swift - CountableRange的startIndex和lowerBound有什么区别？

Question

Documentation says :

The associated Bound type is both the element and index type of CountableRange. Each element of the range is its own corresponding index. The lower bound of a CountableRange instance is its start index, and the upper bound is its end index.

var lowerBound: Bound { get } 已声明

var lowerBound: Bound
The range's lower bound.

In an empty range, lowerBound is equal to upperBound.

Declaration
var lowerBound: Bound { get }

var startIndex: Bound { get } 已声明

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吗？我如何决定使用哪一个？

Answer 1

lowerBound 和 startIndex 对于 CountableRange 来说是完全等价的，upperBound 和 endIndex，如文档所述:

The lower bound of a CountableRange instance is its start index, and the upper bound is its end index.

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)。