Panic：assignment to entry in nil map

7月 23, 2020

参考以下代码，运行时会报 panic: assignment to entry in nil map。

package main

import "fmt"

func main() {
	var m map[string]int
	m["a"] = 1
	fmt.Println(m["a"])
}
//panic: assignment to entry in nil map

查了一下原因，发现是通过 var m map[string]int 得到的值是 nil, 不指向任何内存地址。需要通过 make 方法才可以分配确定的内存地址。

这个问题官方博客中解释如下：

This variable m is a map of string keys to int values:

var m map[string]int

Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn’t point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don’t do that. To initialize a map, use the built in make function:

m = make(map[string]int)

同为引用类型的 slice，在使用 append 向 nil slice 追加新元素就可以，原因是 append 方法在底层为 slice 重新分配了相关数组让 nil slice 指向了具体的内存地址。

nil map doesn’t point to an initialized map. Assigning value won’t reallocate point address. The append function appends the elements x to the end of the slice s, If the backing array of s is too small to fit all the given values a bigger array will be allocated. The returned slice will point to the newly allocated array.

同样的道理，结构体通过 var 初始化并赋值也会报 Panic：

package main

type ListNode struct {
	Value int
	Next  *ListNode
}

func main() {
	var head *ListNode
	head.Value = 10
}
// panic: runtime error: invalid memory address or nil pointer dereference

var 没有初始化结构体的实例，故没有地址，所以在给 head.value 赋值的时候触发了 nil pointer 的 panic。