How to serialize boost::dynamic_bitset?

Question

How to serialize a class with a boost::dynamic_bitset member?

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/dynamic_bitset.hpp>
#include <boost/serialization/bitset.hpp>
#include <sstream>

class A
{
    friend class boost::serialization::access;
    boost::dynamic_bitset<> x;
    template<class Archive>
    void serialize(Archive & ar, const unsigned int){
        ar & x;
    }
};

int main()
{
    A a;
    std::stringstream ss;
    boost::archive::text_oarchive oa(ss);
    oa << a;
    return 0;
}

Compiling gives an error (boost 1.57)

In file included from /usr/include/boost/serialization/extended_type_info_typeid.hpp:37:0,
                 from /usr/include/boost/archive/detail/oserializer.hpp:38,
                 from /usr/include/boost/archive/detail/interface_oarchive.hpp:23,
                 from /usr/include/boost/archive/detail/common_oarchive.hpp:22,
                 from /usr/include/boost/archive/basic_text_oarchive.hpp:32,
                 from /usr/include/boost/archive/text_oarchive.hpp:31,
                 from dynamic_bitset_setial.cpp:1:
/usr/include/boost/serialization/access.hpp: In static member function ‘static void boost::serialization::access::serialize(Archive&, T&, unsigned int) [with Archive = boost::archive::text_oarchive, T = boost::dynamic_bitset<>]’:
/usr/include/boost/serialization/serialization.hpp:69:5:   instantiated from ‘void boost::serialization::serialize(Archive&, T&, unsigned int) [with Archive = boost::archive::text_oarchive, T = boost::dynamic_bitset<>]’
/usr/include/boost/serialization/serialization.hpp:128:9:   instantiated from ‘void boost::serialization::serialize_adl(Archive&, T&, unsigned int) [with Archive = boost::archive::text_oarchive, T = boost::dynamic_bitset<>]’
/usr/include/boost/archive/detail/oserializer.hpp:148:5:   instantiated from ‘void boost::archive::detail::oserializer<Archive, T>::save_object_data(boost::archive::detail::basic_oarchive&, const void*) const [with Archive = boost::archive::text_oarchive, T = boost::dynamic_bitset<>]’
dynamic_bitset_setial.cpp:25:1:   instantiated from here
/usr/include/boost/serialization/access.hpp:118:9: error: ‘class boost::dynamic_bitset<>’ has no member named ‘serialize’

Answer 1

dynamic_bitset<> is not serializable, as you've found out (std::bitset<N> is different type).

Not to worry, though, you can add it without too much effort:

namespace boost { namespace serialization {

    template <typename Ar, typename Block, typename Alloc>
        void save(Ar& ar, dynamic_bitset<Block, Alloc> const& bs, unsigned) {
            size_t num_bits = bs.size();
            std::vector<Block> blocks(bs.num_blocks());
            to_block_range(bs, blocks.begin());

            ar & num_bits & blocks;
        }

    template <typename Ar, typename Block, typename Alloc>
        void load(Ar& ar, dynamic_bitset<Block, Alloc>& bs, unsigned) {
            size_t num_bits;
            std::vector<Block> blocks;
            ar & num_bits & blocks;

            bs.resize(num_bits);
            from_block_range(blocks.begin(), blocks.end(), bs);
            bs.resize(num_bits);
        }

    template <typename Ar, typename Block, typename Alloc>
        void serialize(Ar& ar, dynamic_bitset<Block, Alloc>& bs, unsigned version) {
            split_free(ar, bs, version);
        }

} }

This works e.g. Live On Coliru

int main() {
    A a;
    for (int i=0; i<128; ++i)
        a.x.resize(11*i, i%2);

    std::stringstream ss;
    {
        boost::archive::text_oarchive oa(ss);
        oa << a;
    }
    std::cout << ss.str();
    {
        boost::archive::text_iarchive ia(ss);
        A b;
        ia >> b;

        assert(a.x == b.x);
    }
}

Note that if you can't afford to copy the blocks vector, it's equally easy to add serialization directly on the m_bits level, but that requires intrusive changes (friend access required at a minimum).

Such a thing would easily be added to boost in a pull request.

Update added that pull request

Answer 2

I went ahead and filed the pull request to add Serialization support to Boost Dynamic Bitset

Serialization using the public interface isn't optimal as to_block_range()/from_block_range() require copying of m_bits (and subsequent resize()).

I added a generic implementation to Boost Dynamic Bitset. The changes merge cleanly against develop or master (1_58_0).

Changes

Implementation added with

• minimal intrusiveness, only a nested friend (class serialization_impl;) has been forward declared to "key-hole" the required friend access through
• This class, as well as the actual ADL hook for Boost Serialization are implemented in a separate header (dynamic_bitset/serialization.hpp, similar to other boost libraries with serialization support).
• This means that zero dependencies on Boost Serialization stuff exists unless boost/dynamic_bitset/serialization.hpp is actually included
• Zero copy is achieved (leveraging std::vector<Block>'s builtin support in Boost Serialization)

Tests

The second commit in the pull request adds tests for this feature. I'm not sure how to add the dyn_bitset_unit_tests5.cpp to the Jamfile. I suppose something else must be done to ensure linking to Boost System and Boost Serialization. I have run the tests myself using a simple wrapper:

#include <modular-boost/libs/dynamic_bitset/dyn_bitset_unit_tests5.cpp>

int main() {
    test_main(0, nullptr);
}

Which can then be compiled and run with e.g.

g++ main.cpp -lboost_system -lboost_serialization && ./a.out

No output means no errors.