2016-11-12 03:19
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I am fairly new to Golang. I know there is Struct in Golang. But for all I know, you have to define struct

type Circle struct{
    x,y,r float64

I am wondering how you can declare a new variable that doesn't exist in the Struct

circle := new(Cirlce)
circle.color = "black"

Thx in advance.

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我刚接触Golang。 我知道Golang中有Struct。 但据我所知,您必须定义struct

  type Circle struct {
x,y,r float64 
   \  n 


  circle:= new(Cirlce)
circle.color =  ”“ black” 


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  • dongxian0421 2016-11-12 23:33

    You will need to use a map (of type map[string]interface{}) to work with dynamic JSON. Here is an example of creating a new map:

    // Initial declaration
    m := map[string]interface{}{
        "key": "value",
    // Dynamically add a sub-map
    m["sub"] = map[string]interface{}{
        "deepKey": "deepValue",

    Unmarshalling JSON into a map looks like:

    var f interface{}
    err := json.Unmarshal(b, &f)

    The code above would leave you with a map in f, with a structure resembling:

    f = map[string]interface{}{
        "Name": "Wednesday",
        "Age":  6,
        "Parents": []interface{}{

    You will need to use a type assertion to access it, otherwise Go won't know it's a map:

    m := f.(map[string]interface{})

    You will also need to use assertions or type switches on each item you pull out of the map. Dealing with unstructured JSON is a hassle.

    More information:

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  • dqpwdai095465 2016-11-12 08:32

    You can't. Go is statically typed, and does not allow such constructs.

    Structs have a layout in memory that directly related to the definition, and there's no where to store such additional fields.

    You can use a map instead. Moreover, you can use &circle as a key or part of a key, to associate map elements with arbitrary structs.

    type key struct {
        target interface{}
        field string
    x := make(map[key]string)
    x[key{ target: circle, field: "color" }] = "black"
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  • doyz51819 2019-01-05 21:14

    I've started to work on this small repository

    It's possible at this point to extend existing struct in runtime, by passing a instance of struct and modifying fields (adding, removing, changing types and tags).

    Still in progress, so don't expect something huge :)

    EDIT: At this point, work on library is done, and it looks stable for last a couple of months :)

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