go语言 nil使用避坑指南

var nil = errors.New("nil")
fmt.Printf("%#v\n", nil)//&errors.errorString{s:"nil"}

package main
import (
   "fmt"
   "unsafe"
)
func main() {
   var p *struct{} = nil
   fmt.Println(unsafe.Sizeof(p)) // 8
   var s []int = nil
   fmt.Println(unsafe.Sizeof(s)) // 24
   var m map[int]bool = nil
   fmt.Println(unsafe.Sizeof(m)) // 8
   var c chan string = nil
   fmt.Println(unsafe.Sizeof(c)) // 8
   var f func() = nil
   fmt.Println(unsafe.Sizeof(f)) // 8
   var i interface{} = nil
   fmt.Println(unsafe.Sizeof(i)) // 16
}

//不同类型的nil指针是一样的
package main
import (
   "fmt"
)
func main() {
   var arr []int
   var num *int
   fmt.Printf("%p\n", arr)    //0x0
   fmt.Printf("%p", num)  //0x0
}

//两个相同类型的 nil 值也无法比较
package main
import (
   "fmt"
)
func main() {
   var s1 []int
   var s2 []int
   fmt.Printf(s1 == s2) //invalid operation: s1 == s2 (slice can only be compared to nil)
   var s3 = []int{1}
   var s4 = []int{1}
   var s5 []int
   copy(s5, s3)
   fmt.Printf(s3 == s4) //invalid operation: s3 == s4 (slice can only be compared to nil)
   fmt.Printf(s3 == s5) //invalid operation: s3 == s5 (slice can only be compared to nil)
}

//对nil channel，map，slice和array 指针进行range操作也是合法的
package main
import "fmt"
func main() {
   for range []int(nil) { //循环次数将是0
      fmt.Println("Hello")
   }
   for range map[string]string(nil) { //循环次数将是0
      fmt.Println("world")
   }
   for i := range (*[5]int)(nil) {
      fmt.Println(i) // 0 1 2 3 4
   }
   for range chan bool(nil) { // block here
      fmt.Println("Bye") //fatal error: all goroutines are asleep - deadlock!
   }
}

//如果类型T的零值是用预先定义的nil来表示的话，*new(T)产生一个nil T类型的值
package main
import "fmt"
func main() {
   fmt.Println(*new(*int) == nil)         // true
   fmt.Println(*new([]int) == nil)        // true
   fmt.Println(*new(map[int]bool) == nil) // true
   fmt.Println(*new(chan string) == nil)  // true
   fmt.Println(*new(func()) == nil)       // true
   fmt.Println(*new(interface{}) == nil)  // true
}