我期望从 is_bitwise_serializable 特性中序列化如下类(没有序列化函数):

class A { int a; char b; };
BOOST_IS_BITWISE_SERIALIZABLE(A);
A a{2, 'x'};
some_archive << a; // serializes a bitwisely

我想知道，为什么需要为bitwise_serializable类提供序列化功能？

最佳答案

来自文档:

Some simple classes could be serialized just by directly copying all bits of the class. This is, in particular, the case for POD data types containing no pointer members, and which are neither versioned nor tracked. Some archives, such as non-portable binary archives can make us of this information to substantially speed up serialization.

To indicate the possibility of bitwise serialization the type trait defined in the header file is_bitwise_serializable.hpp is used:

这里是重点:这个优化

• 在适用的存档类型中是可选的

• 并不适用于所有存档类型，例如

  • 无法通过复制原始内存表示来实现需要可移植的二进制存档(因为它依赖于实现和平台)

  • 文本存档可能不希望对此进行优化(例如，它有不同的目标，例如“人类可读的 XML”，他们可能不想将您的 vector<A> 编码为一个大文件bas64 编码的 blob)。

请注意，这也解释了 is_bit_wise_serializable<T>不部分专用于具有 is_pod<T>::value == true 的任何类型(这在技术上很容易完成):

• 有些类可能对序列化它们的所有状态不感兴趣(因此使用按位复制比仅选择有趣的位(双关语意)会占用更多空间)

你没有具体问，但这是工作实现的样子:

#include <boost/archive/binary_oarchive.hpp>
#include <boost/serialization/serialization.hpp>
#include <sstream>

struct A { int a; char b;
    template <typename Ar> void serialize(Ar& ar, unsigned) {
        ar & a;
        ar & b;
    }
};

BOOST_IS_BITWISE_SERIALIZABLE(A)

int main() {
    std::ostringstream oss;
    boost::archive::binary_oarchive oa(oss);

    A data { 1, 'z' };
    oa << data;
}

更新

作为对基本上再次提出相同问题的评论者的回应，我想出了一个演示，说明按位序列化将 (a) 产生明显的差异 (b) 不会导致更小的存档:

Live On Coliru

#include <boost/archive/binary_oarchive.hpp>
#include <boost/serialization/array.hpp>
#include <boost/serialization/serialization.hpp>

#include <array>
#include <fmt/ranges.h>
#include <span>
#include <sstream>

struct A {
    int  a;
    char b;
    void serialize(auto& ar, unsigned) { ar& a& b; }
};

#ifdef BITWISE
BOOST_IS_BITWISE_SERIALIZABLE(A)
#endif

int main() {
    std::ostringstream oss;
    {
        boost::archive::binary_oarchive oa(oss,
                boost::archive::no_header |
                boost::archive::no_tracking |
                boost::archive::no_codecvt);

        std::array<A, 26> data{{
            {1, 'z'},  {2, 'y'},  {3, 'x'},  {4, 'w'},  {5, 'v'},  {6, 'u'},  {7, 't'},
            {8, 's'},  {9, 'r'},  {10, 'q'}, {11, 'p'}, {12, 'o'}, {13, 'n'}, {14, 'm'},
            {15, 'l'}, {16, 'k'}, {17, 'j'}, {18, 'i'}, {19, 'h'}, {20, 'g'}, {21, 'f'},
            {22, 'e'}, {23, 'd'}, {24, 'c'}, {25, 'b'}, {26, 'a'},
        }};

        oa << data;
    }

    auto raw = oss.str();
    fmt::print("raw serialized form {} bytes: {::#04x}\n", //
               raw.size(), std::vector(raw.begin(), raw.end()));
}

构建运行时:

for def in REGULAR BITWISE; do g++ -std=c++20 -O2 -Wall -pedantic main.cpp -lboost_serialization -lfmt -o $def -D$def & done; wait
set -x; ./REGULAR; ./BITWISE

