c++ - boost 序列化: read varying type of data

Question

我有一个 C++/CLI 项目，它使用 boost 序列化来序列化三个不同的类。我想知道是否可以解析 boost 序列化存档的第一行，以便知道在这个存档中序列化了什么类，然后创建一个适当类的对象并将存档反序列化到对象中。该行将包含一个 ID(可能是一个 int 或枚举类的值)以标识哪个类被序列化。

Answer 1

文件格式已由您选择的存档实现处理。

实际上是 boost::archive::text_oarchive , boost::archive::binary_oarchive , boost::archive::xml_oarchive .

只要您的存档类型本身没有变化，您就可以非常轻松地使用 Boost Variant 来区分您的有效负载。换句话说，让序列化框架为您完成工作，而不是围绕它进行“管道录音”:

这是一个序列化 3 种不同(复合)有效负载和往返的演示，无需实际有效负载的外部知识:

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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" } } });
}

输出为:

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