在Swift中将nil分配给泛型可选

Question

I've created a simple generic grid data structure in Swift as follows. Basically it creates an Array of optionals with type T? and initialises the array with with nil. But when I try to explicitly set one grid element back to nil the compiler complains with something I don't really understand.

struct Grid<T> {
    let columns: Int, rows: Int
    var grid: [T?]
    init(columns: Int, rows: Int) {
        self.rows = rows
        self.columns = columns
        grid = Array(count: rows * columns, repeatedValue: nil)
    }

    func test() {
        grid[0] = nil
    }
}

Compiler's outcry when test() function is added:

Grid.swift:26:13: '@lvalue $T7' is not identical to 'T?'

Answer 1

The error message is misleading. The test() method modifies the value of a property in the structure, therefore you have to mark it as "mutating":

struct Grid<T> {
    // ...

    mutating func test() {
        grid[0] = nil
    }
}

See Modifying Value Types from Within Instance Methods in the Swift book:

Modifying Value Types from Within Instance Methods

Structures and enumerations are value types . By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. ...