Re: [Boost-users] serilalization of union

Subject: Re: [Boost-users] serilalization of union
From: Andrew Holden (aholden_at_[hidden])
Date: 2009-01-07 13:42:31

Robert Ramey wrote:
> Steven Watanabe wrote:
>> AMDG
>>
>> niranjan bangera wrote:
>>> union myunion
>>> {
>>>
>>> uint32_t a;
>>> float t;
>>>
>>> template <class Archive>
>>> void serialize(Archive & ar, const unsigned int file_version)
>>> {
>>>
>>> ar & a;
>>> ar & t;
>>>
>>> }
>>
>> This is undefined behavior.  a and t cannot both be valid at the same
>> time.
>
> LOL - makes me wonder if I'm even reading these emails.
>
> One could do try:
>
> struct  myunion {
>     union {
>         uint32_t a;
>         float t;
>     } x;
>     operator (uint32_t &)(){ // i forget the exact casting operator syntax
>         return a;
>     }
>     operator(float &t)(){
>         return t;
>     }
>     template<class Archive>
>     void serialize(Archive & ar, const unsigned int file_version){
>         ar & binary_object(sizeof(x), &x);
>     }
> };
>
> crude - but effective.
>
> Robert Ramey

The best solutions would become more obvious if you know which type is valid at a higher level.  For example, if you have a structure like this:

struct UnionWrapper
{
  myunion  the_union;
  bool     is_float;   /* Set to true if and only if the float member is used */

  template <class Archive>
  void serialize (Archive &ar, unsigned int const version)
  {
    ar & is_float;    /*Do this first so deserialization knows which union member to get. */

    if (is_float)
    {
      ar & the_union.t;
    }
    else
    {
      ar & the_union.a;
    }
  }
};