CountableRange的startIndex和lowerBound有什么区别？

Question

Documentation says：

  

关联的Bound类型是CountableRange的元素和索引类型。范围的每个元素都是其自己的对应索引。 CountableRange实例的下限是其起始索引，上限是其结束索引。

宣布

var lowerBound: Bound { get }

var lowerBound: Bound
The range's lower bound.

In an empty range, lowerBound is equal to upperBound.

Declaration
var lowerBound: Bound { get }

宣布

var startIndex: Bound { get }

var startIndex: Bound
The position of the first element in a nonempty collection.

If the collection is empty, startIndex is equal to endIndex.

Declaration
var startIndex: Bound { get }

这两者之间有什么区别，我该如何决定使用哪一个？

Answer 1

lowerBound和startIndex完全等同于CountableRange，upperBound和endIndex也是如此，如文档中所述：

  

CountableRange实例的下限是其起始索引，上限是其结束索引。

CountableRange提供这两个属性的唯一原因是startIndex and endIndex are required by the Collection protocol而lowerBound and upperBound are common to all Range types（即：CountableRange，CountableClosedRange等）。