Boost serialization : read varying type of data

问题

I have a C++ / CLI project that uses boost serialization to serialize three different classes. I would like to know if it is possible to parse the first line of the boost serialization archive in order to know what class was serialized in this archive, and then create an object of the appropriate class and deserialize the archive into the object. That line would contain an ID (maybe a int or value of an enum class) to identify which class was serialized.

回答1:

The file format is already handled by your choice of Archive implementation.

In practice that would be boost::archive::text_oarchive, boost::archive::binary_oarchive, boost::archive::xml_oarchive.

As long as your archive type itself doesn't vary, you can very easily use a Boost Variant to distinguish your payloads. In other words, make the serialization framework do the work for you, instead of "duct taping" around it:

Here's a demo that serializes 3 different (compound) payloads and roundtrips just fine without external knowledge of the payload actually there:

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#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>

#include <boost/serialization/variant.hpp>
#include <boost/serialization/vector.hpp>
#include <boost/serialization/string.hpp>

#include <boost/serialization/access.hpp>

struct A {
    int simple;

  private:
    friend class boost::serialization::access;
    template <typename Ar> void serialize(Ar& ar, unsigned) {
        ar & simple;
    }
};

struct B {
    std::string text;

  private:
    friend class boost::serialization::access;
    template <typename Ar> void serialize(Ar& ar, unsigned) {
        ar & text;
    }
};

struct C {
    A composed_a;
    B composed_b;

  private:
    friend class boost::serialization::access;
    template <typename Ar> void serialize(Ar& ar, unsigned) {
        ar & composed_a & composed_b;
    }
};

struct FileContents { // conventions...
    boost::variant<A, B, C> payload;

  private:
    friend class boost::serialization::access;
    template <typename Ar> void serialize(Ar& ar, unsigned) {
        ar & payload;
    }
};


#include <sstream>
#include <boost/lexical_cast.hpp>

//////////////////////////////////////////////////////////////////////////////////////////////////////////////
// For our roundtrip test, implement streaming as well so we can independently check equivalence
inline static std::ostream& operator<<(std::ostream& os, A const& v) {
    return os << "A{" << v.simple << "}";
}
inline static std::ostream& operator<<(std::ostream& os, B const& v) {
    return os << "B{" << v.text << "}";
}
inline static std::ostream& operator<<(std::ostream& os, C const& v) {
    return os << "C{" << v.composed_a << ", " << v.composed_b << "}";
}

void roundtrip_test(FileContents const& original) {
    std::stringstream ss;
    {
        boost::archive::text_oarchive oa(ss);
        oa << original;
    }

    {
        boost::archive::text_iarchive ia(ss);

        FileContents clone;
        ia >> clone;

        std::string const before = boost::lexical_cast<std::string>(original.payload);
        std::string const after  = boost::lexical_cast<std::string>(clone.payload);

        std::cout << "Roundtrip '" << before << "': " << std::boolalpha << (before == after) << "\n";
    }
}

int main() {
    roundtrip_test({ A { 42 } });
    roundtrip_test({ B { "Life The Universe And Everything" } });
    roundtrip_test({ C { {42}, { "Life The Universe And Everything" } } });
}

The output being:

Roundtrip 'A{42}': true
Roundtrip 'B{Life The Universe And Everything}': true
Roundtrip 'C{A{42}, B{Life The Universe And Everything}}': true

来源：https://stackoverflow.com/questions/31548328/boost-serialization-read-varying-type-of-data

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c++

class

parsing

boost

boost-serialization