Question

boost::mpl算法似乎无法在开箱即用的std::tuple类型上运行，例如，以下内容无法编译（boost-1.46.0，g ++ snapshot 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？

evtl。 boost::fusion提供此功能（正如boost::tuple）所做的那样？
如果没有，是否可以轻松地将boost::tuple的融合实施转移到std::tuple？
如果不是，我是否真的必须实施all the intrinsic metafunctions listed in the MPL documentation或哪些是足够的？（文档只说“许多内在元函数提供了一个默认的实现，它可以在大多数情况下工作”，但目前尚不清楚究竟是哪些。而一些只提供开始和结束的测试没有带我到任何地方）。

Answer 1

如果你不想将std :: tuple转换为mpl类型，你可以重载标签调度boost mpl使用：

#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;
};

} }

相关测试：

#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进行了测试。它应该包含在mpl中使用任何东西所需的一切（实际上它比它需要的更多）。您可以将元组视为mpl :: list（转发迭代器编译类型）。所以，为了让它发挥得更好你应该实现boost :: mpl :: list的功能。快速浏览boost / mpl / list / aux_目录： begin_end.hpp empty.hpp iterator.hpp pop_front.hpp push_back.hpp size.hpp clear.hpp front.hpp push_front.hpp tag.hpp 是需要实施的相关文件。

Answer 2

从std :: tuple转换为boost类型并返回似乎是最简单的方法

#include <iostream>
#include <tuple>
#include <type_traits>
#include <boost/mpl/if.hpp>
#include <boost/mpl/not.hpp>
#include <boost/mpl/vector.hpp>

namespace mpl = boost::mpl;

template<typename Sequence, typename T>
struct push_front;

template<template<typename...> class Sequence, typename T, typename ... Args>
struct push_front< Sequence<Args...>,T> {
  typedef Sequence<T, Args...> type;
};

template<template<typename...> class To, typename From> struct tuple_change;

template<template<typename...> class To, template<typename...> class From, typename ... Args>
struct tuple_change<To, From<Args...>>
{
  typedef To<Args...> type;
};

template<typename Sequence, size_t N>
struct at : std::tuple_element<N,Sequence> { };

template<typename Sequence>
struct empty;

template<template<typename...> class Sequence, typename ... Args>
struct empty<Sequence<Args...>> {
  typedef Sequence<> type;
};

template<
  size_t N,
  typename Sequence,
  template<typename> class Pred,
  typename ... Args >
struct while_impl
{
  typedef typename mpl::if_c<
    Pred<
        typename at<Sequence, sizeof...(Args) - N -1>::type
    >::value,
    typename push_front<
        typename while_impl<N-1, Sequence, Pred, Args...>::type, 
            typename at<Sequence,sizeof...(Args)-N-1>::type
    >::type,
    typename empty< Sequence > ::type
  >::type type;
};

template<
  typename Sequence,
  template<typename> class Pred,
  typename ... Args >
struct while_impl<-1, Sequence, Pred, Args...>
: empty<Sequence> {
};


template<
  typename Sequence,
  template<typename> class Pred>
struct while_;

template<
  template<typename...> class Sequence,
  template<typename> class Pred,
  typename ... Args >
struct while_< Sequence<Args...>, Pred >
{
  typedef typename while_impl<sizeof...(Args)-1, Sequence<Args...>, Pred, Args...>::type type;
};

template<typename T>
struct not_na : mpl::not_< std::is_same<mpl_::na, T> >
{ };

template<template<typename...> class To, typename From>
struct to_boost;

template<template<typename...> class To, typename...Args >
struct to_boost<To, std::tuple<Args...> > :
  tuple_change< mpl::vector, std::tuple<Args...> >
{ };

template< typename From >
struct to_std;

template<template<typename...> class From, typename...Args >
struct to_std< From<Args...> > :
   while_<typename tuple_change< std::tuple, From<Args...> >::type, not_na>
{ };

static_assert(
std::is_same<
    mpl::vector< char, int, bool>,
    typename to_boost<mpl::vector, std::tuple<char, int, bool> >::type
  >::value,
"tuple_change to boost failed");

static_assert(
  std::is_same<
    std::tuple< char, int, bool>,
    typename to_std< mpl::vector<char, int, bool> >::type
  >::value,
"tuple_change from boost failed");

int main(){ return 0;}

*测试用：
MacOSx上的boost_1_46_0和g ++ - 4.5 Ubuntu 10.10上的boost_1_45_0和g ++ - 4.5

Answer 3

这是我在std :: tuple和boost类型之间进行转换的版本，但正如上面的注释中所述，转换可能不是非常有效的编译时间，即会导致（不必要的）长编译时间。肯定会避免转换的解决方案......

#include <tuple>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/at.hpp>

namespace mpl=boost::mpl;

//_ 1. vector_size and vector_at for std::tuple and mpl sequences
template <typename SEQ> struct vector_size 
: mpl::size<SEQ>
{};

template <typename... TYPES> struct vector_size<std::tuple<TYPES...>> 
: std::tuple_size<std::tuple<TYPES...>>
{};

template <typename SEQ, size_t N> struct vector_at 
: mpl::at_c<SEQ, N>
{};

template <typename... TYPES, size_t N> struct vector_at<std::tuple<TYPES...>, N> 
: std::tuple_element<N, std::tuple<TYPES...>>
{};

//_ 2. convert
template <template <typename...> class COLLECT,
      typename SEQ, size_t N, typename... ARGS>
struct convert_helper 
  : convert_helper<COLLECT, SEQ, N-1, typename vector_at<SEQ, N-1>::type, ARGS...>
{};

template <template <typename...> class COLLECT, typename SEQ, typename... ARGS>
struct convert_helper<COLLECT, SEQ, 0, ARGS...> {
  typedef COLLECT<ARGS...> type;
};

template <template <typename...> class COLLECT, typename SEQ>
struct convert 
  : convert_helper<COLLECT, SEQ, vector_size<SEQ>::value>
{};

//_ 3. tests
typedef std::tuple<int, float, bool> types;
typedef mpl::vector<int, float, bool> types_v;

static_assert(std::is_same<convert<std::tuple, types_v>::type, types>::value, "boost2std works");
static_assert(std::is_same<convert<mpl::vector,types>::type, types_v>::value, "std2boost works");

int main() {}

如何在boost :: mpl算法中使用std :: tuple类型？

3 个答案: