Swift中的append函数如何工作？

Question

Can someone explain me what is wrong with this statement.

var someString = "Welcome"
someString.append("!")

However this works when I replace the code with,

var someString = "Welcome"
let exclamationMark : Character = "!"
someString.append(exclamationMark)

Thanks in advance

Answer 1

In Swift, there is no character literal (such as 'c' in C-derived languages), there are only String literals.

Then, you have two functions defined on String s: append , to append a single character, and extend , to append a whole String. So this works:

var someString = "Welcome"
someString.extend("!")

If you really want to use append , you can force a one-char String literal to be turned into a Character either by calling Character 's constructor:

someString.append(Character("!"))

or by using a type conversion:

someString.append("!" as Character)

or by using a type annotation as you did with an extra variable:

let exclamationMark: Character = "!"
someString.append(exclamationMark)

Answer 2

String has 2 overloaded append(_:)

     mutating func append(x: UnicodeScalar)
     mutating func append(c: Character)

and both Character and UnicodeScalar conforms UnicodeScalarLiteralConvertible

enum Character : ExtendedGraphemeClusterLiteralConvertible, Equatable, Hashable, Comparable {

    /// Create an instance initialized to `value`.
    init(unicodeScalarLiteral value: Character)
}

struct UnicodeScalar : UnicodeScalarLiteralConvertible {

    /// Create an instance initialized to `value`.
    init(unicodeScalarLiteral value: UnicodeScalar)
}

"!" in this case is a UnicodeScalarLiteral . So, the compiler can not determine "!" is Character or UnicodeScalar , and which append(_:) method should be invoked. That's why you must specify it explicitly.

---

You can see: "!" can be a UnicodeScalar literal by this code:

struct MyScalar: UnicodeScalarLiteralConvertible {
    init(unicodeScalarLiteral value: UnicodeScalar) {
        println("unicode \(value)")
    }
}

"!" as MyScalar // -> prints "unicode !"