《从零开始学Swift》学习笔记（Day 46）——下标重写

下标是一种特殊属性。子类属性重写是重写属性的getter和setter访问器，对下标的重写也是重写下标的getter和setter访问器。
下面看一个示例：
class DoubleDimensionalArray {
    let rows: Int, columns: Int
    var grid: [Int]
    init(rows: Int, columns: Int) {
        self.rows = rows
        self.columns = columns
        grid = Array(count: rows * columns, repeatedValue: 0)
    }
    subscript(row: Int, col: Int) -> Int { //定义下标
        get {
            return grid[(row * columns) + col]
        }
        set {
            grid[(row * columns) + col] = newValue
        }
    } //定义下标
}
class SquareMatrix: DoubleDimensionalArray {
    
    override subscript(row: Int, col: Int) -> Int { //重写父类下标
        get {
            return super.grid[(row * columns) + col]
        }
        set {
            super.grid[(row * columns) + col] = newValue * newValue
        }
    }
}
var ary2 = SquareMatrix(rows: 5, columns: 5)
for var i = 0; i < 5; i++ {
    for var j = 0; j < 5; j++ {
        ary2[i,j] = i + j
    }
}
for var i = 0; i < 5; i++ {
    for var j = 0; j < 5; j++ {
        print("\t\t \(ary2[i,j])")
    }
    print("\n")
}
其中super.grid[(row * columns) + col]语句中使用super调用父类的grid属性。
其中super.grid[(row * columns) + col] = newValue * newValue语句是给父类的grid属性赋值。