Golang embedded struct type

Question

I have these types:

type Value interface{}

type NamedValue struct {
    Name  string
    Value Value
}

type ErrorValue struct {
    NamedValue
    Error error
}

I can use v:= NamedValue{Name: "fine", Value: 33} , but I am not able to use e:= ErrorValue{Name: "alpha", Value: 123, Error: err}

Seems that embedding syntax was ok, but using it doesn't work?

Answer 1

Embedded types are (unnamed) fields, referred to by the unqualified type name.

Spec: Struct types:

A field declared with a type but no explicit field name is an anonymous field , also called an embedded field or an embedding of the type in the struct. An embedded type must be specified as a type name T or as a pointer to a non-interface type name *T , and T itself may not be a pointer type. The unqualified type name acts as the field name.

So try:

e := ErrorValue{NamedValue: NamedValue{Name: "fine", Value: 33}, Error: err}

Also works if you omit the field names in the composite literal:

e := ErrorValue{NamedValue{"fine", 33}, err}

Try the examples on the Go Playground .

Answer 2

For deeply nested structs, the accepted answer's syntax is a little verbose. For example, this :

package main

import (
    "fmt"
)

type Alternative struct {
    Question
    AlternativeName string
}

type Question struct {
    Questionnaire
    QuestionName  string
}

type Questionnaire struct {
    QuestionnaireName string
}

func main() {
    a := Alternative{
        Question: Question{
            Questionnaire: Questionnaire{
                QuestionnaireName: "q",
            },
        },
    }
    fmt.Printf("%v", a)
}

( Go playground )

Could be rewritten like this:

a := Alternative{}
a.QuestionnaireName = "q"

Answer 3

In addition to the wonderful answer by icza.

you can simply do this:

v := NamedValue{Name: "fine", Value: 33}
e := ErrorValue{NamedValue:v, Error: err}

and it works just fine. checkout the example Here