Boost.Tuple是否与C ++ 0x可变参数模板兼容？ [英] Is Boost.Tuple compatible with C++0x variadic templates?

问题描述

我在玩可变模板（gcc 4.5），遇到这个问题：

  template< typename ... Args> 
 boost :: tuple< Args ...> 
 my_make_tuple（Args ... args）
 {
 return boost :: tuple< Args ...>（args ...）; 
} 
 
 int main（void）
 {
 boost :: tuple< int，char> t = my_make_tuple（8，'c'）; 
} 
  

GCC错误讯息：

 抱歉，未实现：无法将Arg ...展开成固定长度的参数列表
在函数'int my_make_tuple（Arg ...）'
  

如果我替换每次出现的 boost :: tuple std :: tuple ，它编译得很好。

boost元组实现有问题吗？或者是一个gcc bug？

现在我必须坚持使用Boost.Tuple。你知道任何解决方法吗？

谢谢。

解决方案

C $ c> Args ... 到 T1，T2，T3，...，T9 b
$ b

作为解决方法，使用不需要此扩展的构造：

  #include < boost / tuple / tuple.hpp> 
 
 template< typename ... Args> 
 auto my_make_tuple（Args ... args） - > decltype（boost :: make_tuple（args ...））
 {
 return {args ...}; 
} 
 
 int main（void）
 {
 boost :: tuple< int，char> t = my_make_tuple（8，'c'）; 
} 
  

---

另一种选择是手动扩展，看到 boost :: tuple 最多支持10个参数。

  #include< boost / tuple / tuple.hpp> 
 
 template< unsigned，class，class ...> struct nth_argument; 
 
 template< unsigned N，class Default，class T，class ... Args> 
 struct nth_argument< N，Default，T，Args ...> 
 {
 typedef typename nth_argument< N  -  1，Default，Args ...> :: type type; 
}; 
 
 template< class Default，class T，class ... Args> 
 struct nth_argument< 0，Default，T，Args ...> 
 {
 typedef T type; 
}; 
 
 template< unsigned N，class Default> 
 struct nth_argument< N，默认> 
 {
 typedef默认类型; 
}; 
 
 template< typename ... Args> 
 struct tuple_from_var_template 
 {
 typedef boost :: tuple< 
 typename nth_argument< 0，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 1，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 2，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 3，boost :: tuples :: null_type，Args。 ..> :: type，
 typename nth_argument< 4，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 5，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 6，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< ：tuples :: null_type，Args ...> :: type，
 typename nth_argument< 8，boost :: tuples :: null_type，Args ...> :: type，
 typename nth_argument< 9，boost :: tuples :: null_type，Args ...> :: type 
>类型; 
}; 
 
 template< typename ... Args> 
 typename tuple_from_var_template< Args ...> :: type my_make_tuple（Args ... args）
 {
 return typename tuple_from_var_template< Args ...> :: type（args .. 。）; 
} 
 
 int main（void）
 {
 boost :: tuple< int，char> t = my_make_tuple（8，'c'）; 
} 
  

I was playing around with variadic templates (gcc 4.5) and hit this problem :

template <typename... Args>
boost::tuple<Args...>
my_make_tuple(Args... args)
{
   return boost::tuple<Args...>(args...);
}

int main (void)
{
    boost::tuple<int, char> t = my_make_tuple(8, 'c');
}

GCC error message :

sorry, unimplemented: cannot expand 'Arg ...' into a fixed-length argument list
In function 'int my_make_tuple(Arg ...)'

If I replace every occurrence of boost::tuple by std::tuple, it compiles fine.
Is there a problem in boost tuple implementation? Or is this a gcc bug ?

I must stick with Boost.Tuple for now. Do you know any workaround ?
Thanks.

解决方案

It doesn't seem to like expanding Args... to T1, T2, T3, ..., T9 as Boost has it.

As a workaround, use constructs that don't require this expansion:

#include <boost/tuple/tuple.hpp>

template <typename... Args>
auto my_make_tuple(Args... args) -> decltype(boost::make_tuple(args...))
{
   return {args...};
}

int main (void)
{
    boost::tuple<int, char> t = my_make_tuple(8, 'c');
}

---

Another option might be to do the expanding manually, seeing that boost::tuple supports up to 10 arguments.

#include <boost/tuple/tuple.hpp>

template <unsigned, class, class...> struct nth_argument;

template <unsigned N, class Default, class T, class... Args>
struct nth_argument<N, Default, T, Args...>
{
    typedef typename nth_argument<N - 1, Default, Args...>::type type;
};

template <class Default, class T, class... Args>
struct nth_argument<0, Default, T, Args...>
{
    typedef T type;
};

template <unsigned N, class Default>
struct nth_argument<N, Default>
{
    typedef Default type;
};

template <typename ...Args>
struct tuple_from_var_template
{
    typedef boost::tuple<
        typename nth_argument<0, boost::tuples::null_type, Args...>::type,
        typename nth_argument<1, boost::tuples::null_type, Args...>::type,
        typename nth_argument<2, boost::tuples::null_type, Args...>::type,
        typename nth_argument<3, boost::tuples::null_type, Args...>::type,
        typename nth_argument<4, boost::tuples::null_type, Args...>::type,
        typename nth_argument<5, boost::tuples::null_type, Args...>::type,
        typename nth_argument<6, boost::tuples::null_type, Args...>::type,
        typename nth_argument<7, boost::tuples::null_type, Args...>::type,
        typename nth_argument<8, boost::tuples::null_type, Args...>::type,
        typename nth_argument<9, boost::tuples::null_type, Args...>::type
    > type;
};

template <typename... Args>
typename tuple_from_var_template<Args...>::type my_make_tuple(Args... args)
{
   return typename tuple_from_var_template<Args...>::type(args...);
}

int main (void)
{
    boost::tuple<int, char> t = my_make_tuple(8, 'c');
}

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