How to write dynamically typed data to field within a struct

Question

I have the following response struct that I want to use as a base wrapper for responding to API calls users send to me.

type Response struct {
  Data      ???                     `json:"data,omitempty"`
  Time      int64                   `json:"time,omitempty"`
  Message   string                  `json:"message,omitempty"`
}

The type of the Data field is varying and could be a map[string]*CustomStruct1 map[string*CustomStruct2 or an []CustomStruct3.

What is the best way to attack this kind of problem?

You're probably looking for the empty interface interface{} it works for any type because every type satisfies the requirement of having at least zero methods. Looking at your struct, you might want to get your Time and Message fields through embedding instead and use a separate struct for each response type. — duckbrain
– duckbrain, Commented Dec 14, 2017 at 22:49

maerics · Accepted Answer · 2017-12-15 00:17:19Z

One option is to simply treat "Data" as the interface{} (any) type, instead of using your custom structs, and handle the resulting values based on inspection of what actually got unmarshaled. Of course, once you've inspected the data to determine what type it should be you could convert it into the appropriate strong type after the fact.

type Response struct {
  Data      interface{} `json:"data,omitempty"`
  Time      int64       `json:"time,omitempty"`
  Message   string      `json:"message,omitempty"`
}

Another option is to embed the "Response" struct into specialized structs that look for your custom types and unmarshal into the appropriate one, assuming you know which one you've got ahead of time:

type BaseResponse struct {
  Time      int64  `json:"time,omitempty"`
  Message   string `json:"message,omitempty"`
}

type Response1 struct {
  BaseResponse
  Data map[string]*CustomStruct1 `json:"data"`
}

type Response2 struct {
  BaseResponse
  Data map[string]*CustomStruct2 `json:"data"`
}

// etc...

Ultimately, the unmarshaler cannot pick a varying type based on the document that gets unmarshaled, it only deserializes JSON values into structures either defined explicitly by you or into generic ones.

Victor Martinez · Accepted Answer · 2017-12-15 01:30:39Z

0

You could try to use reflection, but it wouldn't be very idiomatic.

answered Dec 15, 2017 at 1:30

Victor Martinez

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