可变参数可变参数模板模板 [英] Variadic variadic template templates

问题描述

我目前正在努力处理以下代码，其目的是实现可变参数模板模板:

template
<
  template <typename... HeadArgs> class Head,
  template <typename... TailArgs> class...
>
struct join<Head<typename HeadArgs...>, Head<typename TailArgs...>...>
{
  typedef Head<typename HeadArgs..., typename TailArgs......> result;
};

理想情况下，我可以使用此模板元功能来实现以下目标:

template <typename...> struct obj1 {};
template <typename...> struct obj2 {};

typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj1<char*, int, double, const char*>
>::result new_obj1;

typedef join
<
  obj2<int, int, double>, 
  obj2<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj2;

/* This should result in an error, because there are 
   different encapsulating objects
typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj;
*/

上面的输出有望以template<int, int, double, double, char, char*, int, double, const char*> struct new_obj[1|2] {};

的形式创建new_obj1和new_obj2

我在Windows上使用的是gcc 4.6.2，它会在扩展" Head<typename TailArgs...> ..."时输出'...'之前的预期参数包".

使用gcc 4.5.1 可重现此错误.

解决方案

尝试如下操作:

template <typename...> struct join;

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2>
struct join<Tpl<Args1...>, Tpl<Args2...>>
{
    typedef Tpl<Args1..., Args2...> type;
};

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2,
          typename ...Tail>
struct join<Tpl<Args1...>, Tpl<Args2...>, Tail...>
{
     typedef typename join<Tpl<Args1..., Args2...>, Tail...>::type type;
};

I'm currently struggling with the following code, the intent of which is to implement variadic variadic template templates:

template
<
  template <typename... HeadArgs> class Head,
  template <typename... TailArgs> class...
>
struct join<Head<typename HeadArgs...>, Head<typename TailArgs...>...>
{
  typedef Head<typename HeadArgs..., typename TailArgs......> result;
};

Ideally, I'd be able to use this template metafunction to achieve the following:

template <typename...> struct obj1 {};
template <typename...> struct obj2 {};

typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj1<char*, int, double, const char*>
>::result new_obj1;

typedef join
<
  obj2<int, int, double>, 
  obj2<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj2;

/* This should result in an error, because there are 
   different encapsulating objects
typedef join
<
  obj1<int, int, double>, 
  obj1<double, char>,
  obj2<char*, int, double, const char*>
>::result new_obj;
*/

The output of the above would hopefully create new_obj1 and new_obj2 in the form template<int, int, double, double, char, char*, int, double, const char*> struct new_obj[1|2] {};

I'm using gcc 4.6.2 on Windows, which outputs an "expected parameter pack before '...'" for the expansion of "Head<typename TailArgs...>...".

This error is reproducable with gcc 4.5.1.

解决方案

Try something like this:

template <typename...> struct join;

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2>
struct join<Tpl<Args1...>, Tpl<Args2...>>
{
    typedef Tpl<Args1..., Args2...> type;
};

template <template <typename...> class Tpl,
          typename ...Args1,
          typename ...Args2,
          typename ...Tail>
struct join<Tpl<Args1...>, Tpl<Args2...>, Tail...>
{
     typedef typename join<Tpl<Args1..., Args2...>, Tail...>::type type;
};

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