协变类型T发生在相反位置 [英] covariant type T occurs in contravariant position

问题描述

我知道之前曾有人问过这个问题，但是要么答案不适用于这种情况，要么我不理解。

I know this question has been asked before, but either the answers don't apply to this case, or I don't understand them.

基本上，为什么不以下（重新创建我的问题的简单示例）工作？

Basically, why doesn't the following (simple example that recreates my problem) work ?

class Test[+T] {
    var list: List[T] = _
}

我遇到的问题是我有一个对象，我想在其中传递 Test [Nothing] （空的 Test ）的实例，除非我使 T 中的$ c> Test 协变量。

The problem I am having is that I have an object, where I want to pass in an instance of Test[Nothing] (the empty Test), and this doesn't work unless I make Test co-variant in T.

推荐答案

在 T 中进行检验协变量意味着 Test [A] 是<$ c $的子类型c> Test [Any] 中的任何 A 。因此，让我们创建一个 Test ：

Making test covariant in T means that Test[A] is a subtype of Test[Any] for any A. So lets create a Test:

val test_string = new Test[String]

现在我们有一个 Test [String] ，并且包含的​​列表的类型为 List [String] 。

Now we have a Test[String] and the contained list is type List[String].

由于 Test [String] 是 Test [Any] 的子类型，因此应允许以下内容：

Since Test[String] is a subtype of Test[Any], the following should be allowed:

val test_any : Test[Any] = test_string

现在，我们有一个 Test [Any] ，因此 test_any.list 的类型为 List [Any] ，这意味着以下内容应有效：

And now, we have a Test[Any], and therefore test_any.list is type List[Any], which means the following should be valid:

test_any.list = List[Any]()

这意味着我们刚刚分配了一个 List [Any] 到test_strings列表成员，不应该被允许，因为这应该是 List [String] ，而不是 List [Any] 。这也意味着您可以在列表之前添加任何内容，因为它是 List [Any] 的类型。

Which means we just assigned a List[Any] to test_strings list member, which shouldn't be allowed, since that is supposed to be a List[String], not a List[Any]. It also means you could prepend anything at all to the list, since it is type List[Any].

这篇关于协变类型T发生在相反位置的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！