[boost] [serialization] Limited Forward Compatibility

Subject: [boost] [serialization] Limited Forward Compatibility
From: Dmitry Shesterkin (dfb_at_[hidden])
Date: 2013-02-06 18:27:01

Hello,

We use xml serialization for saving and loading several types of metadata  
and we have to support possibility of reading new versions of files with  
old versions of soft. Boost::serialization is very good at backward  
compatibility but does not provide anything with opposite problem.
So, This patch introduces new flag boost::archive::skip_unknown_tail_tags.  
When this flag is set and basic_xml_iarchive is faced with unknown tags at  
the end of element it will skip them (include nested).

E.g., the first version of some file was:
<Test class_id="0" tracking_level="0" version="0">
     <a>31</a>
     <t2 class_id="1" tracking_level="0" version="0">
         <a>11</a>
         <b>12</b>
     </t2>
     <b>32</b>
</Test>

After a while, the second version could be:
<Test class_id="0" tracking_level="0" version="0">
     <a>31</a>
     <t2 class_id="1" tracking_level="0" version="1">
         <a>11</a>
         <b>12</b>
         <c>13</c>
         <d>14</d>
         <t1 class_id="2" tracking_level="0" version="0">
             <a>1</a>
             <b>2</b>
         </t1>
     </t2>
     <b>32</b>
</Test>
Therefore, if the first version software set skip_unknown_tail_tags it can  
read second version of file (if does not set then exception will be  
thrown).

See patch in attachments for details.
Could it be useful, and should it be merged in trunk?

Thanks in advance,
Dmitry