c++ - 如何将 std::tuple 类型与 boost::mpl 算法一起使用？

Question

boost::mpl 算法似乎无法在开箱即用的 std::tuple 类型上工作，例如，以下不编译(boost -1.46.0, g++ 快照 2011-02-19):

#include <tuple>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/contains.hpp>

namespace mpl=boost::mpl;

typedef mpl::vector<int,float,bool> types;
static_assert(mpl::contains<types, float>::value, "vector contains bool");

typedef std::tuple<int,float,bool> types2;
// the following does not compile:
// error: no class template named ‘apply’ in ‘struct boost::mpl::contains_impl<boost::mpl::non_sequence_tag>’
static_assert(mpl::contains<types2, float>::value, "tuple contains bool");

使 boost::mpl 算法在 std::tuple 上运行的最简单方法是什么？

• 做 evtl。 boost::fusion 提供此功能(就像它为 boost::tuple 所做的那样)？
• 如果不是，是否可以轻松地将 boost::tuple 的融合实现转移到 std::tuple？
• 如果两者都不是，我真的必须实现all the intrinsic metafunctions listed in the MPL documentation吗？或者哪些就足够了？ (文档只说“许多内在元函数提供了在大多数情况下都可以工作的默认实现”，但不清楚到底是哪些。一些只提供开始和结束的测试并没有带我去任何地方)。

Answer 1

如果你不想将 std::tuple 转换成 mpl 类型，你可以重载调度 boost mpl 使用的标签:

#include <tuple>
#include <boost/mpl/sequence_tag.hpp>
#include <boost/mpl/pop_front_fwd.hpp>
#include <boost/mpl/push_front_fwd.hpp>
#include <boost/mpl/push_back_fwd.hpp>
#include <boost/mpl/front_fwd.hpp>
#include <boost/mpl/empty_fwd.hpp>
#include <boost/mpl/size_fwd.hpp>
#include <boost/mpl/at_fwd.hpp>
#include <boost/mpl/back_fwd.hpp>
#include <boost/mpl/clear_fwd.hpp>
#include <boost/mpl/pop_back_fwd.hpp>
#include <boost/mpl/iterator_tags.hpp>
#include <boost/mpl/next_prior.hpp>
#include <boost/mpl/deref.hpp>
#include <boost/mpl/begin_end_fwd.hpp>


namespace boost { namespace mpl {
  namespace aux { struct std_tuple; }

  template<class ... Args>
  struct sequence_tag<std::tuple<Args...> >
  {
    typedef aux::std_tuple type;
  };

  template<>
  struct front_impl< aux::std_tuple >
  {
    template< typename Tuple > struct apply
        : std::tuple_element<0, Tuple>
    {
    };
  };

  template<>
  struct empty_impl< aux::std_tuple >
  {
      template< typename Tuple > struct apply
          : std::integral_constant<bool, std::tuple_size<Tuple>::value == 0>
      {
      };
  };

  template<>
  struct pop_front_impl< aux::std_tuple >
  {
    template< typename Tuple > struct apply;

    template< class First, class ... Types > struct apply<std::tuple<First, Types...>>
    {
      typedef std::tuple<Types...> type;
    };
  };

  template<>
  struct push_front_impl< aux::std_tuple >
  {
    template< typename Tuple, typename T > struct apply;

    template< typename T, typename ... Args >
    struct apply<std::tuple<Args...>, T>
    {
        typedef std::tuple<T, Args...> type;
    };
  };

  template<>
  struct push_back_impl< aux::std_tuple >
  {
    template< typename Tuple, typename T > struct apply;

    template< typename T, typename ... Args  >
    struct apply<std::tuple<Args...>, T>
    {
      typedef std::tuple<Args..., T> type;
    };
  };


  template<>
  struct size_impl< aux::std_tuple >
  {
    template< typename Tuple > struct apply
        : std::tuple_size<Tuple>
    {
    };
  };

  template<>
  struct at_impl< aux::std_tuple >
  {
    template< typename Tuple, typename N > struct apply
        : std::tuple_element<N::value, Tuple>
    {
    };
  };

  template<>
  struct back_impl< aux::std_tuple >
  {
    template< typename Tuple > struct apply
        : std::tuple_element<std::tuple_size<Tuple>::value - 1, Tuple>
    {
    };
  };

