Use describe to serialize/unserialize xml (xml_oarchive/xm_iarchive is too limited)

[vricosti]

Hi,

I am using describe to serialize.unserialize some structs to json (thanks to boost::json) but now I would also be able to do it in xml. I tried boost::archive::: (xml_oarchive/xml_iarchive) but the resulting xml always have some useless nodes ( ) that cannot be easily removed from what I understand. Let's consider the following code:

namespace JsonSerializer
{
   struct Item
   {
      std::string id;
      std::string label;
      std::string iconFile;
   };
   BOOST_DESCRIBE_STRUCT(Item, (), (id, label, iconFile))

   struct Items
   {
      using Type = std::vector<Item>;
      Type items;
   };
   BOOST_DESCRIBE_STRUCT(Items, (), (items));

} // namespace JsonSerializer

// Lines below allows to not add bunch of ugly tags/info when serializing in xml
BOOST_CLASS_IMPLEMENTATION(JsonSerializer::Item, boost::serialization::object_serializable);
BOOST_CLASS_IMPLEMENTATION(JsonSerializer::Items::Type, boost::serialization::object_serializable);
BOOST_CLASS_IMPLEMENTATION(JsonSerializer::Items, boost::serialization::object_serializable);

When I try fill an Items object and I serialize here is the result:

Items items;
// ... some code to fill items list ...
std::ostringstream os;
unsigned int flags = boost::archive::no_header;
boost::archive::xml_oarchive ar(os, flags);
ar << boost::make_nvp("items", items.items);
std::string sXml = os.str();

<items>
    <count>1</count>
    <item_version>0</item_version>
    <item>
        <id>standalone</id>
        <label>StandAlone</label>
        <iconFile>Logo_StandAlone.png</iconFile>
    </item>
</items>

Even with all the ugly BOOST_CLASS_IMPLEMENTATION macros and the use of boost::archive::no_header, the result is not satisfying (useless item_version, count), so where should I start to get a clean xml ?

[pdimov]

<count> and <item_version> are emitted by the serialization of std::vector. Looking at the Boost.Serialization source, I don't see any way to turn them off. You might want to ask this question in an issue in boostorg/serialization and see if @robertramey has something to offer as an advice.

I'm not sure that it would be possible to avoid the <count> due to the architecture of Boost.Serialization, which on reading needs to know how many elements to load. I don't know what's the purpose of <item_version>. It's possible that it's needed for optimized saving and loading of arrays of primitive types, but only Robert can say for sure.

[vricosti]

Ok thanks

[vricosti]

I have modified a bit my struct definitions like this:

struct Item
   {
      std::string id;
      std::string label;
      std::string iconFile;
   };
   BOOST_DESCRIBE_STRUCT(Item, (), (id, label, iconFile))

   using Items = std::vector<Item>;

   struct ItemInfo : public Item
   {
      std::string accesLevel;
      std::string isEnableWhenOffline;
      std::string isEnableWhenDetectFwUpdate;
   };
   BOOST_DESCRIBE_STRUCT(ItemInfo, (Item), (accesLevel, isEnableWhenOffline, isEnableWhenDetectFwUpdate))

   using ItemsInfo = std::vector<ItemInfo>;

and when I serialize in xml my object with the code below:

#include <boost/archive/xml_oarchive.hpp>
std::ostringstream os;
boost::archive::xml_oarchive ar(os);
ar << boost::make_nvp("itemsInfo", itemsInfo);
std::string s = os.str();

I get the following result:

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!DOCTYPE boost_serialization>
<boost_serialization signature="serialization::archive" version="19">
<itemsInfo>
    <count>1</count>
    <item_version>0</item_version>
    <item>
        <base.1>
            <id>standalone</id>
            <label>StandAlone</label>
            <iconFile>Logo_StandAlone.png</iconFile>
        </base.1>
        <accesLevel>CUSTOMER</accesLevel>
        <isEnableWhenOffline>true</isEnableWhenOffline>
        <isEnableWhenDetectFwUpdate>true</isEnableWhenDetectFwUpdate>
    </item>
</itemsInfo>

Is it possible to not have a node and put , at the same level as the other nodes (accessLevel ,...) ?

[pdimov]

It's possible; you need to change the definition of serialize to

template<class Archive, class T,
    class D1 = boost::describe::describe_members<T,
        boost::describe::mod_public | boost::describe::mod_inherited>>
    void serialize( Archive & ar, T & t, boost::serialization::version_type )
{
    boost::mp11::mp_for_each<D1>([&](auto D){

        ar & boost::make_nvp( D.name, t.*D.pointer );

    });
}

Using mod_inherited enumerates all members, including those from the base classes.

[robertramey]

I'm not sure that it would be possible to avoid the due to the architecture of Boost.Serialization, which on reading needs to know how many elements to load. I don't know what's the purpose of . It's possible that it's needed for optimized saving and loading of arrays of primitive types, but only Robert can say for sure.

which on reading needs to know how many elements to load

This is correct. with out this "useless" element how many elements should one load?

I don't know what's the purpose of

This holds the version number of the element value type of the vector. Should you change the value of type of the vector, the library will need this it to correctly load the saved vector back into memory.

I think that BOOST_CLASS_IMPLEMENTATION(JsonSerializer::Item, boost::serialization::object_serializable); is on the right track - but it seems to that it wouldn't work with xml_archive. xml_archive serializes through the wrapper BOOST_NVP( ... So something along the lines of alias item = boost::serialization::nap; BOOST_CLASS_IMPLEMENTATION(item, boost::serialization::object_serializable);