如何在 BTreeSet 中使用自定义比较器函数? [英] How do I use a custom comparator function with BTreeSet?

问题描述

在 C++ 中，可以自定义代码 std::set 用于对其参数进行排序.默认情况下，它使用 std::less，但可以使用 Compare 模板进行更改参数.

In C++, it is possible to customize the code std::set uses to sort its arguments. By default it uses std::less, but that can be changed with the Compare template parameter.

Rust 的 BTreeSet 使用Ord trait 对类型.我不知道有什么方法可以覆盖这种行为——它内置在容器存储的类型的类型约束中.

Rust's BTreeSet uses the Ord trait to sort the type. I don't know of a way to override this behavior -- it's built into the type constraint of the type stored by the container.

然而，构建一个按一些本地有用的指标排序的项目列表通常是有意义的，但这并不是总是比较项目的最佳方式.或者，假设我想对 used 类型的项目进行排序；在这种情况下，即使我想要，也不可能自己为该类型实现 Ord.

However, it often makes sense to build a list of items that are sorted by some locally-useful metric that nevertheless is not the best way to always compare the items by. Or, suppose I would like to sort items of a used type; in this case, it's impossible to implement Ord myself for the type, even if I want to.

解决方法当然是构建一个普通的旧 Vec 项目，然后 sort 它.但在我看来，这不像在插入时自动排序那么干净.

The workaround is of course to build a plain old Vec of the items and sort it afterward. But in my opinion, this is not as clean as automatically ordering them on insertion.

有没有办法在 Rust 的容器类型中使用替代比较器?

Is there a way to use alternative comparators with Rust's container types?

推荐答案

Rust 标准集合中目前不存在自定义比较器.解决问题的惯用方法是定义一个 newtype:

Custom comparators currently do not exist in the Rust standard collections. The idiomatic way to solve the issue is to define a newtype:

struct Wrapper(Wrapped);

然后你可以定义一个自定义的Ord Wrapper 的实现，具有您想要的语义.

You can then define a custom Ord implementation for Wrapper with exactly the semantics you want.

此外，由于您有一个 newtype，您还可以轻松实现其他 trait 以方便转换:

Furthermore, since you have a newtype, you can also easily implement other traits to facilitate conversion:

• convert::From 可以实现，给你 convert::Into 免费
• ops::Deref 可以实现，减少由于 auto-deref 导致的映射需求

请注意，访问包装实体在语法上是轻量级的，因为它只有两个字符:.0.

Note that accessing the wrapped entity is syntactically lightweight as it's just two characters: .0.

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