插入的项目从boost :: mpl :: map中消失

问题描述

以下示例演示了我的意思：

The following example demonstrates what I mean:

    #include <boost/mpl/map.hpp>
    #include <boost/mpl/for_each.hpp>
    #include <boost/mpl/pair.hpp>
    #include <boost/mpl/at.hpp>
    #include <boost/mpl/insert.hpp>
    #include <iostream>

    using namespace boost::mpl;

    template <int n1, int n2>
    struct entry
    {
        typedef pair<int_<n1>, int_<n2> > type;
    };

    typedef map<entry<1,1>::type> entries;

    typedef insert<
        entries, entry<4,4>::type>::type update;

    typedef insert<
        update,
        entry<5,5>::type>::type update2;

    struct print_values
    {
        template <class I>
        void operator()(I)
        {
            std::cout << first<I>::type::value << ", " 
                      << second<I>::type::value << std::endl;
        }
    };

    int main()
    {
        for_each<update2>(print_values());
        std::cout << "Next:" << std::endl;
        for_each<update2::type>(print_values());
    }

输出：

    1, 1
    4, 4
    5, 5
    Next:
    1, 1

当我通过访问 update2 :: type评估 update2 时我插入的项目消失了。

When I evaluate update2 by accessing update2::type the items I inserted disappear.

为什么会发生这种情况，我该怎么做才能确保评估的 update2 不会删除插入的元素吗？

Why does this happen and what can I do to to make sure the evaluating update2 doesn't remove the inserted elements?

推荐答案

我确信这是<$ c中的错误$ c> boost :: mpl 。

在 mpl :: map中插入元素的结果不是 mpl :: map 类型的项目，而是 mpl :: m_item 类型的项目。此 m_item 包含新插入的键和值以及它们插入到的映射。

现在， boost中的每个类:: mpl 是一个元函数，容器是一个返回自身的元函数。

The result of inserting an element into an mpl::map is not an item of type mpl::map but of mpl::m_item. This m_item holds the newly inserted key and value as well as the map they were inserted into.
Now every class in boost::mpl is a metafunction, containers are a metafunction that return themselves. This is useful for situations like this

    typedef map<pair<int,int> > i_map;
    eval_if<true_,
            i_map,
            at<i_map, float> >::type type;

所以 mpl :: map 具有typedef在其中只是 typedef映射类型； ， m_item ，但是缺少此 typedef ，因此由于它是从映射派生而来的，因此它被插入到 m_item :: type 实际上是 map :: type 。

So mpl::map has a typedef inside it that is simply typedef map type;, m_item, however, lacks this typedef and so since it derives from the map it's inserted into m_item::type is actually map::type.

这表示插入后的示例（删除 int_<> ；为简洁起见，）如下：

This means my example after insertion (dropping the int_<> for brevity) looks like:

   m_item<5, 5, m_item<4, 4, map1<pair<1, 1> > > >

并访问其类型一直返回到 map1< pair< 1、1> > :: type 返回 map< pair< 1、1> > ，这就是所有插入的项目都消失的原因。

and accessing its type reaches all the way back up to map1<pair<1, 1> >::type which returns map<pair<1, 1> > which is why all the inserted items are disappearing.

我登录了，最初在发布此答案之前等待响应，但是4周后，我决定在没有任何回答的情况下发布此内容。

I logged a bug and was originally waiting for a response before posting this answer but after 4 weeks I decided to post this without any answer from them.

解决方案是简单地添加 typedef m_item type; 到 boost :: mpl :: m_item 在 boost / mpl / map / aux_ / item.hpp 中。这将使 m_item 成为可正确返回自身而不返回原始地图的元函数。

The solution is to simply add typedef m_item type; to boost::mpl::m_item in boost/mpl/map/aux_/item.hpp. This will make m_item a metafunction that correctly returns itself and not return the original map.

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