如何将std::chrono时区代码迁移至Howard Hinnant日期库？解决编译错误

现有一段在MSVC下运行正常的C++20代码，使用std::chrono实现UTC与本地时间转换，但因GCC版本不支持timezone组件，改用Howard Hinnant日期库后，MSVC出现类型转换和函数匹配错误，具体信息如下：

原代码 #

#include <chrono>

using LocalTime = std::chrono::time_point<std::chrono::local_t, std::chrono::system_clock::duration>;
//using LocalTime = std::chrono::local_time<std::chrono::system_clock::duration>;

using UtcTime = std::chrono::time_point<std::chrono::system_clock>;

LocalTime UtcToLocal(UtcTime t)
{       
    auto tz = std::chrono::current_zone();
    LocalTime tLocal = tz->to_local(t);
    return tLocal;
}

UtcTime LocalToUtc(LocalTime t)
{
    auto tz = chronoExt::current_zone();
    UtcTime tUtc = tz->to_sys(t);

    return tUtc;
}

修改后代码 #

#include "tz.h"
#include "date.h"

LocalTime UtcToLocal(UtcTime t)
{       
    auto tz = date::current_zone();
    LocalTime tLocal = tz->to_local(t);
    return tLocal;
}

UtcTime LocalToUtc(LocalTime t)
{
    auto tz = date::current_zone();
    UtcTime tUtc = tz->to_sys(t);

    return tUtc;
}

编译错误信息 #

error C2440: 'initializing': cannot convert from 'std::chrono::time_point<date::local_t,std::chrono::duration<std::chrono::system_clock::rep,std::chrono::system_clock::period>>' to 'std::chrono::time_point<std::chrono::local_t,std::chrono::system_clock::duration>'
message : No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
error C2672: 'date::time_zone::to_sys': no matching overloaded function found
message : could be 'std::chrono::time_point<std::chrono::system_clock,common_type<_Duration,std::chrono::seconds>::type> date::time_zone::to_sys(std::chrono::time_point<date::local_t,Duration>,date::choose) const'
message : 'std::chrono::time_point<std::chrono::system_clock,common_type<_Duration,std::chrono::seconds>::type> date::time_zone::to_sys(std::chrono::time_point<date::local_t,Duration>,date::choose) const': expects 2 arguments - 1 provided
message : or       'std::chrono::time_point<std::chrono::system_clock,common_type<_Duration,std::chrono::seconds>::type> date::time_zone::to_sys(std::chrono::time_point<date::local_t,Duration>) const'
message : 'std::chrono::time_point<std::chrono::system_clock,common_type<_Duration,std::chrono::seconds>::type> date::time_zone::to_sys(std::chrono::time_point<date::local_t,Duration>) const': could not deduce template argument for 'std::chrono::time_point<date::local_t,Duration>' from 'LocalTime'

错误原因与修复方案 #

错误根源 #

std::chrono::local_t和date::local_t是不同命名空间下的独立类型，无法隐式转换。原代码的LocalTime基于std::chrono::local_t，而Howard Hinnant库的to_local/to_sys接口使用date::local_t，导致类型不匹配。

修复后的完整代码 #

#include "tz.h"
#include "date.h"

// 使用date库的local_t统一本地时间类型，与库接口匹配
using LocalTime = std::chrono::time_point<date::local_t, std::chrono::system_clock::duration>;
// 等价写法：直接使用date库提供的local_time模板
// using LocalTime = date::local_time<std::chrono::system_clock::duration>;

using UtcTime = std::chrono::time_point<std::chrono::system_clock>;

LocalTime UtcToLocal(UtcTime t)
{       
    auto tz = date::current_zone();
    // 类型完全匹配，无需额外转换
    return tz->to_local(t);
}

UtcTime LocalToUtc(LocalTime t)
{
    auto tz = date::current_zone();
    // 传入的LocalTime现在符合to_sys的参数类型要求
    return tz->to_sys(t);
}

可选兼容方案（保留std::chrono::local_t） #

如果必须保留原代码中std::chrono::local_t的定义（比如其他代码依赖），可以通过duration显式转换类型：

// UtcToLocal修改如下
LocalTime UtcToLocal(UtcTime t)
{       
    auto tz = date::current_zone();
    auto local_time = tz->to_local(t);
    // 用获取到的duration构造std::chrono::local_t的time_point
    return LocalTime{local_time.time_since_epoch()};
}

// LocalToUtc修改如下
UtcTime LocalToUtc(LocalTime t)
{
    auto tz = date::current_zone();
    // 先将std::chrono::local_t转换为date::local_t的time_point
    date::local_time<std::chrono::system_clock::duration> date_local{t.time_since_epoch()};
    return tz->to_sys(date_local);
}

内容的提问来源于stack exchange，提问作者Martin Perry