rust 如何将一个性状的类型删除并与类型相关联？

cu6pst1q 于 2022-12-19 发布在其他

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假设有一个集合trait，它的项有一个关联的类型：

trait CollectionItem {
    // ...
}

trait Collection {
    type Item: CollectionItem;
    
    fn get(&self, index: usize) -> Self::Item;
    // ...
}

我能不能把它类型擦除到一个对Collection和CollectionItem trait都使用动态调度的类型中？也就是说，把它 Package 成如下所示的形式：

struct DynCollection(Box<dyn Collection<Item=Box<dyn CollectionItem>>>);
impl DynCollection {
  fn get(&self, index: usize) -> Box<dyn CollectionItem> {
    // ... what to do here?
  }
}
impl <C: Collection> From<C> for DynCollection {
  fn from(c: C) -> Self {
    // ... what to do here?
  }
}

Playground

rust

来源：https://stackoverflow.com/questions/74806682/how-to-make-type-erased-version-of-a-trait-with-associated-type

1条答案

按热度按时间

bvjveswy1#

你可以添加一个私有的、类型擦除的辅助特征：

trait DynCollectionCore {
    fn get_dyn(&self, index: usize) -> Box<dyn CollectionItem>;
}

impl<C> DynCollectionCore for C
where
    C: ?Sized + Collection,
    C::Item: 'static,
{
    fn get_dyn(&self, index: usize) -> Box<dyn CollectionItem> {
        Box::new(self.get(index))
    }
}

然后使用以下代码构建一个 Package 器类型：

struct DynCollection(Box<dyn DynCollectionCore>);

impl DynCollection {
    fn new<C>(inner: C) -> Self
    where
        C: Collection + 'static,
        C::Item: 'static,
    {
        Self(Box::new(inner))
    }
}

impl Collection for DynCollection {
    type Item = Box<dyn CollectionItem>;

    fn get(&self, index: usize) -> Box<dyn CollectionItem> {
        self.0.get_dyn(index)
    }
}

// note: something like this is also needed for `Box<dyn CollectionItem>:
//       CollectionItem` to be satisfied
impl<T: ?Sized + CollectionItem> CollectionItem for Box<T> {
    // ...
}

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rust 如何将一个性状的类型删除并与类型相关联？

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