Unmarshaling a JSON integer to an empty interface results in wrong type assertion

Question

I have this code. I expect the interface to be type asserted to int. However, the type of the interface is float64 instead. Can anyone explain why this is happening? And what are the best ways to circumvent it.

package main

import (
    "fmt"
    "encoding/json"
)

type obj struct {
    X interface{}
}

func main() {
    var x int
    x = 5
    o := &obj {
        X: x,
    }
    b, _ := json.Marshal(o)
    var newObj obj
    json.Unmarshal(b, &newObj)
    if _, ok := newObj.X.(int); ok {
        fmt.Println("X is an int")
    } else if _, ok := newObj.X.(float64); ok {
        fmt.Println("X is a float64")
    } else {
        fmt.Println("This does not make any sense")
    }
}

This code prints "X is a float64". You can run the code there https://play.golang.org/p/9L9unW8l3n

`float64` is the default numeric type for JSON. See https://stackoverflow.com/questions/35583735/unmarshaling-into-an-interface-and-then-performing-type-assertion/35584188#35584188 — JimB, Aug 25 '16 at 18:37

score 5 · Accepted Answer · answered Aug 25 '16 at 18:38

Numbers are encoded as "Json numbers" Unmarshal decodes Json numbers as floats. From the docs:

Marshal

Floating point, integer, and Number values encode as JSON numbers.

Unmarshal

To unmarshal JSON into an interface value, Unmarshal stores one of these in the interface value:

bool, for JSON booleans float64, for JSON numbers string, for JSON strings []interface{}, for JSON arrays map[string]interface{}, for JSON objects nil for JSON null

Unmarshaling a JSON integer to an empty interface results in wrong type assertion

1 Answers1