Add support for Boost.Variant2

include/boost/serialization/variant2.hpp

@@ -0,0 +1,176 @@
+#ifndef BOOST_SERIALIZATION_VARIANT2_HPP
+#define BOOST_SERIALIZATION_VARIANT2_HPP
+
+// MS compatible compilers support #pragma once
+#if defined(_MSC_VER)
+# pragma once
+#endif
+
+/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
+// variant.hpp - non-intrusive serialization of variant types
+//
+// copyright (c) 2019 Samuel Debionne, ESRF
+//
+// Use, modification and distribution is subject to the Boost Software
+// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org for updates, documentation, and revision history.
+//
+// Widely inspired form boost::variant serialization
+//
+
+#include <boost/serialization/throw_exception.hpp>
+
+#include <variant>
+
+#include <boost/archive/archive_exception.hpp>
+
+#include <boost/mp11/list.hpp>
+
+#include <boost/serialization/split_free.hpp>
+#include <boost/serialization/serialization.hpp>
+#include <boost/serialization/nvp.hpp>
+
+namespace boost {
+namespace serialization {
+
+template<class Archive>
+struct variant2_save_visitor
+{
+    variant2_save_visitor(Archive& ar) :
+        m_ar(ar)
+    {}
+    template<class T>
+    void operator()(T const & value) const
+    {
+        m_ar << BOOST_SERIALIZATION_NVP(value);
+    }
+private:
+    Archive & m_ar;
+};
+
+
+template<class Archive>
+struct variant2_load_visitor
+{
+    variant2_load_visitor(Archive& ar) :
+        m_ar(ar)
+    {}
+    template<class T>
+    void operator()(T & value) const
+    {
+        m_ar >> BOOST_SERIALIZATION_NVP(value);
+    }
+private:
+    Archive & m_ar;
+};
+
+template<class Archive, class ...Types>
+void save(
+    Archive & ar,
+    variant2::variant<Types...> const & v,
+    unsigned int /*version*/
+){
+    const std::size_t which = v.index();
+    ar << BOOST_SERIALIZATION_NVP(which);
+    variant2_save_visitor<Archive> visitor(ar);
+    std::visit(visitor, v);
+}
+
+template<std::size_t N, class Seq>
+struct variant_impl
+{
+    template<class Archive, class V>
+    static void load (
+        Archive & ar,
+        std::size_t which,
+        V & v,
+        const unsigned int version
+    ){
+        if(which == 0){
+            // note: A non-intrusive implementation (such as this one)
+            // necessary has to copy the value.  This wouldn't be necessary
+            // with an implementation that de-serialized to the address of the
+            // aligned storage included in the variant.
+            using type = mp11::mp_front<Seq>;
+            type value;
+            ar >> BOOST_SERIALIZATION_NVP(value);
+            v = std::move(value);
+            type * new_address = & variant2::get<type>(v);
+            ar.reset_object_address(new_address, & value);
+            return;
+        }
+        //typedef typename mpl::pop_front<S>::type type;
+        using types = mp11::mp_pop_front<Seq>;
+        variant_impl<N - 1, types>::load(ar, which - 1, v, version);
+    }
+};
+
+template<class Seq>
+struct variant_impl<0, Seq>
+{
+    template<class Archive, class V>
+    static void load (
+        Archive & /*ar*/,
+        std::size_t /*which*/,
+        V & /*v*/,
+        const unsigned int /*version*/
+    ){}
+};
+
+template<class Archive, class... Types>
+void load(
+    Archive & ar, 
+    variant2::variant<Types...> & v,
+    const unsigned int version
+){
+    std::size_t which;
+    ar >> BOOST_SERIALIZATION_NVP(which);
+    if(which >=  sizeof...(Types))
+        // this might happen if a type was removed from the list of variant types
+        boost::serialization::throw_exception(
+            boost::archive::archive_exception(
+                boost::archive::archive_exception::unsupported_version
+            )
+        );
+    variant_impl<sizeof...(Types), mp11::mp_list<Types...>>::load(ar, which, v, version);
+}
+
+template<class Archive,class... Types>
+inline void serialize(
+    Archive & ar,
+    variant2::variant<Types...> & v,
+    const unsigned int file_version
+){
+    split_free(ar,v,file_version);
+}
+
+// Specialization for std::monostate
+template<class Archive>
+void serialize(Archive &ar, variant2::monostate &, const unsigned int /*version*/)
+{}
+
+} // namespace serialization
+} // namespace boost
+
+//template<typename T0_, BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename T)>
+
+#include <boost/serialization/tracking.hpp>
+
+namespace boost {
+    namespace serialization {
+
+template<class... Types>
+struct tracking_level<
+    variant2::variant<Types...>
+>{
+    typedef mpl::integral_c_tag tag;
+    typedef mpl::int_< ::boost::serialization::track_always> type;
+    BOOST_STATIC_CONSTANT(int, value = type::value);
+};
+
+} // namespace serialization
+} // namespace boost
+
+#endif //BOOST_SERIALIZATION_VARIANT2_HPP