打印

+ ./REGULAR
raw serialized form 148 bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x1a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x7a, 0x02, 0x00, 0x00, 0x00, 0x79, 0x03, 0x00, 0x00, 0x00, 0x78, 0x04, 0x00, 0x00, 0x00, 0x77, 0x05, 0x00, 0x00, 0x00, 0x76, 0x06, 0x00, 0x00, 0x00, 0x75, 0x07, 0x00, 0x00, 0x00, 0x74, 0x08, 0x00, 0x00, 0x00, 0x73, 0x09, 0x00, 0x00, 0x00, 0x72, 0x0a, 0x00, 0x00, 0x00, 0x71, 0x0b, 0x00, 0x00, 0x00, 0x70, 0x0c, 0x00, 0x00, 0x00, 0x6f, 0x0d, 0x00, 0x00, 0x00, 0x6e, 0x0e, 0x00, 0x00, 0x00, 0x6d, 0x0f, 0x00, 0x00, 0x00, 0x6c, 0x10, 0x00, 0x00, 0x00, 0x6b, 0x11, 0x00, 0x00, 0x00, 0x6a, 0x12, 0x00, 0x00, 0x00, 0x69, 0x13, 0x00, 0x00, 0x00, 0x68, 0x14, 0x00, 0x00, 0x00, 0x67, 0x15, 0x00, 0x00, 0x00, 0x66, 0x16, 0x00, 0x00, 0x00, 0x65, 0x17, 0x00, 0x00, 0x00, 0x64, 0x18, 0x00, 0x00, 0x00, 0x63, 0x19, 0x00, 0x00, 0x00, 0x62, 0x1a, 0x00, 0x00, 0x00, 0x61]
+ ./BITWISE
raw serialized form 221 bytes: [0x00, 0x00, 0x00, 0x00, 0x00, 0x1a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x7a, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x79, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x78, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x77, 0x7f, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x76, 0x7f, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00, 0x75, 0x7f, 0x00, 0x00, 0x07, 0x00, 0x00, 0x00, 0x74, 0x7f, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00, 0x73, 0x7f, 0x00, 0x00, 0x09, 0x00, 0x00, 0x00, 0x72, 0x7f, 0x00, 0x00, 0x0a, 0x00, 0x00, 0x00, 0x71, 0x7f, 0x00, 0x00, 0x0b, 0x00, 0x00, 0x00, 0x70, 0x00, 0x00, 0x00, 0x0c, 0x00, 0x00, 0x00, 0x6f, 0x00, 0x00, 0x00, 0x0d, 0x00, 0x00, 0x00, 0x6e, 0x00, 0x00, 0x00, 0x0e, 0x00, 0x00, 0x00, 0x6d, 0x00, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x6c, 0x7f, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x6b, 0x00, 0x00, 0x00, 0x11, 0x00, 0x00, 0x00, 0x6a, 0x00, 0x00, 0x00, 0x12, 0x00, 0x00, 0x00, 0x69, 0x7f, 0x00, 0x00, 0x13, 0x00, 0x00, 0x00, 0x68, 0x00, 0x00, 0x00, 0x14, 0x00, 0x00, 0x00, 0x67, 0x00, 0x00, 0x00, 0x15, 0x00, 0x00, 0x00, 0x66, 0x7f, 0x00, 0x00, 0x16, 0x00, 0x00, 0x00, 0x65, 0x7f, 0x00, 0x00, 0x17, 0x00, 0x00, 0x00, 0x64, 0x7f, 0x00, 0x00, 0x18, 0x00, 0x00, 0x00, 0x63, 0x7f, 0x00, 0x00, 0x19, 0x00, 0x00, 0x00, 0x62, 0x00, 0x00, 0x00, 0x1a, 0x00, 0x00, 0x00, 0x61, 0x00, 0x00, 0x00]

关于c++ - boost 序列化按位序列化能力，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28121545/