类型为[SuperClass]的Swift数组和类型为[Subclass]的元素 [英] Swift array of type [SuperClass] with elements of type [Subclass]

问题描述

有人可以解释为什么此代码引发错误吗?

Can someone explain why this code throws error?

class Base {}
class SubclassOfBase: Base {}

let baseItems = [Base](count: 1, repeatedValue: Base())
let subclassItems = [SubclassOfBase](count: 3, repeatedValue: SubclassOfBase())

var items = [Base]()
items.append(SubclassOfBase()) //OK
items.appendContentsOf(baseItems) //OK
items.appendContentsOf(subclassItems) //cannot invoke with argument of type [SubclassOfBase]
items.append(subclassItems.first!) //OK

接下来的问题:添加子类元素的唯一方法是在for循环中一个接一个地添加子类元素吗?

And next question: Is the only way to add subclass elements is by adding them one by one in for loop?

推荐答案

如果您检查标头，则:

public mutating func append(newElement: Element)

public mutating func appendContentsOf<C : CollectionType where C.Generator.Element == Element>(newElements: C)

请注意类型说明符之间的差异.虽然 append 使您可以添加 Element 的任何内容，即包括子集，但 appendContentsOf 会强制您使用具有完全相同的数组元素类型(不允许子类).

Note the difference in type specifiers. While append enables you to add anything that is an Element, that is, including sublasses, appendContentsOf forces you to use an array with exactly the same element type (subclasses not allowed).

它适用于:

let subclassItems = [Base](count: 3, repeatedValue: SubclassOfBase())

我认为这是一个错误，因为可以通过改进函数标头轻松解决此问题(嗯，这还需要扩展到 where 子句，因为现在无法检测泛型子类型).

I would consider this to be a bug because this could be easily fixed by improving the function header (well, this also needs an extension to where clauses because detecting generic subtypes is impossible right now).

一些可能的解决方法:

1. 直接为每个项目添加

1. Append directly for every item

subclassItems.forEach {items.append($0)}

为数组声明一个辅助方法(将对 Array 使用，不适用于通用的 CollectionType )

Declare a helper method for Arrays (will work for Array, not for generic CollectionType)

extension Array {
    public mutating func appendContentsOf(newElements: [Element]) {
        newElements.forEach {
            self.append($0)
        }
    }
}

直接投射

Direct cast

items.appendContentsOf(subclassItems as [Base])

这篇关于类型为[SuperClass]的Swift数组和类型为[Subclass]的元素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！