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C++ 用 来定义时间格式化函数strftime，它需要一个struct tm“分解的时间”记录。但是，C 和 C++03 语言没有提供线程安全的方式来获取这样的记录；整个程序只有一位大师struct tm。

在 C++03 中，这或多或少是可以的，因为该语言不支持多线程；它仅支持支持多线程的平台，然后提供诸如 POSIX 之类的工具localtime_r。

C++11 还定义了新的时间实用程序，它与非分解time_t类型接口，这将用于重新初始化全局struct tm. 但是获得atime_t不是问题。

我是否遗漏了什么或者这项任务仍然需要依赖 POSIX？

编辑：这是一些解决方法代码。它保持与提供的多线程环境::localtime_r和仅提供std::localtime. 它也可以很容易地适用于检查其他功能，例如posix::localtime_ror::localtime_s或 what-have-you。

namespace query {
    char localtime_r( ... );

    struct has_localtime_r
        { enum { value = sizeof localtime_r( std::declval< std::time_t * >(), std::declval< std::tm * >() )
                        == sizeof( std::tm * ) }; };


    template< bool available > struct safest_localtime {
        static std::tm *call( std::time_t const *t, std::tm *r )
            { return localtime_r( t, r ); }
    };

    template<> struct safest_localtime< false > {
        static std::tm *call( std::time_t const *t, std::tm *r )
            { return std::localtime( t ); }
    };
}
std::tm *localtime( std::time_t const *t, std::tm *r )
    { return query::safest_localtime< query::has_localtime_r::value >().call( t, r ); }
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1 回答 1

28

You're not missing anything.

The next C standard (due out probably this year) does have defined in Annex K:

struct tm *localtime_s(const time_t * restrict timer,
                       struct tm * restrict result);

And this new function is thread safe! But don't get too happy. There's two major problems:

  1. localtime_s is an optional extension to C11.

  2. C++11 references C99, not C11. local_time_s is not to be found in C++11, optional or not.

Update

In the 4 years since I answered this question, I have also been frustrated by the poor design of C++ tools in this area. I was motivated to create modern C++ tools to deal with this:

http://howardhinnant.github.io/date/tz.html

#include "tz.h"
#include <iostream>

int
main()
{
    using namespace date;
    auto local_time = make_zoned(current_zone(), std::chrono::system_clock::now());
    std::cout << local_time << '\n';
}

This just output for me:

2015-10-28 14:17:31.980135 EDT

local_time is a pairing of std::chrono::system_clock::time_point and time_zone indicating the local time.

There exists utilities for breaking the std::chrono::system_clock::time_point into human-readable field types, such as year, month, day, hour, minute, second, and subseconds. Here is a presentation focusing on those (non-timezone) pieces:

https://www.youtube.com/watch?v=tzyGjOm8AKo

All of this is of course thread safe (it is modern C++).

Update 2

The above is now part of C++20 with this slightly altered syntax:

#include <chrono>
#include <iostream>

int
main()
{
    namespace chr = std::chrono;

    chr::zoned_time local_time{chr::current_zone(), chr::system_clock::now()};
    std::cout << local_time << '\n';
}
于 2011-09-06T02:44:14.727 回答