初始化嵌套的结构图

Question

The problem:

I've got a map of structs in another struct and I'd like to initialize the nested map of structs, but apparently that is not possible.

Code:

type Exporter struct {
    TopicsByName      map[string]Topic
}

type Topic struct {
    Name       string
    Partitions map[int32]Partition
}

type Partition struct {
    PartitionID   int32
    HighWaterMark int64
}

// Eventually I want to do something like:
e := Exporter{ TopicsByName: make(map[string]Topic) }
for _, topicName := range topicNames {
  // This does not work because "cannot assign to struct field e.TopicsByName[topicName].Partitions in map"
  e.TopicsByName[topicName].Partitions = make(map[int32]Partition)
}

// I wanted to initialize all these maps so that I can do
e.TopicsByName[x.TopicName].Partitions[x.PartitionID] = Partition{...}

I don't understand why I can not initialize the nested struct map above. Is it so bad to nest maps with struct as values? How can I fix this?

Answer 1

It is not possible to assign to a field in a map value. The fix is to assign a struct value to the map value:

for _, topicName := range []string{"a"} {
    e.TopicsByName[topicName] = Topic{Partitions: make(map[int32]Partition)}
}