Kotlin中另一个类内的密封类无法编译:无法访问'< init>'它是私人的 [英] Sealed classes inside another class in Kotlin can't be compiled: cannot access '<init>' it is private

问题描述

如果我使用了 docs

class SomeActivity : AppCompatActivity() {
    sealed class Expr
    data class Const(val number: Double) : Expr()
    data class Sum(val e1: Expr, val e2: Expr) : Expr()
    object NotANumber : Expr()
}

它无法编译，并显示错误:

it does not compile, with the error:

Cannot access '<init>', it is private in 'Expr'.

但是，将其移到封闭类之外会使它编译:

However, moving it outside the enclosing class makes it compile:

sealed class Expr
data class Const(val number: Double) : Expr()
data class Sum(val e1: Expr, val e2: Expr) : Expr()
object NotANumber : Expr()

class SomeActivity : AppCompatActivity() {
}

为什么会这样?这是预期的行为吗?文档似乎没有提到这一点.

Why is this so? Is this an intended behavior? The docs does not seem to mention this.

推荐答案

是的，事实证明这是预期的行为.根据允许非嵌套子类的建议 :

Yes, it turns out to be intended behavior. According to the proposal allowing non-nested subclasses:

建议:在同一文件中允许顶级子类用于顶级密封类.

对于非顶级密封类，应在其内部声明所有子类.因此，对于这些类，没有任何变化.

For a non top-level sealed class all subclasses should be declared inside it. So, for such classes nothing changes.

您想要的方案被列为一个悬而未决的问题.在 https://youtrack.jetbrains.com/issue/KT-13495 .目前似乎没有人对此进行研究.在讨论该提案时，开发人员说:

The scenario you want is listed as an open question. There is a ticket for it at https://youtrack.jetbrains.com/issue/KT-13495. Nobody seems to be working on it at the moment. In the discussion of the proposal, the developer says:

好吧，这里有一些不平凡的实现细节(关于一代合成构造函数)

Well, there is some not-trivial implementations details(about generation synthetic constructors) which was solved for top-level classes but how do it in general is not clear.

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