function for converting a struct to map in Golang

Question 1

I also had need for something like this. I was using an internal package which was converting a struct to a map. I decided to open source it with other struct based high level functions. Have a look:

https://github.com/fatih/structs

It has support for:

Convert struct to a map
Extract the fields of a struct to a []string
Extract the values of a struct to a []values
Check if a struct is initialized or not
Check if a passed interface is a struct or a pointer to struct

You can see some examples here: http://godoc.org/github.com/fatih/structs#pkg-examples For example converting a struct to a map is a simple:

type Server struct {
    Name    string
    ID      int32
    Enabled bool
}

s := &Server{
    Name:    "gopher",
    ID:      123456,
    Enabled: true,
}

// => {"Name":"gopher", "ID":123456, "Enabled":true}
m := structs.Map(s)

The structs package has support for anonymous (embedded) fields and nested structs. The package provides to filter certain fields via field tags.

Question 2

From struct to map[string]interface{}

package main

import (
    "fmt"
    "encoding/json"
)

type MyData struct {
    One   int
    Two   string
    Three int
}

func main() {   
    in := &MyData{One: 1, Two: "second"}

    var inInterface map[string]interface{}
    inrec, _ := json.Marshal(in)
    json.Unmarshal(inrec, &inInterface)

    // iterate through inrecs
    for field, val := range inInterface {
            fmt.Println("KV Pair: ", field, val)
    }
}

go playground here

Question 3

Here is a function I've written in the past to convert a struct to a map, using tags as keys

// ToMap converts a struct to a map using the struct's tags.
//
// ToMap uses tags on struct fields to decide which fields to add to the
// returned map.
func ToMap(in interface{}, tag string) (map[string]interface{}, error){
    out := make(map[string]interface{})

    v := reflect.ValueOf(in)
    if v.Kind() == reflect.Ptr {
        v = v.Elem()
    }

    // we only accept structs
    if v.Kind() != reflect.Struct {
        return nil, fmt.Errorf("ToMap only accepts structs; got %T", v)
    }

    typ := v.Type()
    for i := 0; i < v.NumField(); i++ {
        // gets us a StructField
        fi := typ.Field(i)
        if tagv := fi.Tag.Get(tag); tagv != "" {
            // set key of map to value in struct field
            out[tagv] = v.Field(i).Interface()
        }
    }
    return out, nil
}

Runnable example here.

Note, if you have multiple fields with the same tag value, then you will obviously not be able to store them all within a map. It might be prudent to return an error if that happens.

Question 4

I like the importable package for the accepted answer, but it does not translate my json aliases. Most of my projects have a helper function/class that I import.

Here is a function that solves my specific problem.


// Converts a struct to a map while maintaining the json alias as keys
func StructToMap(obj interface{}) (newMap map[string]interface{}, err error) {
    data, err := json.Marshal(obj) // Convert to a json string

    if err != nil {
        return
    }

    err = json.Unmarshal(data, &newMap) // Convert to a map
    return
}

And in the main, this is how it would be called...

package main

import (
    "fmt"
    "encoding/json"
    "github.com/fatih/structs"
)

type MyStructObject struct {
    Email string `json:"email_address"`
}

func main() {
    obj := &MyStructObject{Email: "test@test.com"}

    // My solution
    fmt.Println(StructToMap(obj)) // prints {"email_address": "test@test.com"}

    // The currently accepted solution
    fmt.Println(structs.Map(obj)) // prints {"Email": "test@test.com"}
}

Question 5

package main

import (
    "fmt"
    "reflect"
)

type bill struct {
    N1 int
    N2 string
    n3 string
}

func main() {
    a := bill{4, "dhfthf", "fdgdf"}

    v := reflect.ValueOf(a)

    values := make(map[string]interface{}, v.NumField())

    for i := 0; i < v.NumField(); i++ {
        if v.Field(i).CanInterface() {
            values[v.Type().Field(i).Name] = v.Field(i).Interface()
        } else {
            fmt.Printf("sorry you have a unexported field (lower case) value you are trying to sneak past. I will not allow it: %v\n", v.Type().Field(i).Name)
        }
    }

    fmt.Println(values)

    passObject(&values)
}

func passObject(v1 *map[string]interface{}) {
    fmt.Println("yoyo")
}

Question 6

I'm a bit late but I needed this kind of feature so I wrote this. Can resolve nested structs. By default, uses field names but can also use custom tags. A side effect is that if you set the tagTitle const to json, you could use the json tags you already have.

package main

import (
    "fmt"
    "reflect"
)

func StructToMap(val interface{}) map[string]interface{} {
    //The name of the tag you will use for fields of struct
    const tagTitle = "kelvin"

    var data map[string]interface{} = make(map[string]interface{})
    varType := reflect.TypeOf(val)
    if varType.Kind() != reflect.Struct {
        // Provided value is not an interface, do what you will with that here
        fmt.Println("Not a struct")
        return nil
    }

    value := reflect.ValueOf(val)
    for i := 0; i < varType.NumField(); i++ {
        if !value.Field(i).CanInterface() {
            //Skip unexported fields
            continue
        }
        tag, ok := varType.Field(i).Tag.Lookup(tagTitle)
        var fieldName string
        if ok && len(tag) > 0 {
            fieldName = tag
        } else {
            fieldName = varType.Field(i).Name
        }
        if varType.Field(i).Type.Kind() != reflect.Struct {
            data[fieldName] = value.Field(i).Interface()
        } else {
            data[fieldName] = StructToMap(value.Field(i).Interface())
        }

    }

    return data
}

Question 7

map := Structpb.AsMap()

// map is the map[string]interface{}

function for converting a struct to map in Golang

Edit Dec 14, 2020

Edit TL;DR version, Jun 15, 2015

Original Post