'11年代のMPL

この記事はC++11 Advent Calendar 2011の参加記事です。

※注: やたらと長いコードが貼ってありますが、実装についての解説は無いので読み飛ばしてください。

Boost.MPL

Boost.MPLはBoostを使っている人なら(魔クロ界の住人を除く)ほぼ全員が間接的にお世話になっているライブラリです。
これはC++03で Variadic Templates をエミュレートするためにいろいろエグいことをやっているのですが、これをそのままC++11で書き直すとどうなるか少しだけ見てみましょう。

シーケンス

まずはMPLシーケンスのmpl::vectorを見てみましょう。C++03では次のようになっています。

// boost/mpl/vector/vector10.hpp
#   include <boost/mpl/aux_/config/typeof.hpp>
#   include <boost/mpl/aux_/config/ctps.hpp>
#   include <boost/preprocessor/iterate.hpp>

namespace boost { namespace mpl {

#   define BOOST_PP_ITERATION_PARAMS_1 \
    (3,(0, 10, <boost/mpl/vector/aux_/numbered.hpp>))
#   include BOOST_PP_ITERATE()

}}

おっと、これは単にPP_ITERATEしているだけですね。実体はこっちです。

// boost/mpl/vector/aux_/numbered.hpp
#   define AUX778076_VECTOR_TAIL(vector, i_, T) \
    BOOST_PP_CAT(vector,i_)< \
          BOOST_PP_ENUM_PARAMS(i_, T) \
        > \
    /**/

#if i_ > 0
template<
      BOOST_PP_ENUM_PARAMS(i_, typename T)
    >
struct BOOST_PP_CAT(vector,i_)
    : v_item<
          BOOST_PP_CAT(T,BOOST_PP_DEC(i_))
        , AUX778076_VECTOR_TAIL(vector,BOOST_PP_DEC(i_),T)
        >
{
    typedef BOOST_PP_CAT(vector,i_) type;
};

簡単に見えますか? だとしたらそれは全て魔クロの成果です。ちなみに、この実装はtypeof拡張が使えるときのもの(つまりgcc限定)で、それ以外の場合はこれの10倍の記述が必要です。また、実際にこのPP_ITERATEを毎回行うとそれだけでコンパイル時間がBoostするので、通常使う場合はあらかじめマクロ展開されたヘッダが使われます。ちょっとプリプロセスされたものを見てみましょう。

// boost/mpl/vector/aux_/preprocessed/typeof_based/vector10.hpp
namespace boost { namespace mpl {

template<
      typename T0
    >
struct vector1
    : v_item<
          T0
        , vector0<  >
        >
{
    typedef vector1 type;
};

template<
      typename T0, typename T1
    >
struct vector2
    : v_item<
          T1
        , vector1<T0>
        >
{
    typedef vector2 type;
};

template<
      typename T0, typename T1, typename T2
    >
struct vector3
    : v_item<
          T2
        , vector2< T0,T1 >
        >
{
    typedef vector3 type;
};

template<
      typename T0, typename T1, typename T2, typename T3
    >
struct vector4
    : v_item<
          T3
        , vector3< T0,T1,T2 >
        >
{
    typedef vector4 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    >
struct vector5
    : v_item<
          T4
        , vector4< T0,T1,T2,T3 >
        >
{
    typedef vector5 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    , typename T5
    >
struct vector6
    : v_item<
          T5
        , vector5< T0,T1,T2,T3,T4 >
        >
{
    typedef vector6 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename T6
    >
struct vector7
    : v_item<
          T6
        , vector6< T0,T1,T2,T3,T4,T5 >
        >
{
    typedef vector7 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename T6, typename T7
    >
struct vector8
    : v_item<
          T7
        , vector7< T0,T1,T2,T3,T4,T5,T6 >
        >
{
    typedef vector8 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename T6, typename T7, typename T8
    >
struct vector9
    : v_item<
          T8
        , vector8< T0,T1,T2,T3,T4,T5,T6,T7 >
        >
{
    typedef vector9 type;
};

template<
      typename T0, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename T6, typename T7, typename T8, typename T9
    >
struct vector10
    : v_item<
          T9
        , vector9< T0,T1,T2,T3,T4,T5,T6,T7,T8 >
        >
{
    typedef vector10 type;
};

}}

(´・_・`)

※これは0-10要素版で、11-20要素版...41-50要素版に同じだけの記述があります。

さて、これをC++11で書き直してみましょう。

namespace boost { namespace mpl {
template <typename ...Types>
struct vector;
template <>
struct vector<> { // from boost/mpl/vector/aux_/vector0.hpp
    typedef aux::vector_tag tag;
    typedef vector          type;
    typedef long_<32768>    lower_bound_;
    typedef lower_bound_    upper_bound_;
    typedef long_<0>        size;

    static aux::type_wrapper<void_> item_(...);
};
template <typename T0>
struct vector<T0>: v_item<T0, vector<>> { typedef vector type; };
template <typename T0, typename ...Types, typename Tn>
struct vector<T0, Types..., Tn>: v_item<Tn, vector<T0, Types...>> { typedef vector type; };

template <typename = na>
using vector0 = vector<>;

