以无视HashMap值的通用方式公开HashMap [英] Expose a HashMap in a generic way that disregards the HashMap value

问题描述

我有不同的结构，它们都包含一个以String为键的HashMap，但是具有不同的值类型.例如，一个结构的成员类型为HashMap<String, String>，另一个结构的成员类型为HashMap<String, u8>，依此类推.

我想定义一个方法，该方法可以访问这些HashMap成员并对不涉及该值的成员执行一般操作.例如，我想计算键的数量，删除键，检查键是否存在等.我不确定如何实现此行为.

到目前为止，我想到的最好的方法是定义一个特征，该特征具有公开HashMap的方法，并允许每个结构实现它.但是，我不知道如何以忽略"值类型的方式编写此特征和方法.我尝试使用通配符(_)，但是它不起作用.我该如何执行呢?

这是我的代码(无法编译):

use std::collections::HashMap;

pub trait HashMapContainer {
    fn get_hash_map(&self) -> HashMap<String, _>;
}

struct HashMapContainerImpl1 {
    map: HashMap<String, String>,
}

impl HashMapContainerImpl1 {
    pub fn new() -> HashMapContainerImpl1 {
        HashMapContainerImpl1 {
            map: HashMap::new(),
        }
    }

    fn internal_logic_on_map(&mut self) {
        //....
    }
}

impl HashMapContainer for HashMapContainerImpl1 {
    fn get_hash_map(&self) -> HashMap<String, _> {
        self.map
    }
}

struct HashMapContainerImpl2 {
    map: HashMap<String, u8>,
}

impl HashMapContainerImpl2 {
    pub fn new() -> HashMapContainerImpl2 {
        HashMapContainerImpl2 {
            map: HashMap::new(),
        }
    }

    fn internal_logic_on_map(&mut self) {
        //....
    }
}

impl HashMapContainer for HashMapContainerImpl2 {
    fn get_hash_map(&self) -> HashMap<String, _> {
        self.map
    }
}

fn do_generic_actions_on_map(hm_container: &HashMapContainer) {
    println!("key count: {}", hm_container.get_hash_map().len());
    println!(
        "key esists? {}",
        hm_container.get_hash_map().get("key1").is_some()
    );
    hm_container.get_hash_map().remove("key2");
}

fn main() {
    let cont1 = HashMapContainerImpl1::new();
    let cont2 = HashMapContainerImpl2::new();
    do_generic_actions_on_map(cont1);
    do_generic_actions_on_map(cont2);
}

解决方案

具有关联的类型

下面使用泛型的代码是正确的，但是考虑一下之后，我认为在此处使用关联类型可能更合适.特质应如下所示:

pub trait HashMapContainer {
    type Value;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value>;
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value>;
}

区别在于，您现在只能为一个结构实现一次特征，而不能多次实现，在这种情况下，更正确.

实现与通用类型参数大致相同.

impl HashMapContainer for HashMapContainerImpl1 {
    type Value = String;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value> {
        &mut self.map
    }
}

impl HashMapContainer for HashMapContainerImpl2 {
    type Value = u8;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value> {
        &mut self.map
    }
}

(游乐场 )

您还可以查看_) but it doesn't work. How do I implement this?

Here is my code (which doesn't compile):

use std::collections::HashMap;

pub trait HashMapContainer {
    fn get_hash_map(&self) -> HashMap<String, _>;
}

struct HashMapContainerImpl1 {
    map: HashMap<String, String>,
}

impl HashMapContainerImpl1 {
    pub fn new() -> HashMapContainerImpl1 {
        HashMapContainerImpl1 {
            map: HashMap::new(),
        }
    }

    fn internal_logic_on_map(&mut self) {
        //....
    }
}

impl HashMapContainer for HashMapContainerImpl1 {
    fn get_hash_map(&self) -> HashMap<String, _> {
        self.map
    }
}

struct HashMapContainerImpl2 {
    map: HashMap<String, u8>,
}

impl HashMapContainerImpl2 {
    pub fn new() -> HashMapContainerImpl2 {
        HashMapContainerImpl2 {
            map: HashMap::new(),
        }
    }

    fn internal_logic_on_map(&mut self) {
        //....
    }
}

impl HashMapContainer for HashMapContainerImpl2 {
    fn get_hash_map(&self) -> HashMap<String, _> {
        self.map
    }
}

fn do_generic_actions_on_map(hm_container: &HashMapContainer) {
    println!("key count: {}", hm_container.get_hash_map().len());
    println!(
        "key esists? {}",
        hm_container.get_hash_map().get("key1").is_some()
    );
    hm_container.get_hash_map().remove("key2");
}

fn main() {
    let cont1 = HashMapContainerImpl1::new();
    let cont2 = HashMapContainerImpl2::new();
    do_generic_actions_on_map(cont1);
    do_generic_actions_on_map(cont2);
}

解决方案

With an associated type

The code below using a generic is correct, but after thinking about it, I think that using an associated type might be more suitable here. The trait should look like this:

pub trait HashMapContainer {
    type Value;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value>;
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value>;
}

The difference is, that you now can only implement the trait once for a struct and not multiple times, which is more correct in this case.

The implementations are roughly the same as with the generic type parameter.

impl HashMapContainer for HashMapContainerImpl1 {
    type Value = String;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value> {
        &mut self.map
    }
}

impl HashMapContainer for HashMapContainerImpl2 {
    type Value = u8;
    fn get_hash_map(&self) -> &HashMap<String, Self::Value> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, Self::Value> {
        &mut self.map
    }
}

(Playground)

You can also look at When is it appropriate to use an associated type versus a generic type? which explains the difference between those two pretty good.

With a generic type

This is solvable by introducing a generic type in your HashMapContainer trait.

pub trait HashMapContainer<T> {
    fn get_hash_map(&self) -> &HashMap<String, T>;
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, T>;
}

I changed the signature to return a reference to the HashMap. It would be possible to do it without, e.g. by using clone or taking self as value, instead of as reference. I also introduced a _mut version.

The implementation is straight foreword:

impl HashMapContainer<String> for HashMapContainerImpl1 {
    fn get_hash_map(&self) -> &HashMap<String, String> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, String> {
        &mut self.map
    }
}

impl HashMapContainer<u8> for HashMapContainerImpl2 {
    fn get_hash_map(&self) -> &HashMap<String, u8> {
        &self.map
    }
    fn get_hash_map_mut(&mut self) -> &mut HashMap<String, u8> {
        &mut self.map
    }
}

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