Multiple Instance of an object in a FlatBuffers UNION

Question

I am new to FlatBuffers and unfortunately, the Google Tutorial is not very clear about it, So here are my questions:

• Is it possible to have multiple object instances in a FlatBuffer Union without using a table?
• If Yes, How to retrieve them?

I tried the following code based on the tutorial:

import os
import sys
sys.path.append(os.path.join(os.path.dirname(__file__), '../python'))

import flatbuffers
import MyGame.Sample.Color
import MyGame.Sample.Equipment
import MyGame.Sample.Monster
import MyGame.Sample.Vec3
import MyGame.Sample.Weapon

# Example of how to use FlatBuffers to create and read binary buffers.

def main():
  builder = flatbuffers.Builder(0)

  # Create some weapons for our Monster ('Sword' and 'Axe').
  weapon_one = builder.CreateString('Sword')
  weapon_two = builder.CreateString('Axe')

  MyGame.Sample.Weapon.WeaponStart(builder)
  MyGame.Sample.Weapon.WeaponAddName(builder, weapon_one)
  MyGame.Sample.Weapon.WeaponAddDamage(builder, 3)
  sword = MyGame.Sample.Weapon.WeaponEnd(builder)

  MyGame.Sample.Weapon.WeaponStart(builder)
  MyGame.Sample.Weapon.WeaponAddName(builder, weapon_two)
  MyGame.Sample.Weapon.WeaponAddDamage(builder, 5)
  axe = MyGame.Sample.Weapon.WeaponEnd(builder)

  # Serialize the FlatBuffer data.
  name = builder.CreateString('Orc')

  MyGame.Sample.Monster.MonsterStartInventoryVector(builder, 10)
  # Note: Since we prepend the bytes, this loop iterates in reverse order.
  for i in reversed(range(0, 10)):
    builder.PrependByte(i)
  inv = builder.EndVector(10)

  MyGame.Sample.Monster.MonsterStartWeaponsVector(builder, 2)
  # Note: Since we prepend the data, prepend the weapons in reverse order.
  builder.PrependUOffsetTRelative(axe)
  builder.PrependUOffsetTRelative(sword)
  weapons = builder.EndVector(2)

  pos = MyGame.Sample.Vec3.CreateVec3(builder, 1.0, 2.0, 3.0)

  MyGame.Sample.Monster.MonsterStart(builder)
  MyGame.Sample.Monster.MonsterAddPos(builder, pos)
  MyGame.Sample.Monster.MonsterAddHp(builder, 300)
  MyGame.Sample.Monster.MonsterAddName(builder, name)
  MyGame.Sample.Monster.MonsterAddInventory(builder, inv)
  MyGame.Sample.Monster.MonsterAddColor(builder,
                                        MyGame.Sample.Color.Color().Red)
  MyGame.Sample.Monster.MonsterAddWeapons(builder, weapons)
  # Equiping the first Weapon
  MyGame.Sample.Monster.MonsterAddEquippedType(
      builder, MyGame.Sample.Equipment.Equipment().Weapon)
  MyGame.Sample.Monster.MonsterAddEquipped(builder, axe)
  # Equiping the second Weapon
  MyGame.Sample.Monster.MonsterAddEquippedType(
      builder, MyGame.Sample.Equipment.Equipment().Weapon)
  MyGame.Sample.Monster.MonsterAddEquipped(builder, sword)
  orc = MyGame.Sample.Monster.MonsterEnd(builder)

  builder.Finish(orc)

The following code does not generate an error, but when reading the data it seems that I can get only the last added Weapon (The variable is overwritten). So is it the expected behaviour?

Thanks :)

Answer 1

No, a union represents one value out of a range of options (much like in Python a single variable can't hold 2 subclass instances, unless you make it a list).

To achieve what you want you'd need a vector of unions (which sadly is not implemented in Python FlatBuffers yet), or otherwise a vector of tables, each of which holds a union.