encoding: empty values are not considered during unmarshal

[azr]

Hello there !

On any golang version (1.5 or 1.6 ).

# http://play.golang.org/p/71ZtR0mX9K
package main

import (
    "encoding/json"
    "fmt"
)

type FooBar struct {
    Foo int `json:"Foo,omitempty"`
    Bar int `json:"Bar,omitempty"`
}

var (
    foo1 = `{"Foo": 1}`
    bar1 = `{"Bar": 1}`
)

func main() {
    fb := FooBar{}

    json.Unmarshal([]byte(foo1), &fb)
    fmt.Printf("%+v\n", fb)

    json.Unmarshal([]byte(bar1), &fb)
    fmt.Printf("%+v\n", fb)
}

outputs

{Foo:1 Bar:0}
{Foo:1 Bar:0}

I assume it's because as its not set it's considered empty and omitted.

But this can provoque weird behaviours when you reuse structs twice or have an unmarshal loop that read a file. Or worse, when you rewrite a that file with a loop in a different format.

---

Possible solutions:

• add a reset() builtin, the one that's called just after allocation ? How would that work with pointers ?
• make the encoding pkgs reset unset fields in Unmarshal using reflection ?
• just tell people to be careful

cheers !

[ALTree]

Dup of #14640 I believe.

[azr]

oops sorry about that !

[ianlancetaylor]

Closing as duplicate.