C ++ 11替代localtime_r [英] C++11 alternative to localtime_r

问题描述

C ++根据 strftime 定义时间格式化函数，这需要一个 struct tm 分解时间记录。然而，C和C ++ 03语言不提供线程安全的方式来获得这样的记录;对于整个程序只有一个主 struct tm 。

在C ++ 03中，不太好，因为语言不支持多线程;它只支持支持多线程的平台，然后提供像POSIX这样的工具 localtime_r 。

C ++ 11还定义了新的时间实用程序，它与未分解的 time_t 类型，其中是将用于重新初始化全局 struct tm 。但是获得 time_t 不是问题。

我缺少一些东西或者这个任务仍然需要依靠POSIX？

EDIT：这是一些解决方法代码。它保持与提供 :: localtime_r 的多线程环境和只提供 std :: localtime 的单线程环境的兼容性。它可以很容易地适用于检查其他函数，如 posix :: localtime_r 或 :: localtime_s 或

 命名空间查询{
 char localtime_r（...）; 
 
 struct has_localtime_r 
 {enum {value = sizeof localtime_r（std :: declval< std :: time_t *>（），std :: declval< std :: tm *> ））
 == sizeof（std :: tm *）}; }; 
 
 
模板< bool可用> struct safest_localtime {
 static std :: tm * call（std :: time_t const * t，std :: tm * r）
 {return localtime_r（t，r）; } 
}; 
 
模板<> 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）; } 
  

解决方案

>

下一个C标准（可能是今年到期）确实已在附件K中定义：

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

这个新函数是线程安全的！但不要太高兴。有两个主要问题：

1. localtime_s 对C11的扩展。




2. C ++ 11引用C99，而不是C11。 local_time_s 在C ++ 11中找不到，是否可选。

更新

在我回答这个问题后的4年里，我也对C ++的糟糕设计感到失望工具在这方面。我有动机创建现代C ++工具来处理这个问题：

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

  #include tz.h
 #include< iostream> 
 
 int 
 main（）
 {
使用命名空间date; 
 auto local_time = make_zoned（current_zone（），std :: chrono :: system_clock :: now（））; 
 std :: cout<< local_time<< '\\\
'; 
} 
  

这只是输出给我：

2015-10-28 14：17：31.980135 EDT

local_time code> std :: chrono :: system_clock :: time_point 和 time_zone 表示本地时间。

存在用于将 std :: chrono :: system_clock :: time_point 打成人类可读字段类型的实用程序，例如年，月，日，小时，分钟，秒和子秒。以下是关于这些（非时区）作品的演示：

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

这当然是线程安全的是现代C ++）。

C++ defines time formatting functions in terms of strftime, which requires a struct tm "broken-down time" record. However, the C and C++03 languages provide no thread-safe way to obtain such a record; there is just one master struct tm for the whole program.

In C++03, this was more or less OK, because the language didn't support multithreading; it merely supported platforms supporting multithreading, which then provided facilities like POSIX localtime_r.

C++11 also defines new time utilities, which interface with the non-broken-down time_t type, which is what would be used to reinitialize the global struct tm. But obtaining a time_t isn't the problem.

Am I missing something or does this task still require reliance on POSIX?

EDIT: Here is some workaround code. It maintains compatibility with multithreaded environments that provide ::localtime_r and single-threaded environments that provide only std::localtime. It can easily be adapted to check for other functions as well, such as posix::localtime_r or ::localtime_s or 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 ); }

解决方案

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++).

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