#define DECL_VECTOR_N(n) \
  template <BOOST_PP_ENUM_PARAMS(n, typename T)> \
  using vector##n = vector<BOOST_PP_ENUM_PARAMS(n, T)>

DECL_VECTOR_N(1);
DECL_VECTOR_N(2);
DECL_VECTOR_N(3);
DECL_VECTOR_N(4);
DECL_VECTOR_N(5);
DECL_VECTOR_N(6);
DECL_VECTOR_N(7);
DECL_VECTOR_N(8);
DECL_VECTOR_N(9);
DECL_VECTOR_N(10);

#undef DECL_VECTOR_N

}}

素晴らしいですね。どうして今までこれが出来なかったんでしょうか。

論理演算

今度はmpl::and_を見てみましょう。

// boost/mpl/and.hpp
#   define AUX778076_OP_NAME and_
#   define AUX778076_OP_VALUE1 false
#   define AUX778076_OP_VALUE2 true
#   include <boost/mpl/aux_/logical_op.hpp>

おっと、これも実体は別のヘッダですね。

// boost/mpl/aux_/logical_op.hpp
namespace boost { namespace mpl {

#   define AUX778076_PARAMS(param, sub) \
    BOOST_MPL_PP_PARAMS( \
          BOOST_MPL_PP_SUB(BOOST_MPL_LIMIT_METAFUNCTION_ARITY, sub) \
        , param \
        ) \
    /**/

#   define AUX778076_SHIFTED_PARAMS(param, sub) \
    BOOST_MPL_PP_EXT_PARAMS( \
          2, BOOST_MPL_PP_SUB(BOOST_PP_INC(BOOST_MPL_LIMIT_METAFUNCTION_ARITY), sub) \
        , param \
        ) \
    /**/

#   define AUX778076_SPEC_PARAMS(param) \
    BOOST_MPL_PP_ENUM( \
          BOOST_PP_DEC(BOOST_MPL_LIMIT_METAFUNCTION_ARITY) \
        , param \
        ) \
    /**/

namespace aux {

template< bool C_, AUX778076_PARAMS(typename T, 1) >
struct BOOST_PP_CAT(AUX778076_OP_NAME,impl)
    : BOOST_PP_CAT(AUX778076_OP_VALUE1,_)
{
};

template< AUX778076_PARAMS(typename T, 1) >
struct BOOST_PP_CAT(AUX778076_OP_NAME,impl)< AUX778076_OP_VALUE2,AUX778076_PARAMS(T, 1) >
    : BOOST_PP_CAT(AUX778076_OP_NAME,impl)<
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value
        , AUX778076_SHIFTED_PARAMS(T, 1)
        , BOOST_PP_CAT(AUX778076_OP_VALUE2,_)
        >
{
};

template<>
struct BOOST_PP_CAT(AUX778076_OP_NAME,impl)<
          AUX778076_OP_VALUE2
        , AUX778076_SPEC_PARAMS(BOOST_PP_CAT(AUX778076_OP_VALUE2,_))
        >
    : BOOST_PP_CAT(AUX778076_OP_VALUE2,_)
{
};

} // namespace aux

template<
      typename BOOST_MPL_AUX_NA_PARAM(T1)
    , typename BOOST_MPL_AUX_NA_PARAM(T2)
    BOOST_MPL_PP_DEF_PARAMS_TAIL(2, typename T, BOOST_PP_CAT(AUX778076_OP_VALUE2,_))
    >
struct AUX778076_OP_NAME
    : aux::BOOST_PP_CAT(AUX778076_OP_NAME,impl)<
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value
        , AUX778076_SHIFTED_PARAMS(T,0)
        >
{
    BOOST_MPL_AUX_LAMBDA_SUPPORT(
          BOOST_MPL_LIMIT_METAFUNCTION_ARITY
        , AUX778076_OP_NAME
        , (AUX778076_PARAMS(T, 0))
        )
};

BOOST_MPL_AUX_NA_SPEC2(
      2
    , BOOST_MPL_LIMIT_METAFUNCTION_ARITY
    , AUX778076_OP_NAME
    )

}}

#undef AUX778076_SPEC_PARAMS
#undef AUX778076_SHIFTED_PARAMS
#undef AUX778076_PARAMS
#undef AUX778076_OP_NAME
#undef AUX778076_OP_VALUE1
#undef AUX778076_OP_VALUE2

