Hi Robert, On Sat, Sep 24, 2011 at 10:08 AM, Robert Ramey <ramey@rrsd.com> wrote:
Current, serializing quantity<si::length> q produces:
<q class_id="0" tracking_level="1" version="0" object_id="_0"> <value>5</value> </q>
Which doesn't give any information about the quantity at hand, not even for the human reading it. So, I tried to use BOOST_CLASS_EXPORT_GUID to register a name for the serialized class, but it is ignored. This is a feature of the xml_?archive implementation. In general, the archive implementations include the minimum information required to re-constitute the original data - and no more.
Ah! that may also explain why in all the examples I see of BOOST_CLASS_EXPORT, it only worked (saved a string with the name of the class) for derived classes saves from their base class. Good. Having settled on that, I think it would be cool to have the *option* to save the class name (or other tag information) with the object. Just a suggestion for Boost.Serialization.XML I think of the quantity example because all quantities saved to xml_archive look exactly the same regardless of whether they are the same dimension or not, so it exists the possibility that quantity<si::length> is saved, but then read as quantity<si::time>. Now about a workaround from the point of view of Boost.Units, suppose I want to ensure that additional information (spurious for reconstruction but useful as a check and human reader) with the current implementation. Currently the serialization of boost.unit.quantity is: // boost/units/io.hpp template<class Archive,class Unit,class Y> inline void serialize(Archive& ar,boost::units::quantity<Unit,Y>& q,const unsigned int /*version*/){ ar & boost::serialization::make_nvp("value", q.value); //basically this in the code } if changed to template<class Archive,class Unit,class Y> inline void serialize(Archive& ar,boost::units::quantity<Unit,Y>& q,const unsigned int /*version*/){ ar & boost::serialization::make_nvp("value", q.value); //basically this in the code #ifdef BOOST_UNIT_SERIALIZATION_ADD_INFO std::string unit_info = boost::to_string(Unit()); ar & boost::serialization::make_nvp("unit", unit_info ); #ifdef BOOST_UNIT_SERIALIZATION_CHECK_INFO assert( unit_info == boost::to_string(Unit()); // or throw SOMETHING (see below)? #endif #endif } then I expect the output to be: <q class_id="0" tracking_level="1" version="0" object_id="_0"> <value>5</value> <unit>m</unit> </q> Which provides information to the human and and also can be used do a basic check at reading the archive. The logic would be that nobody ever will want to read a saved quantity of a given system and dimension as another quantity in anothe system or dimension. In this case this is a suggestion for the Boost.Units serialization implementation. If double check is not desired the human can still benefit, as it can if the tag had the following info <q unit = "m" class_id="0" tracking_level="1" version="0" object_id="_0"> BTW, Robert, what would be the canonical exception to throw from the assert line in the proposed code (which I plan to implement internally)? something that can by useful for the error handling of boost.serialization? (I now understand that boost.serialization was not designed with this in mind, but I need to do something like this to add this little piece of information to the xml.) Thanks, Alfredo