go 中 slice 和 map 的 默认值都是 “nil”

package main

import “fmt”
func main() {
var (
s []int
m map[int]bool
)
if s == nil {
fmt.Println(“The value of s is nil”)
}
if m == nil {
fmt.Println(“The value of m is nil”)
}
} 

结果：

The value of s is nil
The value of m is nil

区别是当slice 为nil 时可以进行操作,如append, 而map 却不能存key-value

package main

import "fmt"

func main() {
    var s []int
    s = append(s, 1)
    fmt.Println("Is s a nil? ", s == nil)
    fmt.Println(s)

    var m map[int]bool
    fmt.Println("Is m a nil? ", m == nil)
    fmt.Println("m[1] is ", m[1])
    m[1] = true

}

结果：

Is s a nil?  false
[1]
Is m a nil?  true
m[1] is  false
panic: assignment to entry in nil map
goroutine 1 [running]:
…

所以我们在使用map之前最后实例化一个map

m := make(map[int]bool)