C++17：跨Win/Linux如何将std::filesystem::file_time_type存为整数？

背景 #

开发一款跨Windows和Linux的程序，需收集目录文件的时间并以数字形式存入文本文件；后续程序读取这些文件，将存储的数字与新获取的时间数值对比，且读写程序可能运行在不同平台，文件来自共享网络驱动器。

核心问题 #

如何在C++17中以最优且可移植的方式，将std::filesystem::last_write_time()返回的时间输出为单个整数？

要求 #

• 基于std::filesystem::file_time_type
• 兼容C++17标准
• 跨平台可移植
• 输出为单个整数

注：本问题非重复提问！

已尝试的方案（存在缺陷） #

以下两种方案通过时间点转换实现，但两次调用now()会导致结果抖动：

template < typename TP >
std::time_t
to_time_tnew( TP tp )
{
    using namespace std::chrono;
    auto sctp = time_point_cast< system_clock::duration >( tp - TP::clock::now() + system_clock::now() );
    return system_clock::to_time_t( tp );
}

template < typename TP >
std::time_t
to_time_t( TP tp )
{
    //    using namespace std::chrono;

    auto sctp = std::chrono::time_point_cast< std::chrono::system_clock::duration >( tp - TP::clock::now() + std::chrono::system_clock::now() );
    return std::chrono::system_clock::to_time_t( sctp );
}

当前临时适配方案（Windows + 树莓派Linux） #

虽依赖平台宏和常量仅作为原型实现，代码如下：

#include <cstdarg>
#include <inttypes.h>
#include <filesystem>
#include <chrono>

#include <fstream>
#include <sstream>
#include <iostream>
#include <iomanip>

#ifdef WIN32
#define FILE_TIME__FILE_TIME_OFFSET ( std::filesystem::__std_fs_file_time_epoch_adjustment ) // "( 116444736000000000ull )"
#define FILE_TIME__FILE_TIME_GAIN   ( 10000000ull )
#else
#define FILE_TIME__FILE_TIME_OFFSET ( 12009080074ull )
#define FILE_TIME__FILE_TIME_GAIN   ( 1000000000ull )
#endif

#ifdef WIN32
std::time_t
TimeHelper::to_time_t( const std::filesystem::file_time_type file_time ) const
{
    std::time_t                            rv = 0ull;
    const std::filesystem::file_time_type *ptr = &file_time;
    uint64_t *                             timeptr = ( uint64_t * ) ptr;
    uint64_t                               time = *timeptr;
    if ( time >= FILE_TIME__FILE_TIME_OFFSET )
    {
        time -= FILE_TIME__FILE_TIME_OFFSET;
        time += ( FILE_TIME__FILE_TIME_GAIN >> 1 ); // do correct rounding by adding halv of dividend before division
        time /= FILE_TIME__FILE_TIME_GAIN;
        rv = time;
    }
    return rv;
}
std::filesystem::file_time_type
TimeHelper::to_FileTimeType( const std::time_t time ) const
{
    uint64_t                         time_t1 = time * FILE_TIME__FILE_TIME_GAIN;      // time_t to 100ns
    uint64_t                         time_t2 = time_t1 + FILE_TIME__FILE_TIME_OFFSET; // time_t to offset 01.01.1601
    std::filesystem::file_time_type *ptr = ( std::filesystem::file_time_type * ) &time_t2;
    std::filesystem::file_time_type  rv = *ptr;

    return rv;
}
#else

std::time_t
TimeHelper::to_time_t( const std::filesystem::file_time_type file_time ) const
{
    const std::filesystem::file_time_type *ftptr = &file_time;
    const uint64_t                         ft = *( ( uint64_t * ) ftptr );
    const uint64_t                         offs = FILE_TIME__FILE_TIME_OFFSET;
    const uint64_t                         ts = ft / FILE_TIME__FILE_TIME_GAIN;
    std::time_t                            rv = 0;
    if ( offs < ts )
    {
        rv = ( std::time_t ) ( ts - offs );
    }
    return rv;
}
std::filesystem::file_time_type
TimeHelper::to_FileTimeType( const std::time_t time ) const
{
    uint64_t                        offs = FILE_TIME__FILE_TIME_OFFSET;
    uint64_t                        t1 = time + offs;
    uint64_t                        t2 = t1 * FILE_TIME__FILE_TIME_GAIN;
    std::filesystem::file_time_type ft = *( ( std::filesystem::file_time_type * ) &t2 );
    return ft;
}
#endif

内容的提问来源于stack exchange，提问作者Jesko