How can I serialize std::type_info using Boost serialization?

Question

I want to record the std::type_info of a variable so that on load I will be able to re-create the appropriate type variable.

Saving will look like this:

friend class boost::serialization::access;
template<class Archive>
void save(Archive & ar, const unsigned int version) const
{
    ...
    ar & BOOST_SERIALIZATION_NVP(typeid(value)); 
    ar & BOOST_SERIALIZATION_NVP(value);
    ...
}

Loading will look like this:

template<class Archive>
void load(Archive & ar, const unsigned int version)
{
    ...
    std::type_info currentType;
    ar & BOOST_SERIALIZATION_NVP(currentType);

    if(currentType.name() == typeid(std::string).name())
    {
        std::string value;
        ar & BOOST_SERIALIZATION_NVP(value);

        //this next line is why I need to read back the type
        someObject.setValue<std::string>(value);
    }
    ... //else if for all other types
}

Or, if you have any other ideas for a way to solve this I am open to suggestions. I feel like I am going about this in the wrong way.

Answer 1

I don't know if this is exactly what you're looking for, and this is certainly a different approach from what you have so far, so YMMV... but in the past I have accomplished a similar goal by creating a type hierarchy and then using Boost.Serialization's ability to automatically serialize/deserialize a pointer to a polymorphic type. This has the benefit of not having to muck with any RTTI in your code (the library may be doing RTTI under the hood - I'm not sure), but does have a few drawbacks.

For one, it certainly makes the code more complicated IMO. You may be dealing with types in the code that you can't easily modify or work into a hierarchy. You also may not be willing to take the (relatively small) performance hit of polymorphism and heap allocation.

Here's a more or less complete example. The visitor pattern is useful here since you may want just a very thin wrapper around existing types that might not have anything in common:

#include <boost/serialization/export.hpp>

class Visitor;

class Base
{
public:
    virtual ~Base() { }
    virtual void accept(const Visitor & v) = 0;

protected:
    friend class boost::serialization::access;
    template <typename Archive>
    void serialize(Archive & ar, const unsigned int version)
    { }
};

/* specialization for one set of types */
class Type1 : public Base
{
public:
    virtual ~Type1() { }
    virtual void accept(const Visitor & v) { ... }

protected:
    friend class boost::serialization::access;
    template <typename Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
        ar & boost::serialization::base_object<Base>(*this);
        ar & m_dataType1;
        ar & m_dataType2;
        //etc...
    }
    //member data follows...
};

/* specialization for some other set of types */
class Type2 : public Base
{
public:
    virtual ~Type2() { }
    virtual void accept(const Visitor & v) { ... }

protected:
    friend class boost::serialization::access;
    template <typename Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
        ar & boost::serialization::base_object<Base>(*this);
        ar & m_dataType1;
        ar & m_dataType2;
        //etc...
    }
};

BOOST_CLASS_EXPORT_GUID(Type1, "Type1")
BOOST_CLASS_EXPORT_GUID(Type2, "Type2")

The BOOST_CLASS_EXPORT_GUID is required to "register" the derived types with the library so that it can identify them uniquely within the archive. For a more complete explanation, see the section titled "Pointers to Objects of Derived Classes" on this page

Now you can serialize and deserialize using a pointer to the base class (can even used shared_ptr ;), and use the visitor pattern (or some other method) to access the data via polymorphism at runtime, e.g.:

boost::shared_ptr<Base> p;
...
p.reset(new Type1(...));
archive << p;
p.reset(new Type2(...));
archive << p;
...
archive >> p; //p now points to a Type1
archive >> p; //p now points to a Type2

This question and answer may also be useful: http://stackoverflow.com/questions/478668/boost-serialization-using-polymorphic-archives

Hope this helps!

EDIT: fixed my link to the Boost docs...