Re: [Boost-users] serilalization of union

Subject: Re: [Boost-users] serilalization of union
From: Robert Ramey (ramey_at_[hidden])
Date: 2009-01-07 20:34:57

This is what boost variant serialization does.

Robert Ramey

Andrew Holden wrote:
> 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;
>    }
>  }
> };