隐式类是否应该始终扩展AnyVal? [英] Should implicit classes always extend AnyVal?

问题描述

说我在写扩展方法

implicit class EnhancedFoo(foo: Foo) {
  def bar() { /* ... */ }
}

您应该始终在类定义中包含extends AnyVal吗?在什么情况下，您不想将隐式类设为值类?

Should you always include extends AnyVal in the class defininition? Under what circumstances would you not want to make an implicit class a value class?

推荐答案

让我们看看列出了针对值类的限制，并考虑何时可能不适合隐式类:

Let's look at the limitations listed for value classes and think when they may not be suitable for implicit classes:

1. 必须仅具有一个主要构造函数，该构造函数仅具有一个公共的val参数，其类型不是值类."因此，如果要包装的类本身是值类，则不能使用implicit class作为包装器，但是可以这样做:

1. "must have only a primary constructor with exactly one public, val parameter whose type is not a value class." So if the class you are wrapping is itself a value class, you can't use an implicit class as a wrapper, but you can do this:

// wrapped class
class Meters(val value: Int) extends AnyVal { ... }

// wrapper
class RichMeters(val value: Int) extends AnyVal { ... }

object RichMeters { 
  implicit def wrap(m: Meter) = new RichMeter(m.value)
}

如果包装器也具有隐式参数，则可以尝试将其移至方法声明. IE.代替

If your wrapper has implicit parameters as well, you can try to move them to the method declarations. I.e. instead of

implicit class RichFoo[T](foo: Foo[T])(implicit ord: Ordering[T]) {
  def bar(otherFoo: Foo[T]) = // something using ord
}

你有

implicit class RichFoo[T](foo: Foo[T]) extends AnyVal {
  def bar(otherFoo: Foo[T])(implicit ord: Ordering[T]) = // something using ord
}

可能没有专门的类型参数."在包装本身具有专用类型参数的类时，您可能希望包装器是专用的.

"may not have specialized type parameters." You may want the wrapper to be specialized when wrapping a class which itself has specialized type parameters.

此外，使隐式类成为值类可能会使用反射更改代码的某些行为，但反射通常不应看到隐式类.

In addition, making your implicit class a value class could possibly change some behavior of code using reflection, but reflection shouldn't normally see implicit classes.

如果您的隐式类确实满足所有这些限制，那么我就不会想到不将其设为值类的原因.

If your implicit class does satisfy all of those limitations, I can't think of a reason not to make it a value class.

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