Go 变长参数函数的实现示例

func (values ...int64) (sum int64) 
func (sep string, tokens ...string) string

func Sum(values ...int64) (sum int64) {
 // values的类型为[]int64
 sum = 0
 for _, v := range values {
  sum += v
 }
 return
}

func Concat(sep string, tokens ...string) (r string) {
 // tokens的类型为[]string
 for i, t := range tokens {
  if i != 0 {
   r += sep
  }
  r += t
 }
 return r
}

func Print(a ...interface{}) (n int, err error)
func Printf(format string, a ...interface{}) (n int, err error)
func Println(a ...interface{}) (n int, err error)

package main

import "fmt"

func Sum(values ...int64) (sum int64) {
 sum = 0
 for _, v := range values {
  sum += v
 }
 return
}

func main() {
 a0 := Sum()
 a1 := Sum(2)
 a3 := Sum(2, 3, 5)
 // 上面三行和下面三行是等价的。
 b0 := Sum([]int64{}...)
 b1 := Sum([]int64{2}...)
 b3 := Sum([]int64{2, 3, 5}...)
 fmt.Println(a0, a1, a3) // 0 2 10
 fmt.Println(b0, b1, b3) // 0 2 10
}

func (engine *Engine) Run(addr ...string) (err error) {
 defer func() { debugPrintError(err) }()

 address := resolveAddress(addr)
 debugPrint("Listening and serving HTTP on %s\n", address)
 err = http.ListenAndServe(address, engine)
 return
}


func resolveAddress(addr []string) string {
 switch len(addr) {
 case 0:
  if port := os.Getenv("PORT"); port != "" {
   debugPrint("Environment variable PORT="%s"", port)
   return ":" + port
  }
  debugPrint("Environment variable PORT is undefined. Using port :8080 by default")
  return ":8080"
 case 1:
  return addr[0]
 default:
  panic("too many parameters")
 }
}

package main
import "fmt"
type CoffeeOptions struct {
 sugar        int
 milk         int
 coffeePowder int
}
// 这个函数会接受上面结构体 CoffeeOptions 实例的指针 这个函数最终的目的都是为了修改实例
type CoffeeOption func(*CoffeeOptions)
// 下面这三个函数都会返回 CoffeeOption 函数 方便修改 CoffeeOptions 结构体
func CoffeeSugar(sugar int) CoffeeOption {
 return func(opts *CoffeeOptions) {
  opts.sugar = sugar
 }
}
func CoffeeMilk(milk int) CoffeeOption {
 return func(opts *CoffeeOptions) {
  opts.milk = milk
 }
}
func CoffeePowder(coffeePowder int) CoffeeOption {
 return func(opts *CoffeeOptions) {
  opts.coffeePowder = coffeePowder
 }
}
func newDefaultCoffeeOptions() *CoffeeOptions {
 return &CoffeeOptions{
  sugar:        5,
  milk:         0,
  coffeePowder: 60,
 }
}
func NewCoffee(opts ...CoffeeOption) *CoffeeOptions {
 // 新建实例opt
 defaultOptions := newDefaultCoffeeOptions()
 // 根据所传的参数修改opt实例
 for _, o := range opts {
  o(defaultOptions)
 }
 return defaultOptions
}
func main() {
 // 创建函数来修改实例
 opts := NewCoffee(
  CoffeeSugar(10),
  CoffeeMilk(10),
  CoffeePowder(80),
 )
 fmt.Printf("%#v", *opts)
 // main.CoffeeOptions{sugar:10, milk:10, coffeePowder:80}
}