get_or_create("bar");
    map.get_or_create("bar");
    assert_eq!(map.map.len(), 1);
}

游乐场

我的问题是，在 get_or_create 中总是会创建一个 String，从而导致不需要的内存分配，即使占用的条目不需要它也是如此.有可能以任何方式解决这个问题吗?也许以一种简洁的方式使用 Cow?

My problem is that in get_or_create a String will always be created, incurring unneeded memory allocation, even if it's not needed for an occupied entry. Is it possible to fix this in any way? Maybe in a neat way with Cow?

推荐答案

在 nightly Rust 中，你可以使用不稳定的 raw_entry_mut() 允许这样做的功能:

In nightly Rust, you can use the unstable raw_entry_mut() feature that allows this:

为 HashMap 创建一个原始条目构建器.

Creates a raw entry builder for the HashMap.

[...]

原始条目对于以下特殊情况很有用:

Raw entries are useful for such exotic situations as:

• 将自有密钥的创建推迟到已知需要时

在稳定的 Rust 中，您可以添加具有相同 API 但稳定的 hashbrown crate.hashbrown crate 实际上是标准库的 hashmap 的底层实现.

In stable Rust, you can add the hashbrown crate which has the same API but stable. The hashbrown crate is actually the underlying implementation of the standard library's hashmap.

示例:

#![feature(hash_raw_entry)]
use std::collections::HashMap;

#[derive(Hash, PartialEq, Eq, Debug)]
struct NoCopy {
    foo: i32,
}

impl Clone for NoCopy {
    fn clone(&self) -> Self {
        println!("Clone of: {:?}", self);
        Self { foo: self.foo }
    }
}

fn main() {
    let no_copy = NoCopy { foo: 21 };

    let mut map = HashMap::new();

    map.raw_entry_mut()
        .from_key(&no_copy)
        .or_insert_with(|| (no_copy.clone(), 42));

    map.raw_entry_mut()
        .from_key(&no_copy)
        .or_insert_with(|| (no_copy.clone(), 84));

    println!("{:#?}", map);
}

应用于您的原始示例:

fn get_or_create<K>(&mut self, key: K) -> &mut Foo
where
    K: AsRef<str> + Into<String>,
{
    self.map
        .raw_entry_mut()
        .from_key(key.as_ref())
        .or_insert_with(|| (key.into(), Foo))
        .1
}

这篇关于如何将 Entry API 与仅在 Entry 空置时构建的昂贵密钥一起使用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！