Scala类构造函数中的隐式对象 [英] implicits in scala Class constructors

问题描述

我不明白为什么下面三个示例中只有一个在起作用?是什么导致其他两个错误?

I dont understand why only one out of the three examples below are working? What makes the other two faulty?

class H(implicit a:String, b: Int) {
  //Working
}

class H(a:String, implicit b: Int) {
  //Not-Working
}

class H(implicit a:String, implicit b: Int) {
  //Not-Working
}

推荐答案

在第一种情况下，implicit并不引用a，而是引用整个参数列表.这意味着在调用构造函数时可以隐式提供a和b"(并使它们在类体中作为隐式提供).从这种意义上讲，您不能为类或方法implicit设置单个参数.

In the first case implicit doesn't refer to a but to the entire parameter list. It means "a and b can be provided implicitly when calling the constructor" (and also makes them available as implicits in the class body). You can't make a single parameter of a class or a method implicit in this sense.

implicit的第二个用途是标记类型/对象成员.构造函数的参数不是成员，但可以使用val/var使其成为成员，如pamu的回答所示；或者，为避免显示为private[this] val.

The second use of implicit is to mark a type/object member. Constructor parameters aren't members, but can be made into members by using val/var, as in pamu's answer; or, to avoid making it visible, private[this] val.

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