Swift：使用与号（＆）引用对象的属性会导致编译器错误

Question

This is a rather straightforward issue. Consider an array:

var array = [1, 2, 3, 4, 5, 6, 7]

I wish to swap the first and the last element of the array, using the standard swap function:

swap(&array[0], &array[array.count - 1])

However there exists another way to obtain the first and last elements of an array, which is to use Array 's built-in properties first and last . It follows that this code should also work:

swap(&array.first!, &array.last!)

However this time the compiler complains that Type of expression is ambiguous without more context . I wonder how I can modify the expression so that the compiler can parse it correctly.

Answer 1

first is a read-only property, therefore you cannot modify the array through array.first .

swap(&array[0], &array[array.count - 1])

works because subscript() of mutable collection types has both setter and getter.

Not that I recommend it, but if you define your own read-write properties

extension Array {
    var myFirst: T {
        get { return self[0] }
        set { self[0] = newValue }
    }
    var myLast: T {
        get { return self[count - 1] }
        set { self[count - 1] = newValue }
    }
}

(which assume that the array it not empty) then

swap(&array.myFirst, &array.myLast)

works as expected.