go - 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.

question from:https://stackoverflow.com/questions/42605337/cannot-assign-to-struct-field-in-a-map

Answer 1

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.