panic: assignment to entry in nil map

轉載自：https://github.com/kevinyan815/gocookbook/issues/7
golang中map是引用型別，應用型別的變數未初始化時預設的zero value是nil。直接向nil map寫入鍵值資料會導致執行時錯誤

panic: assignment to entry in nil map

看一個例子：

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

執行這段程式會出現執行時從錯誤:

panic: assignment to entry in nil map

因為在宣告alphabetMap後並未初始化它，所以它的值是nil, 不指向任何記憶體地址。需要通過make方法分配確定的記憶體地址。程式修改後即可正常執行:

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"])
}

關於這個問題官方文件中解釋如下：

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)

同為引用型別的slice，在使用append 向nil slice追加新元素就可以，原因是append方法在底層為slice重新分配了相關陣列讓nil slice指向了具體的記憶體地址

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.