如何在go中初始化嵌套结构？ [重复]

Question

This question already has an answer here:

• Initialize a nested struct 8 answers

Hi I am very new to Golang, please help me. I have defined a struct inside a struct. But I get an error when I try to initialise the main struct.

type DetailsFilter struct {
  Filter struct {
    Name    string        
    ID      int                           
  } 
}

var M map[string]interface{}
M = make(map[string]interface{})
M["Filter"] = map[string]interface{}{"Name": "XYZ", "ID": 5}
var detailsFilter = DetailsFilter{Filter: M["Filter"]}}

The error I get is : can not use (type interface {}) as type struct in field value : need type assertion.

Please suggest a way to initialise DetailsFilter. I tried doing the method described in Initialize a nested struct in Golang, but even this is not working.

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Answer 1

Unfortunately if the type of a struct field is an anonymous struct, at construction time you can only initialize it by "duplicating" the anonymous struct type (specifying it again):

type DetailsFilter struct {
    Filter struct {
        Name string
        ID   int
    }
}

df := DetailsFilter{Filter: struct {
    Name string
    ID   int
}{Name: "myname", ID: 123}}
fmt.Println(df)

Output:

{Filter:{Name:myname ID:123}}

Shorter Alternative

So instead I recommend not to initialize it at construction, but rather after the zero-valued struct has been created, like this:

df = DetailsFilter{}
df.Filter.Name = "myname2"
df.Filter.ID = 321
fmt.Printf("%+v
", df)

Output:

{Filter:{Name:myname2 ID:321}}

Try it on the Go Playground.

Naming the anonymous struct type

Or don't use anonymous struct as field type at all, name the type like this:

type Filter struct {
    Name string
    ID   int
}

type DetailsFilter struct {
    Filter Filter
}

And then you can simply initialize it like this:

df := DetailsFilter{Filter: Filter{Name: "myname", ID: 123}}
fmt.Printf("%+v
", df)

Output (try it on the Go Playground):

{Filter:{Name:myname ID:123}}