为什么在golang中打印（nil）映射会产生非“ <nil>”结果？

Question

In golang, if you create a struct with a map in it, which is uninitialized, printing the map yields a non <nil> string.

Why does "print" of a nil map output "map[]" instead of <nil> ?

// Assume above there is a struct, a.AA, which is created but which otherwise
// has not created the map.
//output:
//map[]
//map[]
fmt.Println(a.AA)
fmt.Println(make(map[string]int64))

Answer 1

A nil pointer is not the same as an empty (or zero) map. See https://golang.org/ref/spec#The_zero_value for authoritative information concerning zero values.

If you want a nil, you need a pointer, such as in the following example:

package main

import (
    "fmt"
)

func main() {
    var myMap map[string]int64
    fmt.Printf("myMap before assignment: %v
", myMap)

    myMap = make(map[string]int64)
    fmt.Printf("myMap after assignment : %v
", myMap)

    var myMapPointer *map[string]int64
    fmt.Printf("myMapPointer before assignment: %v
", myMapPointer)

    myMapPointer = new(map[string]int64)
    fmt.Printf("myMapPointer after assignment : %v
", myMapPointer)
}

This gives:

myMap before assignment: map[]
myMap after assignment : map[]
myMapPointer before assignment: <nil>
myMapPointer after assignment : &map[]