Support unpacking object variants

[dchem]

Version: 0.2.9

Commit: 9c4bc6e66d5d193692be0e7c5605166dc61756dc

Observed:
Object variants using standard idiom (with case) packs correctly, but fails to unpack.

Expected:
Object variants with zero-initialized variables should unpack correctly

Steps to Replicate:

import streams, msgpack4nim

type MsgType* = enum
  MSG_FOO,
  MSG_BAR

type Foo* = object
  name: string

type Bar* = object
  price: int

type FooBarMsg* = object
  case msgType*: MsgType
  of MSG_FOO:
    foo*: Foo
  of MSG_BAR:
    bar*: Bar

when isMainModule:
  var fooOriginal = Foo(name: "Baz")
  var barOriginal = Bar(price: 11)
  var outboundMsgFoo = FooBarMsg(
  msgType: MSG_FOO,
  foo: fooOriginal
  )
  var outboundMsgBar = FooBarMsg(
  msgType: MSG_BAR,
  bar: barOriginal
  )

  var sFoo = MsgStream.init()
  sFoo.pack(outboundMsgFoo)

  var sBar = MsgStream.init()
  sBar.pack(outboundMsgBar)

  var incomingFoo: FooBarMsg
  var sFooReception = MsgStream.init(sFoo.data)
  try:
    sFooReception.unpack(incomingFoo)
  except:
    echo "unpack error for foo"

  var incomingBar: FooBarMsg
  var sBarReception = MsgStream.init(sBar.data)
  try:
    sBarReception.unpack(incomingBar)
  except:
    echo "unpack error for bar"

[jangko]

thank you for reporting this. case object/object variants is always an interesting subject for serializer library. it is tricky to do it properly automatically.

work-around: you can provide a custom deserializer for your special object, e.g:

proc unpack_type*[ByteStream](s: ByteStream, x: var FooBarMsg) =
    let numFields = s.unpack_array() # or unpack_map if you use OBJ_TO_MAP version
    let kind = MsgType(s.unpack_imp_int64())
    x = FooBarMsg(kind: kind)
    case kind:
    of MSG_FOO : s.unpack(x.foo)
    of MSG_BAR : s.unpack(x.bar)

EDIT: you can use stringify to debug the msgpack binary data.

[dchem]

@jangko thank you very much for the workaround suggestion. That works.

I agree that object variant handling is tough to implement correctly and generally, so please feel free to close the issue if you feel that it makes sense to do so.