一. 在swift函數的申明語法中，可以給參數設置默認值，代碼如下:

func test(name: String = "") {
    print(#function)
}
test() /// 調用一
test(name: "tom") /// 調用二

• 有默認值的函數，可以省略參數的傳遞 調用一
• 按正常函數調用 調用二

二. 這種函數是怎么實現的呢？

猜測：編譯器會對應生成多個函數：

func test(name: String) {
    /// -> 相當于1種寫法
    /// test(name: String)
    print(#function)
}
func test(name: String, address: String = "") {
    /// -> 相當于2種寫法
    /// test(name: String)
    /// test(name: String, address: String)
    print(#function)
}

從上面的函數來看 test(name: String) 就被重復申明了，那么應該報錯啊！不不不，我們只能說它可能報錯，請看下面的代碼，代碼比較長，請耐心閱讀

• 正確情況:

/// 同名不同參
func test(name: String) {
    /// -> 相當于1種寫法
    /// test(name: String) -> 和自身行式一樣的，假設為特殊，可以覆蓋其他（不能被其它覆蓋）specialFunc
    print(#function)
}
func test(name: String, address: String = "") {
    /// -> 相當于2種寫法
    /// test(name: String)
    /// test(name: String, address: String) specialFunc
    print(#function)
}
func test(name: String, age: Int = 18) {
    /// -> 相當于2種寫法
    /// test(name: String)
    /// test(name: String, age: Int) specialFunc
    print(#function)
}
func test(name: String, age: Int = 18, address: String = "") {
    /// -> 相當于4種寫法
    /// test(name: String)
    /// test(name: String, age: Int)
    /// test(name: String, address: String)
    /// test(name: String, age: Int, address: String) specialFunc
    print(#function)
}

/// 由于specialFunc的覆蓋和唯一性
/// 上面的函數最終得到如下4個(僅有)唯一函數
/// test(name: String) specialFunc
/// test(name: String, address: String) specialFunc
/// test(name: String, age: Int) specialFunc
/// test(name: String, age: Int, address: String) specialFunc

/// 這里的調用一一對應specialFunc函數, 沒有歧義，運行正常
test(name: "tom") /// test(name:)
test(name: "tom", address: "") /// test(name:address:)
test(name: "tom", age: 10) /// test(name:age:)
test(name: "tom", age: 10, address: "") /// test(name:age:address:)

• 會產生錯誤的情況如下：

func parameter(name: String, address: String = "", age: Int = 18) {
    /// -> 相當于4種寫法
    /// parameter(name: String)
    /// parameter(name: String, address: String)
    /// parameter(name: String, age: Int)
    /// parameter(name: String, address: String, age: Int) specialFunc
    print(#function)
}
func parameter(name: String, age: Int = 18, address: String = "") {
    /// -> 相當于4種寫法
    /// parameter(name: String)
    /// parameter(name: String, age: Int)
    /// parameter(name: String, address: String)
    /// parameter(name: String, age: Int, address: String) specialFunc
    print(#function)
}

/// 上面的到6個有歧義的函數:
/// parameter(name: String)
/// parameter(name: String, address: String)
/// parameter(name: String, age: Int)
/// 和
/// parameter(name: String)
/// parameter(name: String, age: Int)
/// parameter(name: String, address: String)

/// 無法確認函數的唯一性，無法正確的調用函數，報錯!
/// parameter(name: "")
/// parameter(name: "", age: 10)
/// parameter(name: "", address: "")

/// 上面的函數得到兩個唯一函數:
/// parameter(name: String, address: String, age: Int) specialFunc
/// parameter(name: String, age: Int, address: String) specialFunc
/// 調用唯一函數，正確
parameter(name: "", address: "", age: 18)
parameter(name: "", age: 18, address: "")

• 如上的代碼可以直接運行來校驗猜測
• 就specialFunc而言，確實是出于對結果的構想（假定就是這樣來推斷后面的結果）

三. 以上是我個人的猜測。如有更好的解釋，請各位多多指教。