"nil slice" vs "nil map" -

Slice and map are all reference types in Go, and their default values are 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")
  }
}

The result is like this：

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

When a slice’s value is nil, you could also do operations on it, such as append:

package main

import "fmt"

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

The result is like this：

Is s a nil?  true
Is s a nil?  false
[1]

A caveat you should notice is the length of both nil and empty slice is 0:

package main

import "fmt"

func main() {
  var s1 []int
  s2 := []int{}
  fmt.Println("Is s1 a nil? ", s1 == nil)
  fmt.Println("Length of s1 is: ", len(s1))
  fmt.Println("Is s2 a nil? ", s2 == nil)
  fmt.Println("Length of s2 is: ", len(s2))
}

The result is like this：

Is s1 a nil?  true
Length of s1 is:  0
Is s2 a nil?  false
Length of s2 is:  0

So you should compare the slice’s value with nil to check whether it is a nil.

Accessing a nil map is OK, but storing a nil map cause program panic:

package main

import "fmt"

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

The result is like this:

  Is m a nil?  true
  m[1] is  false
  panic: assignment to entry in nil map
  
  goroutine 1 [running]:
  panic(0x4cc0e0, 0xc082034210)
    C:/Go/src/runtime/panic.go:481 +0x3f4
  main.main()
    C:/Work/gocode/src/Hello.go:9 +0x2ee
  exit status 2
  
  Process finished with exit code 1

So the best practice is to initialize a map before using it, like this:

  m := make(map[int]bool)

BTW, you should use the following pattern to check whether there is an element in map or not:

if v, ok := m[1]; !ok {
  .....
}

Reference:
The Go Programming Language. https://github.com/NanXiao/golang-101-hacks

See also