  template<>
  struct clear_impl< aux::std_tuple >
  {
    template< typename Tuple > struct apply
    {
      typedef std::tuple<> type;
    };
  };

  template<>
  struct pop_back_impl< aux::std_tuple >
  {
    template<int ...> struct tuple_seq {};
    template<int N, int ...S> struct tuple_gens : tuple_gens<N-1, N-1, S...> {};
    template<int ...S> struct tuple_gens<0, S...>{ typedef tuple_seq<S...> type; };

    template < class Tuple, class Index> struct apply_impl;
    template < class Tuple, int ... S> struct apply_impl<Tuple, tuple_seq<S...>>
    {
      typedef std::tuple<typename std::tuple_element<S, Tuple>::type...> type;
    };

    template< typename Tuple > struct apply : apply_impl<Tuple, typename tuple_gens<std::tuple_size<Tuple>::value - 1>::type> { };
  };

template< class ... Args >
struct tuple_iter;

  template< class ... Args >
  struct tuple_iter<std::tuple<Args...>>
  {
    typedef aux::std_tuple tag;
    typedef forward_iterator_tag category;
  };

template<>
struct begin_impl< aux::std_tuple >
{
  template< class Tuple > struct apply
  {
    typedef tuple_iter<Tuple> type;
  };
};

template<>
struct end_impl< aux::std_tuple >
{
  template< typename > struct apply
  {
    typedef tuple_iter<std::tuple<>> type;
  };
};

template< typename First, class ... Args >
struct deref< tuple_iter<std::tuple<First, Args...> > >
{
  typedef First type;
};

template< typename First, class ... Args >
struct next< tuple_iter<std::tuple<First, Args...>> >
{
  typedef tuple_iter< std::tuple<Args...> > type;
};

} }

和相关的测试:

#include <boost/mpl/pop_front.hpp>
#include <boost/mpl/push_front.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/front.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/back.hpp>
#include <boost/mpl/clear.hpp>
#include <boost/mpl/pop_back.hpp>
#include <boost/mpl/contains.hpp>


#include <boost/mpl/aux_/test.hpp>
MPL_TEST_CASE()
{
  typedef std::tuple<int, char, bool> Tuple;
  MPL_ASSERT((is_same<front<Tuple>::type, int>));
  MPL_ASSERT_RELATION( size<Tuple>::type::value, ==, 3 );
  MPL_ASSERT(( is_same< pop_front<Tuple>::type, std::tuple<char, bool> > ));
  MPL_ASSERT(( is_same< push_front<Tuple, unsigned>::type, std::tuple<unsigned, int, char, bool> > ));
  MPL_ASSERT(( is_same< push_back<Tuple, unsigned>::type, std::tuple<int, char, bool, unsigned> > ));
  MPL_ASSERT_RELATION( empty<Tuple>::type::value, ==, false );
  MPL_ASSERT(( is_same< at_c<Tuple, 0>::type, int > ));
  MPL_ASSERT(( is_same< at_c<Tuple, 1>::type, char > ));
  MPL_ASSERT(( is_same< back<Tuple>::type, bool > ));
  MPL_ASSERT(( is_same< clear<Tuple>::type, std::tuple<> > ));
  MPL_ASSERT(( is_same< pop_back<Tuple>::type, std::tuple<int, char> > ));
  MPL_ASSERT(( contains<Tuple, int> ));
  MPL_ASSERT(( contains<Tuple, char> ));
  MPL_ASSERT(( contains<Tuple, bool> ));
  MPL_ASSERT_NOT(( contains<Tuple, unsigned> ));

}

我用 gcc 4.7.2 和 clang 3.2 测试了这个。它应该包含您在 mpl 中使用任何东西所需的一切(实际上比它需要的多一点)。您可以将元组视为 mpl::list(前向迭代器编译类型)。所以，为了让它发挥得更好，你应该实现 boost::mpl::list 的功能。快速查看 boost/mpl/list/aux_ 目录:begin_end.hpp empty.hpp iterator.hpp pop_front.hpp push_back.hpp size.hppclear.hpp front.hpp push_front.hpp tag.hpp是需要执行的相关文件。