panic: assignment to entry in nil map

转载自：https://github.com/kevinyan815/gocookbook/issues/7
golang中map是引用类型，应用类型的变量未初始化时默认的zero value是nil。直接向nil map写入键值数据会导致运行时错误

panic: assignment to entry in nil map

看一个例子：

package main
const alphabetStr string = "abcdefghijklmnopqrstuvwxyz"
func main() {
  var alphabetMap map[string]bool
  for _, r := range alphabetStr {
    c := string(r)
    alphabetMap[c] = true
  }
}

运行这段程序会出现运行时从错误:

panic: assignment to entry in nil map

因为在声明alphabetMap后并未初始化它，所以它的值是nil, 不指向任何内存地址。需要通过make方法分配确定的内存地址。程序修改后即可正常运行:

package main
import "fmt"
const alphabetStr string = "abcdefghijklmnopqrstuvwxyz"
func main() {
  alphabetMap := make(map[string]bool)
  for _, r := range alphabetStr {
    c := string(r)
    alphabetMap[c] = true
  }
  fmt.Println(alphabetMap["x"])
  alphabetMap["x"] = false
  fmt.Println(alphabetMap["x"])
}

关于这个问题官方文档中解释如下：

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.