Cannot assign to struct field in a map

Question

I have the data structure like this:

type Snapshot struct {
  Key   string
  Users []Users
}

snapshots := make(map[string] Snapshot, 1)

// then did the initialization
snapshots["test"] = Snapshot {
  Key: "testVal",
  Users: make([]Users, 0),
}

Users is another struct.

Then when I tried to append some new Users values in the Users slice like this:

snapshots["test"].Users = append(snapshots["test"].Users, user)

I kept getting this error:

cannot assign to struct field snapshots["test"].Users in map

Also tried the workaround here https://github.com/golang/go/issues/3117 so like this:

tmp := snapshots["test"].Users
tmp = append(tmp, user)
snapshots["test"].Users = tmp

But no luck, still exactly same error.

And also tried to declare the map with pointer, so: snapshots := make(map[string] *Snapshot, 1), still no luck.

Answer 1

For those looking for a simpler example:

This is wrong:

type myStruct struct{
   Field int
}

func main() {
   myMap := map[string]myStruct{
        "key":{
            Field: 1,
        },
   }

   myMap["key"].Field = 5
}

Because myMap["key"] is not "addressable".

This is correct:

type myStruct struct{
   Field int
}

func main(){
   myMap := map[string]myStruct{
       "key":{
           Field: 1,
       },
   }

   // First we get a "copy" of the entry
   if entry, ok := myMap["key"]; ok {

       // Then we modify the copy
       entry.Field = 5
    
       // Then we reassign map entry
       myMap["key"] = entry
   }

   // Now "key".Field is 5
   fmt.Println(myMap) // Prints map[key:{5}]
}

Here you have a working example.

Answer 2

First, for this question, the solution in this post Why do I get a "cannot assign" error when setting value to a struct as a value in a map? works perfectly fine.

Then, finally figured out why after I already changed to use pointer my case still doesn't work, refer to the below very simple code:

a := make([]int, 3)
fmt.Println(len(a))

b := make(map[string]string, 3)
fmt.Println(len(b))

What do think the output will be? I simply thought it is all would be: 3, but actually for the map, the output will be 0

Then later in the map initialization process, i used a for loop and with this value len(snapshots), that means the initialization process will never get run...

Yea, that is the reason.

Answer 3

For my use case, I needed to update the entries fairly often. Thus, modifying the copy and reassigning it to the map entry would have been very inefficient. An alternative way could be to use a pointer to a struct instead. (I know it won't fit for all use cases, but just in case yours is flexible enough to use either a struct or a pointer to it...)

type bigStruct struct {
    val1 int
    val2 bool
    val3 string
}

newMap := make(map[string]*bigStruct)

newMap["struct1"] = &bigStruct{1, true, "example1"}

// and can now modify the entries normally
newMap["struct1"].val1 = 2
newMap["struct1"].val2 = false
newMap["struct1"].val3 = "example2"

See the full code here.