Create a struct by reflection in Go

Question

I want to define a struct completely dynamically, so that I can get the following struct, but without defining it first?

type Data struct {
   a string
   b int `json:"b"`
}
d := Data{}

The main issue here is that you cannot, via reflection, set any unexported field (since Go 1.8). Since a and b are both unexported, you will only be able to create a zero-valued Data object. — torek
– torek, Commented Aug 20, 2019 at 6:08
The function godoc.org/reflect#StructOf will get you most of the way there. The unexported fields shown in the question are not supported. — user5728991
– user5728991, Commented Aug 20, 2019 at 6:25
I'm with @bereal here, why is this needed? There might be another way that doesn't rely on reflection and etc. — mazei513
– mazei513, Commented Aug 20, 2019 at 11:30

asethi · Accepted Answer · 2019-08-20 12:57:02Z

13

An application can create a struct programmatically using reflect.StructOf, but all fields in the struct must be exported.

The question gets the struct as a value, but it's likely that a pointer to the struct is more useful to the application.

Given the above, this answer shows how to do the following without defining the type at compile time:

type Data struct {
   A string `json:"a"`
   B int `json:"b"`
}
var d interface{} = &Data{}

The code is:

t := reflect.StructOf([]reflect.StructField{
    {
        Name: "A",
        Type: reflect.TypeOf(int(0)),
        Tag:  `json:"a"`,
    },
    {
        Name: "B",
        Type: reflect.TypeOf(""),
        Tag:  `json:"B"`,
    },
})
d := reflect.New(t).Interface()

Here's a runnable example that sets some fields: https://play.golang.org/p/uik7Ph8_BRH

edited Aug 20, 2019 at 12:57

answered Aug 20, 2019 at 12:49

asethi

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