Swift 泛型与重载

编译器在编译泛型函数时，会根据实参类型，以泛型函数为模板，合成对应的函数。
如果泛型函数有重载版本，编译器会以最接近实参类型的泛型函数为模板，进行函数合成。

eg1：

func test_func<T>(_ s: T) {
    print(type(of: s), s)
}

func doTest() {
    let a: Int = 1
    let b: Int? = 2
    let c: Int? = nil
    
    test_func(a)
    test_func(b)
    test_func(c)
}

合成的函数是：

func test_func(_ s: Int) {
    // let a: Int = 1 执行这个函数
    print(type(of: s), s) 
}

func test_func(_ s: Optional<Int>) {
    // let b: Int? = 2 和 let c: Int? = nil 执行这个函数
    print(type(of: s), s) 
}

执行结果是：

let a: Int = 1         Int             1
let b: Int? = 2        Optional<Int>   Optional(2)
let c: Int? = nil      Optional<Int>   nil

eg2：

func test_func<T>(_ s: T?) {
    print(type(of: s), s, terminator: "")
    if s != nil {
        print(type(of: s!), s!)
    } else {
        print("error")
    }
}

func doTest() {
    let a: Int = 1
    let b: Int? = 2
    let c: Int? = nil
    
    test_func(a)
    test_func(b)
    test_func(c)
}

合成的函数是：

func test_func(_ s: Int?) {
    print(type(of: s), s, terminator: "")
    if s != nil {
        print(type(of: s!), s!)
    } else {
        print("error")
    }
}

执行结果是：

let a: Int = 1         Optional<Int>   Optional(1) Int 1
let b: Int? = 2        Optional<Int>   Optional(2) Int 2
let c: Int? = nil      Optional<Int>   nil         error

eg3：

func test_func<T>(_ s: T) {
    print(type(of: s), s)
}

func test_func<T>(_ s: T?) {
    print(type(of: s), s, terminator: "")
    if s != nil {
        print(type(of: s!), s!)
    } else {
        print("error")
    }
}

func doTest() {
    let a: Int = 1
    let b: Int? = 2
    let c: Int? = nil
    
    test_func(a)
    test_func(b)
    test_func(c)
}

合成的函数是：

func test_func(_ s: Int) {
    // let a: Int = 1 执行这个函数
    print(type(of: s), s) 
}

func test_func(_ s: Int?) {
    // let b: Int? = 2 和 let c: Int? = nil 执行这个函数
    print(type(of: s), s, terminator: "") 
    if s != nil {
        print(type(of: s!), s!)
    } else {
        print("error")
    }
}

执行结果是：

let a: Int = 1         Int             1
let b: Int? = 2        Optional<Int>   Optional(2) Int 2
let c: Int? = nil      Optional<Int>   nil         error

应用

自定义LSPrint函数，能够准确判断参数items的类型，并且获取到其值。

#if DEBUG
public func LSPrint(file: String = #file, line: UInt = #line, function: String = #function) {
    print("\t" + (file as NSString).lastPathComponent + " [\(line)] " + function)
}
public func LSPrint<T>(_ items: @autoclosure () -> T, file: String = #file, line: UInt = #line, function: String = #function) {
    let result = items()
    print("\t" + (file as NSString).lastPathComponent + " [\(line)] " + function + " 输出: \(type(of: result))\n\(result)")
}
public func LSPrint<T>(_ items: @autoclosure () -> T?, file: String = #file, line: UInt = #line, function: String = #function) {
    let result = items()
    if result != nil {
        print("\t" + (file as NSString).lastPathComponent + " [\(line)] " + function + " 输出: \(type(of: result))\n\(result!)")
    } else {
        print("\t" + (file as NSString).lastPathComponent + " [\(line)] " + function + " 输出: \(type(of: result)) = nil")
    }
}
#else
    public func LSPrint(_ items: @autoclosure () -> Any? = nil) {}
#endif