encoding/json: MarshalJSON isn't called for fields of a non-pointer struct

[gopherbot]

by timothy.stranex:

$ go version
go version go1.2 darwin/amd64

Here is a link to a program on play.golang.org that demonstrates the problem.
http://play.golang.org/p/HXNEgYhZgE

For reference, this is the program:

-------
package main

import "encoding/json"
import "fmt"

type DecimalE8 int64

func (d *DecimalE8) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf("%.8f", float64(*d) / 1e8)), nil
}

type Payment struct {
    Amount DecimalE8
}

func main() {
    var p Payment
    p.Amount = 123456789

    s1, _ := json.Marshal(p)
    fmt.Println(string(s1))

    s2, _ := json.Marshal(&p)
    fmt.Println(string(s2))
}
-------

The actual output from this program is:
{"Amount":123456789}
{"Amount":1.23456789}

The expected output is:
{"Amount":1.23456789}
{"Amount":1.23456789}

Basically it seems that DecimalE8.MarshalJSON isn't being used when the struct is passed
as a value to json.Marshal but it is used when it's passed as a pointer. I would expect
that DecimalE8.MarshalJSON be used in both cases.

[fraenkel]

Comment 1:

The behavior looks correct.
If you change your MarshalJSON to the following you will get your expected behavior.
func (d DecimalE8) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf("%.8f", float64(d) / 1e8)), nil
}

[adg]

Comment 2:

The language specifies that if *T implements the methods of an interface I then only a
*T (pointer) value will satisfy I, and not a T (value) value. So encoding/json can't see
that *DecimalE8 implements MarshalJSON because it only has a DecimalE8 value.
It is important to preserve this property, because a value implemented on *T might
assume that it can modify the underlying T value (which, given a value receiver, it
could not).

Status changed to WorkingAsIntended.