如何指定“自己的类型"?作为Kotlin的返回类型 [英] How to specify "own type" as return type in Kotlin

问题描述

是否可以将函数的返回类型指定为被调用对象的类型?

Is there a way to specify the return type of a function to be the type of the called object?

例如

trait Foo {
    fun bar(): <??> /* what to put here? */ {
        return this
    }
}

class FooClassA : Foo {
    fun a() {}
}

class FooClassB : Foo {
    fun b() {}
}

// this is the desired effect:
val a = FooClassA().bar() // should be of type FooClassA
a.a()                     // so this would work

val b = FooClassB().bar() // should be of type FooClassB
b.b()                     // so this would work

实际上，这大致相当于Objective-C中的instancetype或Swift中的Self.

In effect, this would be roughly equivalent to instancetype in Objective-C or Self in Swift.

推荐答案

没有语言功能支持此功能，但是您始终可以使用递归泛型(许多库使用的模式):

There's no language feature supporting this, but you can always use recursive generics (which is the pattern many libraries use):

// Define a recursive generic parameter Me
trait Foo<Me: Foo<Me>> {
    fun bar(): Me {
        // Here we have to cast, because the compiler does not know that Me is the same as this class
        return this as Me
    }
}

// In subclasses, pass itself to the superclass as an argument:
class FooClassA : Foo<FooClassA> {
    fun a() {}
}

class FooClassB : Foo<FooClassB> {
    fun b() {}
}

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