c++ - 如何将 boost::serialization 与嵌套结构和最少的代码更改一起使用？

Question

目前我们使用存储在嵌套结构中的 POD。例子:

#define MaxNum1 100;
#define MaxNum2 50;

struct A
{
    int Value[MaxNum1]; 
    char SomeChar = 'a';
};

struct B
{
    A data[MaxNum2];
    float SomeFloat = 0.1f;
};


int main()
{
    B StructBObject = {};
}

我们想使用 std::vector 来 boost 我们的数据结构，就像这样:

struct NewA
{
    std::vector<int> Value; 
    char SomeChar = 'a';
};

struct NewB
{
    std::vector<NewA> data;
    float SomeFloat = 0.1f;
};

int main()
{
    NewB StructNewBObject = {};
}

反对这种修改的唯一理由是 NewA和 NewB不再是 POD，这使得读取/写入文件更加复杂。

怎么可能读/写 NewA和 NewB使用 boost::serialization 到文件用最少的
代码更改为 NewA和 NewB ?最少的代码更改很重要，因为我们使用例如具有多达 7 个嵌套级别的大结构。

Answer 1

您可以使用 boost 序列化¹进行序列化:

template <typename Ar> void serialize(Ar& ar, A& a, unsigned) {
    ar & a.Value & a.SomeChar;
}
template <typename Ar> void serialize(Ar& ar, B& b, unsigned) {
    ar & b.data & b.SomeFloat;
}

使用这些，您将已经拥有 C 数组和 std::vector 方法的开箱即用的正确行为。

如果你想继续使用固定大小的普通可复制类型²，你可以使用类似 Boost Container 的 static_vector :它将跟踪当前大小，但数据是在结构内部静态分配的。

三重演示

这是一个包含三个实现的三重演示程序，具体取决于 IMPL多变的。

如您所见，大部分代码保持不变。但是，为了“最佳比较”，我已确保所有容器在序列化之前都处于一半容量 (50/25)。

主程序也反序列化。

Live On Coliru

#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/stream.hpp>

#include <boost/archive/binary_oarchive.hpp>
#include <boost/archive/binary_iarchive.hpp>

#include <boost/serialization/access.hpp>
#include <boost/serialization/is_bitwise_serializable.hpp>
#include <boost/serialization/binary_object.hpp>

#include <iostream>

#if (IMPL==0) // C arrays
    struct A {
        int Value[100]; 
        char SomeChar = 'a';
    };

    struct B {
        A data[50];
        float SomeFloat = 0.1f;
    };

    template <typename Ar> void serialize(Ar& ar, A& a, unsigned) {
        ar & a.Value & a.SomeChar;
    }
    template <typename Ar> void serialize(Ar& ar, B& b, unsigned) {
        ar & b.data & b.SomeFloat;
    }

#elif (IMPL==1) // std::vector
    #include <boost/serialization/vector.hpp>
    struct A {
        std::vector<int> Value;
        char SomeChar = 'a';
    };

    struct B {
        std::vector<A> data;
        float SomeFloat = 0.1f;
    };

    template <typename Ar> void serialize(Ar& ar, A& a, unsigned) {
        ar & a.Value & a.SomeChar;
    }
    template <typename Ar> void serialize(Ar& ar, B& b, unsigned) {
        ar & b.data & b.SomeFloat;
    }

#elif (IMPL==2) // static_vector
    #include <boost/serialization/vector.hpp>
    #include <boost/container/static_vector.hpp>
    struct A {
        boost::container::static_vector<int, 100> Value; 
        char SomeChar = 'a';
    };

    struct B {
        boost::container::static_vector<A, 50> data; 
        float SomeFloat = 0.1f;
    };

    template <typename Ar> void serialize(Ar& ar, A& a, unsigned) {
        ar & boost::serialization::make_array(a.Value.data(), a.Value.size()) & a.SomeChar;
    }
    template <typename Ar> void serialize(Ar& ar, B& b, unsigned) {
        ar & boost::serialization::make_array(b.data.data(), b.data.size()) & b.SomeFloat;
    }

#endif

namespace bio = boost::iostreams;
static constexpr auto flags = boost::archive::archive_flags::no_header;
using BinaryData = std::vector</*unsigned*/ char>;

int main() {
    char const* impls[] = {"C style arrays", "std::vector", "static_vector"};
    std::cout << "Using " << impls[IMPL] << " implementation: ";
    BinaryData serialized_data;

    {
        B object = {};
#if IMPL>0
        {
            // makes sure all containers half-full
            A element;
            element.Value.resize(50);
            object.data.assign(25, element);
        }
#endif

        bio::stream<bio::back_insert_device<BinaryData>> os { serialized_data };
        boost::archive::binary_oarchive oa(os, flags);

        oa << object;
    }

    std::cout << "Size: " << serialized_data.size() << "\n";

    {
        bio::array_source as { serialized_data.data(), serialized_data.size() };
        bio::stream<bio::array_source> os { as };
        boost::archive::binary_iarchive ia(os, flags);

        B object;
        ia >> object;
    }
}

打印

Using C style arrays implementation: Size: 20472
Using std::vector implementation: Size: 5256
Using static_vector implementation: Size: 5039

最后的想法

也可以看看:

Boost serialization bitwise serializability

https://www.boost.org/doc/libs/1_72_0/libs/serialization/doc/wrappers.html#binaryobjects

¹(但请记住可移植性，因为您可能已经了解 POD 方法，请参阅 C++ Boost::serialization : How do I archive an object in one program and restore it in another?)

² 不是 POD，与 NSMI 一样，您的类型不是 POD