# Swift 字面量本质

## 常见的字面量默认类型

- `public typealias IntegerLiteralType = Int`
- `public typealias FloatLiteralType = Float`
- `public typealias BooleanLiteralType = Bool`
- `public typealias StringLiteralType = String`

可以通过 typealias 修改字面量的默认类型，Demo 如下：

```swift
typealias IntegerLiteralType = UInt8
var val = 8
val
```

<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftChangeLiteral.png" style="zoom:25%">



## 字面量协议

Swift 自带的数据类型基本都可通过字面量初始化，本质原因是遵循了对应的协议

|               |                                                        |                                                              |
| ------------- | ------------------------------------------------------ | ------------------------------------------------------------ |
| Bool          | ExpressibleByBooleanLiteral                            | var b:Bool = false                                           |
| Int           | ExpressibleByIntegerLiteral                            | var num:Int = 2                                              |
| Float、Double | ExpressibleByIntegerLiteral、ExpressibleByFloatLiteral | var height:Float = 175  <br>var height1:Float = 175.2 <br>var weight:Double = 130 <br>var weight1:Double = 130.1 |
| Dictionary    | ExpressibleByDictionaryLiteral                         | var dic:Dictionary = ["name": "FantasticLBP"]                |
| String        | ExpressibleByStringLiteral                             | var name:String = "FantasticLBP"                             |
| Array、Set    | ExpressibleByArrayLiteral                              | var arr:Array = [1, 2, 3]                                    |
| Optional      | ExpressibleByNilLiteral                                | var o:Optional<Int> = nil                                    |



## 字面量协议的使用

Demo1

```swift
extension Int : ExpressibleByBooleanLiteral {
    public init(booleanLiteral value: Bool) {
        self = value ? 1 : 0
    }
}
var num: Int = true
print(num)	// 1
num = false
print(num)	// 0
```

Demo2

```swift
class Student: ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral, ExpressibleByStringLiteral, CustomStringConvertible {
    var name: String = ""
    var score: Double = 0
    
    required init(floatLiteral value: FloatLiteralType) {
        self.score = value
    }
    required init(stringLiteral value: StringLiteralType) {
        self.name = value
    }
    required init(integerLiteral value: IntegerLiteralType) {
        self.score = Double(value)
    }
    required init(extendedGraphemeClusterLiteral value: String) {
        self.name = value
    }
    required init(unicodeScalarLiteral value: String) {
        self.name = value
    }
    var description: String {
        "name is \(self.name), score is \(self.score)"
    }
}

var student: Student = "杭城小刘"
print(student)	// name is 杭城小刘, score is 0.0
student = 100
print(student)  // name is , score is 100.0
```