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encoding/json: Go's json decoder should support '*' spill over #17079

liquidgecka opened this issue Sep 12, 2016 · 2 comments


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commented Sep 12, 2016

If the process decoding JSON wants to detect unsupported fields its very difficult. Either the json has to be decoded into a map[string]json.RawMessage then parse out each field into sub object, etc. But this could be codified into the decoder very easily. For example I would love to see this work:

What I would love is to be able to write this as code:

type Object struct {
    Size int64 `json:"size"`
    Name string`json:"name"`
    Misc map[string]interface{} `json:"*"` 

func ParseObject() error {
    input := []byte(`{
        "size": 1,
        "name": "user_name",
        "unsupported_field": 10

    obj := new(Object)
    if err := json.Unmarshal(input, obj); err != nil {
       return err
    if len(obj.Misc) != 0 {
        return fmt.Errorf("Unsupported fields: %s", MakeKeysAList(obj.Misc))

    return nil

In order to work json:"*" should only be used on a map[string]interface{} or a map[string]json.RawMessage. That is the one oddity to this that I can see outright.


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commented Sep 12, 2016

Possibly a duplicate of #6213; see also some relevant discussion in #15314.

@quentinmit quentinmit changed the title Go's json decoder should support '*' spill over encoding/json: Go's json decoder should support '*' spill over Sep 12, 2016


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commented Sep 12, 2016

Ahh yea sure enough! That looks like exactly what I am suggesting.. I will close this out. =)

Thanks! I didn't see that in my initial searches.

@golang golang locked and limited conversation to collaborators Sep 12, 2017

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