De- and encode interface{} with Gob

Question

I'm trying to de- and encode a struct which contains a Interface{} as field.

The problem there is, that the encoding works fine, but if I try to decode the data to data the value gets { <nil>}.

It actually works, if I change Data interface{} to Data substring, but this is not a solution for me because I want to cache the results of a query to a database which have different types depending on the query. (e.g. Users or Cookies)

Minimal working example

Source

http://play.golang.org/p/aX7MIfqrWl

package main

import (
    "bytes"
    "encoding/gob"
    "fmt"
)

type Data struct {
    Name string
    Data interface{}
}

type SubType struct {
    Foo string
}

func main() {
    // Encode
    encodeData := Data{
        Name: "FooBar",
        Data: SubType{Foo: "Test"},
    }
    mCache := new(bytes.Buffer)
    encCache := gob.NewEncoder(mCache)
    encCache.Encode(encodeData)

    fmt.Printf("Encoded: ")
    fmt.Println(mCache.Bytes())

    // Decode
    var data Data
    pCache := bytes.NewBuffer(mCache.Bytes())
    decCache := gob.NewDecoder(pCache)
    decCache.Decode(&data)

    fmt.Printf("Decoded: ")
    fmt.Println(data)
}

Outputs

Expected output

Encoded: [37 255 129 3 1 1 4 68 97 116 97 1 255 130 0 1 2 1 4 78 97 109 101 1 12 0 1 4 68 97 116 97 1 255 132 0 0 0 29 255 131 3 1 1 7 83 117 98 84 121 112 101 1 255 132 0 1 1 1 3 70 111 111 1 12 0 0 0 19 255 130 1 6 70 111 111 66 97 114 1 1 4 84 101 115 116 0 0]

Decoded: {FooBar {Test}}

Current Result

Encoded: [37 255 129 3 1 1 4 68 97 116 97 1 255 130 0 1 2 1 4 78 97 109 101 1 12 0 1 4 68 97 116 97 1 255 132 0 0 0 29 255 131 3 1 1 7 83 117 98 84 121 112 101 1 255 132 0 1 1 1 3 70 111 111 1 12 0 0 0 19 255 130 1 6 70 111 111 66 97 114 1 1 4 84 101 115 116 0 0]

Decoded: { }

How is the decoder supposed to determine the type into which to decode ? — Denys Séguret, Jan 02 '13 at 11:46
`var data Data` and `decCache.Decode(&data)` should handle this, or am I missing something here? — Testuser, Jan 02 '13 at 11:48
This gives the address into which to decode, but not the type if Data is an interface{}. — Denys Séguret, Jan 02 '13 at 11:49

score 44 · Answer 1 · answered Mar 09 '13 at 23:06

The problem is that in your code, there is an error when executing encCache.Encode(encodeData) but since you don't check for error, you don't realize that. The output is blank because encodedData fails to get encoded properly.

If you add error checking,

err := enc.Encode(encodeData)
if err != nil {
    log.Fatal("encode error:", err)
}

Then you'd see something like

2013/03/09 17:57:23 encode error:gob: type not registered for interface: main.SubType

If you add one line to your original code before enc.Encode(encodeData),

gob.Register(SubType{})

Then you get expected output.

Decoded: {FooBar {Test}}

See http://play.golang.org/p/xt4zNyPZ2W

score -7 · Accepted Answer · edited Jan 02 '13 at 20:20

You can't decode into an interface because the decoder has no way to determine what type the field should be.

You can handle this in a few different ways. One is to have Data hold a struct with a field for every type that could be decoded. But the type could be very complicated.

The other way is to implement the GobDecoder and GobEncoder interface for your struct and implement your own serialization for the types. This is probably not ideal though.

Perhaps the best approach is to have the cache store specific types instead and use separate method for each type. To use your example. Your application would have a cache method called GetSubType(key string) (*SubType, error) on the cache. This would return the concrete type or a decoding error instead of an interface. It would be cleaner and more readable as well as more typesafe.

The third approach (`GetSubType`) sounds absolutely reasonable to me. Thanks! — Testuser, Jan 02 '13 at 20:28
Downvoting because this answer is wrong; Dmitri's answer shows the very simple approach to make it work. — Bryan, Feb 10 '15 at 18:20