以相反的顺序迭代可变参数模板参数 [英] Iterating variadic template arguments in reverse order

问题描述

如果我手动颠倒传递给它的模板参数的顺序，下面的代码工作：

  template< typename HeadTag，typename ... TailTag> 
 struct Mapped_scope_deep：public Mapped_scope_deep< TailTag ...> {
 typedef typename boost :: mpl :: at< typename Mapped_scope_deep< TailTag ...> :: type :: type_map，
 HeadTag> :: type type; 
}; 
 
 template< typename HeadTag> 
 struct Mapped_scope_deep< HeadTag> {
 typedef typename boost :: mpl :: at< type_map，HeadTag> :: type type; 
}; 
  

示例：

 code> // typename Mapped_scope_deep< T0，T1，T2，T3> :: type 
 //需要被写为
 typename Mapped_scope_deep< T3，T2，T1，T0& 
  

我已尝试在这里修复：

  template< typename map，typename HeadTag，typename ... TailTag> 
 struct Mapped_scope_deep_r：
 public Mapped_scope_deep_r< typename boost :: mpl :: at< map，HeadTag> :: type :: type_map，TailTag ...> {
 typename Mapped_scope_deep_r< typename boost :: mpl :: at< map，HeadTag> :: type :: type_map，TailTag ...> :: type type; 
}; 
 
 template< typename map，typename HeadTag> 
 struct Mapped_scope_deep_r< map，HeadTag> {
 typedef typename boost :: mpl :: at< map，HeadTag> :: type type; 
}; 
 
 template< typename ... Tags> 
 struct Mapped_scope_deep3：
 public Mapped_scope_deep_r< type_map，Tags ...> {
 typedef typename Mapped_scope_deep_r< type_map，Tags ...> :: type type; 
}; 
  

示例：

 code> typename Mapped_scope_deep< T0，T1，T2，T3> :: type 
  

结束于编译错误：

  ./ compressed_enums.hxx：197：66：error：typename说明符指非类型成员'type'in'Gamblify :: Asdf< unsigned char，CAT> :: Mapped_scope_deep_r< boost :: mpl :: map< boost :: mpl :: pair< Cat，Gamblify :: Category2< unsigned char，1，Cat，B_First ，TagA_array_2，B_Second> > >，Cat，First>'
 typedef typename Mapped_scope_deep_r< type_map，Tags ...> :: type type; 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~ ^ ~~~ 
  

我做错了什么，

c>在 Mapped_scope_deep_r 中。此行声明一个对象，而不是类型：

  typename Mapped_scope_deep_r< typename boost :: mpl :: at< map，HeadTag> :: type :: type_map，TailTag ...> :: type type; 
  

至于颠倒一个包的顺序，有一些肮脏的技巧，但最好的方法是定义元函数 tuple_reverse ，并使用它来过滤模板的输入。

The following code is working if I manual reverse the order of the template arguments which is passed to it:

template<typename HeadTag, typename... TailTag>
struct Mapped_scope_deep : public Mapped_scope_deep<TailTag...> {
    typedef typename boost::mpl::at<typename Mapped_scope_deep<TailTag...>::type::type_map,
                                    HeadTag>::type type;
};

template<typename HeadTag>
struct Mapped_scope_deep<HeadTag> {
    typedef typename boost::mpl::at<type_map, HeadTag>::type type;
};

Example:

// typename Mapped_scope_deep<T0, T1, T2, T3>::type
// needs to be written as
typename Mapped_scope_deep<T3, T2, T1, T0>::type

I have tried to fix this here:

template<typename map, typename HeadTag, typename... TailTag>
struct Mapped_scope_deep_r :
    public Mapped_scope_deep_r< typename boost::mpl::at<map, HeadTag>::type::type_map, TailTag...> {
  typename Mapped_scope_deep_r< typename boost::mpl::at<map, HeadTag>::type::type_map, TailTag...>::type type;
};

template<typename map, typename HeadTag>
struct Mapped_scope_deep_r<map, HeadTag> {
    typedef typename boost::mpl::at<map, HeadTag>::type type;
};

template<typename... Tags>
struct Mapped_scope_deep3 :
    public Mapped_scope_deep_r<type_map, Tags...> {
    typedef typename Mapped_scope_deep_r<type_map, Tags...>::type type;
};

Example:

typename Mapped_scope_deep<T0, T1, T2, T3>::type

But this ends in a compile error:

./compressed_enums.hxx:197:66: error: typename specifier refers to non-type member 'type' in 'Gamblify::Asdf<unsigned char, CAT>::Mapped_scope_deep_r<boost::mpl::map<boost::mpl::pair<Cat, Gamblify::Category2<unsigned char, 1, Cat, B_First, TagA_array_2, B_Second> > >, Cat, First>'
    typedef typename Mapped_scope_deep_r<type_map, Tags...>::type type;
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~

What have I done wrong, and is their an easier way to fold-like operations in the reverse order?

解决方案

You're missing a typedef in Mapped_scope_deep_r. This line declares an object, not a type:

typename Mapped_scope_deep_r< typename boost::mpl::at<map, HeadTag>::type::type_map, TailTag...>::type type;

As for reversing the order of a pack, there are some dirty tricks, but the best way is to define a metafunction tuple_reverse and use it to filter the template's input.

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