谁负责删除构面? [英] Who is responsible for deleting the facet?

问题描述

我有一个函数使用Boost.DateTime库生成当前GMT/UTC日期和时间字符串(

I have a function that uses the Boost.DateTime library for generating the current GMT/UTC date and time string (live example).

std::string get_curr_date() {
    auto date = boost::date_time::second_clock<boost::posix_time::ptime>::universal_time();

    boost::posix_time::time_facet* facet = new boost::posix_time::time_facet("%a, %d %b %Y %H:%M:%S GMT");

    std::ostringstream os;
    os.imbue(std::locale(os.getloc(), facet));
    os << date;

    return os.str();
}

这主要基于 Boost.DateTime的示例:

//example to customize output to be "LongWeekday LongMonthname day, year"
//                                  "%A %b %d, %Y"
date d(2005,Jun,25);
date_facet* facet(new date_facet("%A %B %d, %Y"));
std::cout.imbue(std::locale(std::cout.getloc(), facet));
std::cout << d << std::endl;
// "Saturday June 25, 2005"

我的代码运行良好，但是现在由于包含new的这些特殊行而让我感到不安:

My code worked nicely, but now I'm feeling uneasy because of these particular lines containing new:

• boost::posix_time::time_facet* facet = new boost::posix_time::time_facet("%a, %d %b %Y %H:%M:%S GMT");

date_facet* facet(new date_facet("%A %B %d, %Y"));

如您所见，Boost.DateTime中没有delete，因此我以某种方式认为必须delete date_facet.我用std::unique_ptr包裹了new ed time_facet对象.

As you can see, there is no delete in Boost.DateTime's so I somehow presumed that it is imperative for me to delete the date_facet. I used std::unique_ptr to wrap the newed time_facet object.

std::unique_ptr<boost::posix_time::time_facet> facet(new boost::posix_time::time_facet("%a, %d %b %Y %H:%M:%S GMT"));

但是，我遇到了段错误，您可以在此处中看到.我还尝试过手动delete ed new ed指针，但仍然遇到相同的错误(对不起，无法在Coliru中重现错误).

However, I am getting segfault errors, as you can see in here. I have also tried manually deleteing the newed pointer, and am still getting the same errors (sorry, can't reproduce error in Coliru).

time_facet指针在构造std::locale对象时作为参数传递，所以我很困惑谁是负责delete构面的人.

The time_facet pointer is passed as an argument when constructing an std::locale object, so I'm confused who's the one responsible for deleteing the facet.

这是我的问题的核心:

• 我需要delete time_facet 还是std::locale对象负责delete对其进行操作?

• Am I required to delete the time_facet or is the std::locale object responsible for deleteing it?

请注意，boost::posix_time::time_facet是从boost::date_time::date_facet派生的，而boost::date_time::date_facet又是从std::locale::facet派生的.尽管我的问题特定于time_facet，但是这个问题可能会推广到std::locale::facet.

Please note that boost::posix_time::time_facet is derived from boost::date_time::date_facet which is, in turn, derived from std::locale::facet. This question might generalized to std::locale::facet, though my problem is specific to time_facet.

以下是有关std::locale的构造函数的一些文档:

Here are some docs on std::locale's constructors:

• MSDN
• cppreference.com

• MSDN
• cppreference.com

推荐答案

我需要删除time_facet还是std :: locale对象 负责删除它吗?

Am I required to delete the time_facet or is the std::locale object responsible for deleteing the it?

只要time_facet是从std::locale::facet派生的，就不需要删除time_facet. std::locale::facet是所有方面都应从该类派生的基类，该类实现了引用计数的形式.标准说:

You're not required to delete the time_facet so long as time_facet derives from std::locale::facet, which it should. The std::locale::facet is a base class that all facets should derive from that implement a form of reference counting. The standard says this:

§22.3.1.6

一旦通过调用从语言环境对象获得构面引用 use_facet<>，该引用仍然可用，并且结果来自 它的成员函数可能会被缓存和重复使用，只要某些 语言环境对象是指该构面.

Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.

一旦未使用构面的所有引用，std::locale的析构函数将管理和删除对构面的引用(如果其引用计数为0).

Once all references of the facet are not being used, the destructor of std::locale will manage and delete references to the facet if its ref count is 0.

所有这些都在C ++ 11标准的§22.3.1.1.2中指定.说明:

This is all specified in §22.3.1.1.2 in the C++11 standard. Where it states:

构造函数的refs参数用于生命周期管理.

The refs argument to the constructor is used for lifetime management.

-对于refs == 0，实现将执行delete static_cast<locale::facet*>(f)(其中f是指向构面的指针) 当包含该构面的最后一个语言环境对象被销毁时；为了 refs == 1，该实现永远不会破坏该方面.

— For refs == 0, the implementation performs delete static_cast<locale::facet*>(f) (where f is a pointer to the facet) when the last locale object containing the facet is destroyed; for refs == 1, the implementation never destroys the facet.

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