Mastering Go - Second Edition

Storing to a nil map

The following Go code will work:

aMap := map[string]int{} 
aMap["test"] = 1 

However, the next Go code will not work because you have assigned the nil value to the map you are trying to use:

aMap := map[string]int{} 
// var aMap map[string]int 
aMap = nil 
fmt.Println(aMap) 
aMap["test"] = 1 

Saving the preceding code to failMap.go and trying to compile it will generate the next error message:

$ go run failMap.go
map[]
panic: assignment to entry in nil map
...
  

This means that trying to insert data to a nil map will fail. However, looking up, deleting, finding the length, and using range loops on nil maps will not crash your code.