将boost :: multiprecision数据类型写入二进制文件 [英] Writing boost::multiprecision data type to binary file

问题描述

我正在使用 boost :: multiprecision :: uint128_t 类型以便对128位值执行按位运算.但是我在将128位值写到二进制文件时遇到了麻烦.特别是需要用零填充值.

I was using the boost::multiprecision::uint128_t type in order to perform bitwise operations on a 128 bit value. However I am having trouble writing the 128 bit value out to a binary file. Specifically with the need to pad out the value with zeros.

作为一个示例，如果 uint128_t 的值为 0x123456 ，则在十六进制编辑器中查看文件，我需要以下顺序:

As an example if the uint128_t value was 0x123456 then looking at the file in the hex editor I would want the sequence:

56 34 12 00 00 00 00 00 00 00 00 00 00 00 00 00

#include <boost/multiprecision/cpp_int.hpp>
#include <fstream>

boost::multiprecision::uint128_t temp = 0x123456;
std::ofstream ofile("test.bin", std::ios::binary);
ofile.write((char*)&temp, 16);
ofile.close();

相反，二进制文件以一个值结尾:

Instead the binary file ends up with a value:

56 34 12 00 CC CC CC CC CC CC CC CC CC CC CC CC CC

我可以看到 uint128_t 模板的boost后端似乎将128位存储为四个32位值.并具有"limb"值，该值指示正在使用多少个32位值.当不使用32位值时，它们将用 0xCCCCCCCC 填充.因此， ofstream.write 正在遍历字符数组并写出 0xC .

I can see the the boost backend for the uint128_t template appears to store the 128 bits as four 32 bit values. And has a "limb" value which indicates how many 32 bit values are in use. When the 32 bit values are not in use they are filled with 0xCCCCCCCC. So the ofstream.write is walking through the array of characters and writing out the 0xC's.

boost库中是否缺少某些内容来帮助正确地将其写出，还是需要将 uint128_t 值转换为其他数据类型?

Is there something I am missing in the boost library to help write this out correctly, or will I need to convert the uint128_t value into some another data type?

推荐答案

我深入其中，可以编写一个实用程序将连续的肢体作为POD对象:

I dived into it, you can write a utility to write the contiguous limbs as POD objects:

在Coliru上直播

#include <boost/multiprecision/cpp_int.hpp>
#include <fstream>

template <typename BigInt, typename Backend = typename BigInt::backend_type>
void write_binary(std::ostream& os, BigInt const& number) {
    static_assert(boost::is_pod<typename Backend::local_limb_type>::value, "not allowed");

    os.write(
            reinterpret_cast<char const*>(number.backend().limbs()), 
            number.backend().size()*sizeof(typename Backend::local_limb_type)
        );
}

int main()
{
    using uint128_t = boost::multiprecision::uint128_t;

    std::ofstream ofs("binary.dat", std::ios::binary);
    write_binary(ofs, uint128_t(42));
}

Hexdump:

0000000: 2a00 0000 0000 0000 0000 0000 0000 0000  *...............

恐怕这不是可移植的(它可能取决于128位数字的编译器内在函数的可用性).至少它是安全类型.

I'm afraid this isn't portable (it may depend on the availability of compiler intrinsics for 128 bit numbers). At least it's type safe.

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