Re: [Boost-users] Boost Serialization + Forward compatibility [was Re: boost::serialization and PHP websites]

Subject: Re: [Boost-users] Boost Serialization + Forward compatibility [was Re: boost::serialization and PHP websites]
From: Sohail Somani (sohail_at_[hidden])
Date: 2009-05-31 23:24:27

Robert Ramey wrote:
> Sohail Somani wrote:
>> Still interested in forward compatibility, if you are inclined to
>> elaborate a little.
> 
> Here we go.  Basically, backward compatibility is handled by code
> that looks like the following:
> 
> template<class Archive>
> void save(Archive &ar, const unsigned int version){
>     ar << original_stuff
>     ar << stuff_added_version_3;
>     ar << stuff_added_version_4;
> }
> 
> template<class Archive>
> void load(Archive &ar, const unsigned int version){
>     ar >> original_stuff;
>     if(version > 3)
>         ar >> stuff_added_version_3;
>     if(version > 4)
>         ar >> stuff_added_version_4;
>     ...
> }
> 
> So what we need to do is replace the save above with:
> 
> template<class Archive>
> void save(Archive &ar, const unsigned int version) const {
>     ar << original_stuff
>     if(version > 3)
>         ar << stuff_added_version_3;
>     if(version > 4)
>         ar << stuff_added_version_4;
> }

Just typing out loud: Forward compatibility is the ability to load an
archive with a version of 4 into an application with a version of 3.

So if we have std::vector<SomeClass> and in version 4, I add an int to
SomeClass, I'm pretty sure it would cause the whole thing to go boom
when loading an archive from version 4 into version 3.

I'm not sure if the above avoids this problem, does it?

-- 
Sohail Somani
http://uint32t.blogspot.com