Scala 中的 A<:B 和 +B 有什么区别? [英] What's the difference between A<:B and +B in Scala?

问题描述

和

[A <: B]

和

[+B]

在 Scala 中?

推荐答案

Q[A <: B] 表示 class Q 可以带任何 class A 是 B 的子类.

Q[A <: B] means that class Q can take any class A that is a subclass of B.

Q[+B] 表示 Q 可以取任何类，但如果 A 是子类B 的子类，则 Q[A] 被认为是 Q[B] 的子类.

Q[+B] means that Q can take any class, but if A is a subclass of B, then Q[A] is considered to be a subclass of Q[B].

Q[+A <: B] 表示Q类只能取B的子类，并传播子类关系.

Q[+A <: B] means that class Q can only take subclasses of B as well as propagating the subclass relationship.

当你想做一些通用的事情时，第一个很有用，但你需要依赖 B 中的特定方法集.例如，如果您有一个带有 toFile 方法的 Output 类，您可以在任何可以传递到 Q 的类中使用该方法.

The first is useful when you want to do something generic, but you need to rely upon a certain set of methods in B. For example, if you have an Output class with a toFile method, you could use that method in any class that could be passed into Q.

当您想要创建与原始类行为相同的集合时，第二个很有用.如果你使用 B 并创建一个子类 A，那么你可以在任何需要 B 的地方传递 A.但是如果你取一个B的集合，Q[B]，你是否总是可以传入Q[A] 代替?一般来说，没有；在某些情况下，这是错误的做法.但是你可以说这是正确的做法，使用 +B (covariance; Q covaries--follows with--B'子类的继承关系).

The second is useful when you want to make collections that behave the same way as the original classes. If you take B and you make a subclass A, then you can pass A in anywhere where B is expected. But if you take a collection of B, Q[B], is it true that you can always pass in Q[A] instead? In general, no; there are cases when this would be the wrong thing to do. But you can say that this is the right thing to do by using +B (covariance; Q covaries--follows along with--B's subclasses' inheritance relationship).

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