指定泛型参数属于一小类类型 [英] Specifying generic parameter to belong to a small set of types

问题描述

是否有可能将一个通用参数约束为少数几种类型之一，而又不弄清楚哪些特征可以精确地定义那些类型?例如

Is it possible with to constrain a generic parameter to be one of the select few types without figuring out what traits precisely define those type? e.g.

impl<T> Data<T> where T == u32 || T == u64 

有时候，弄清楚要添加到where的所有特征以获取所需的类型是一件很麻烦的事情，有时，即使由于语义而在语法上有意义，也不允许这种类型.

Sometimes it's tedious to figure out what all traits to add to where to get the types you want, and sometimes one wouldn't want to allow a type even when it makes syntactic sense because of semantics.

推荐答案

您可以对要支持的类型使用标记特征:

You could use a marker trait for the types you want to support:

trait DataSupported {}

impl DataSupported for u64 {}
impl DataSupported for u32 {}

impl<T> Data<T> where T: DataSupported {}

为 Pavel Strakhov提到的，如果您需要在几个impl上使用此特征，并且需要其他特征界限，则可以将这些特征作为标记特征的界限，这样可以保留您的impl简洁:

As Pavel Strakhov mentioned, if you need to use this trait for a few impls and you need other trait bounds, then you can just make those traits as bounds of your marker trait instead, which will keep your impls terse:

trait DataSupported: Num + Debug {}

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