boost Serialisation on a variant containing boost::blank

Question

I have a boost::variant wherein one of the fields has the boost::blank field, using boost::serialise on the same throws an error saying error: no member named 'serialize' in 'boost::blank'.

Is there a way to avoid this by adding a separate function for boost::blank somewhere?

Answer 1

Yeah, just use non-member serialization, a.k.a. non-intrusive serialization:

namespace boost { namespace serialization {
    template <typename Ar> void serialize(Ar&, boost::blank&, unsigned) {}
} }

You could also add the overload in namespace boost as ADL will find it there

Live Demo

Live On Coliru

#include <boost/archive/text_oarchive.hpp>
#include <boost/serialization/variant.hpp>
#include <iostream>

using V = boost::variant<boost::blank, int>;

namespace boost { namespace serialization {
    template <typename Ar> void serialize(Ar&, boost::blank&, unsigned) {}
} }

int main() {
    V a{42}, b;

    boost::archive::text_oarchive oa(std::cout);
    oa << a << b;
}

Prints e.g.

22 serialization::archive 17 1 0
0 1 42
1 0 0 0