封送至JSON时将类型int转换为const名称的字符串

Question

I have a struct that looks like this:

type Type int

const (
    A Type = iota
    B
    C
)

type Character struct {
    Type Type   `json:"type"`
}

When I call json.Marshal(...) on the struct, is there a way that the json:"type" representation is a string called either "A", "B", or "C"?

When this is presented in JSON, nobody is going to know what 0, 1, or 2 is, so the name of the constant is more useful.

Apologies if this has been asked before. I googled all over and couldn't find anything.

Here is an example:

type Type int

const (
    A Type = iota
    B
    C
)

type Character struct {
    Type Type   `json:"type,string"`
}

func main() {
    c := Character{}
    c.Type = A
    j, err := json.Marshal(c)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(j))
}

I want fmt.Println(string(j)) to print {"type":"A"}, not {"type":0}.

Answer 1

You need to define a custom marshaller for your type.

type Type int

const (
    A Type = iota
    B
    C
)

var typeToString = map[Type]string{
    A: "A",
    B: "B",
    C: "C",
}

func (t Type) MarshalJSON() ([]byte, error) {
    return json.Marshal(typeToString[t])
}

type Character struct {
    Type Type `json:"type"`
}

func main() {
    c := Character{}
    c.Type = A
    j, err := json.Marshal(c)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(j))
}

The function MarshalJSON defines how json.Marshal should marshal your type. You can do something similar for unmarshaling if you need to go the other direction as well.

See https://play.golang.org/p/mLxThWA19by.