标

Created: November-22, 2018

扩展可以向现有类型添加新下标。

此示例使用给定索引获取 String 内的字符：

Version = 2.2

extension String {
    subscript(index: Int) -> Character {
        let newIndex = startIndex.advancedBy(index)
        return self[newIndex]
    }
}

var myString = "StackOverFlow"
print(myString[2]) // a
print(myString[3]) // c

Version = 3.0

extension String {
    subscript(offset: Int) -> Character {
        let newIndex = self.index(self.startIndex, offsetBy: offset)
        return self[newIndex]
    }
}

var myString = "StackOverFlow"
print(myString[2]) // a
print(myString[3]) // c