Go structs comparison - reflect.DeepEqual fails on maps?

Question

I'm writing unit test and my goal is to unmarshall data from json to one struct and compare it to the other, mock struct. I'm using reflect.DeepEqual() method but it's returning false on these.

My guess is that it is somehow related to type casting going on in the background, where map[string]interface{} is converted to map[string]int, but that's as far as I got.

type MyStruct struct {
    Cache map[string]interface{} `json:"cache"`
}

var js = `{"cache":{"productsCount":28}}`

func main() {
    var s1, s2 MyStruct
    s1 = MyStruct{
        Cache: map[string]interface{} {
            "productsCount": 28,
        },
    }
    s2 = MyStruct{}
    err := json.Unmarshal([]byte(js), &s2)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }

    fmt.Printf("%#v\n", s1)
    fmt.Printf("%#v\n", s2)
    fmt.Println(reflect.DeepEqual(s1, s2))
}

The output looks like this:

main.MyStruct{Cache:map[string]interface {}{"productsCount":28}}
main.MyStruct{Cache:map[string]interface {}{"productsCount":28}}
false

Possible duplicate of [How to compare struct, slice, map are equal?](https://stackoverflow.com/questions/24534072/how-to-compare-struct-slice-map-are-equal) — Vikash Pathak, Jul 16 '19 at 13:23
I don't see it as duplicate, that thread is much more general — IFeel3, Jul 16 '19 at 13:32

score 9 · Accepted Answer · answered Jul 16 '19 at 13:21

The thing here is how the golang encoding an int, you're initializing it as int, but in the json you provide it is float64.

Here is working example:

package main

import (
    "encoding/json"
    "fmt"
    "os"
    "reflect"
)

type MyStruct struct {
    Cache map[string]interface{} `json:"cache"`
}

var js = `{"cache":{"productsCount":28}}`

func main() {
    var s1, s2 MyStruct
    s1 = MyStruct{
        Cache: map[string]interface{}{
            "productsCount": float64(28),
        },
    }
    s2 = MyStruct{}
    err := json.Unmarshal([]byte(js), &s2)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }

    fmt.Printf("%#v\n", s1)
    fmt.Printf("%#v\n", s2)
    fmt.Println(reflect.DeepEqual(s1, s2))
}

Output:

main.MyStruct{Cache:map[string]interface {}{"productsCount":28}}
main.MyStruct{Cache:map[string]interface {}{"productsCount":28}}
true

thanks, this is excactly the explanation I needed. I was missing knowledge about json marshalling details. — IFeel3, Jul 16 '19 at 13:34

Go structs comparison - reflect.DeepEqual fails on maps?

1 Answers1