こんなん読めるかああああああああ！！！ 幸い、これもプリプロセストヘッダがあるので、そっちを見てみましょう。

// boost/mpl/aux_/preprocessed/gcc/and.hpp
namespace boost { namespace mpl {

namespace aux {

template< bool C_, typename T1, typename T2, typename T3, typename T4 >
struct and_impl
    : false_
{
};

template< typename T1, typename T2, typename T3, typename T4 >
struct and_impl< true,T1,T2,T3,T4 >
    : and_impl<
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value
        , T2, T3, T4
        , true_
        >
{
};

template<>
struct and_impl<
          true
        , true_, true_, true_, true_
        >
    : true_
{
};

} // namespace aux

template<
      typename BOOST_MPL_AUX_NA_PARAM(T1)
    , typename BOOST_MPL_AUX_NA_PARAM(T2)
    , typename T3 = true_, typename T4 = true_, typename T5 = true_
    >
struct and_

    : aux::and_impl<
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value
        , T2, T3, T4, T5
        >

{
    BOOST_MPL_AUX_LAMBDA_SUPPORT(
          5
        , and_
        , ( T1, T2, T3, T4, T5)
        )
};

BOOST_MPL_AUX_NA_SPEC2(
      2
    , 5
    , and_
    )

}}

(´・_・`)

まあ、さっきよりはマシ、ですね。。。これをC++11で書き直してみましょう。

namespace boost { namespace mpl {

namespace detail {

template <bool ...C>
struct and_impl: false_ { };
template <>
struct and_impl<true>: true_ { };
template <bool C0, bool ...C>
struct and_impl<true, C0, C...>: and_impl<C0, C...> { };

}

template <typename ...T>
struct and_: detail::and_impl<T::value...> { };

}}

C++11は素晴らしいですね。何よりも魔クロが消えたのが素晴らしいです。

バインダ

最後にもう一つ、mpl::bindを見てみましょう。これもプリプロセストヘッダがあります。

// boost/mpl/aux_/preprocessed/gcc/bind.hpp
namespace boost { namespace mpl {

namespace aux {

template<
      typename T, typename U1, typename U2, typename U3, typename U4
    , typename U5
    >
struct resolve_bind_arg
{
    typedef T type;
};

template<
      typename T
    , typename Arg
    >
struct replace_unnamed_arg
{
    typedef Arg next;
    typedef T type;
};

template<
      typename Arg
    >
struct replace_unnamed_arg< arg< -1 >, Arg >
{
    typedef typename Arg::next next;
    typedef Arg type;
};

template<
      int N, typename U1, typename U2, typename U3, typename U4, typename U5
    >
struct resolve_bind_arg< arg<N>, U1, U2, U3, U4, U5 >
{
    typedef typename apply_wrap5<mpl::arg<N>, U1, U2, U3, U4, U5>::type type;
};

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename U1, typename U2, typename U3, typename U4
    , typename U5
    >
struct resolve_bind_arg< bind< F,T1,T2,T3,T4,T5 >, U1, U2, U3, U4, U5 >
{
    typedef bind< F,T1,T2,T3,T4,T5 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

template<
      typename F
    >
struct bind0
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
     public:
        typedef typename apply_wrap0<
              f_
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename U1, typename U2, typename U3, typename U4
    , typename U5
    >
struct resolve_bind_arg<
      bind0<F>, U1, U2, U3, U4, U5
    >
{
    typedef bind0<F> f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(1, bind0)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(1, bind0)

template<
      typename F
    >
struct bind< F,na,na,na,na,na >
    : bind0<F>
{
};

template<
      typename F, typename T1
    >
struct bind1
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
     public:
        typedef typename apply_wrap1<
              f_
            , typename t1::type
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename T1, typename U1, typename U2, typename U3
    , typename U4, typename U5
    >
struct resolve_bind_arg<
      bind1< F,T1 >, U1, U2, U3, U4, U5
    >
{
    typedef bind1< F,T1 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(2, bind1)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(2, bind1)

template<
      typename F, typename T1
    >
struct bind< F,T1,na,na,na,na >
    : bind1< F,T1 >
{
};

template<
      typename F, typename T1, typename T2
    >
struct bind2
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
     public:
        typedef typename apply_wrap2<
              f_
            , typename t1::type, typename t2::type
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename T1, typename T2, typename U1, typename U2
    , typename U3, typename U4, typename U5
    >
struct resolve_bind_arg<
      bind2< F,T1,T2 >, U1, U2, U3, U4, U5
    >
{
    typedef bind2< F,T1,T2 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(3, bind2)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(3, bind2)

template<
      typename F, typename T1, typename T2
    >
struct bind< F,T1,T2,na,na,na >
    : bind2< F,T1,T2 >
{
};

template<
      typename F, typename T1, typename T2, typename T3
    >
struct bind3
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
        typedef aux::replace_unnamed_arg< T3,n3 > r3;
        typedef typename r3::type a3;
        typedef typename r3::next n4;
        typedef aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > t3;
        ///
     public:
        typedef typename apply_wrap3<
              f_
            , typename t1::type, typename t2::type, typename t3::type
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename T1, typename T2, typename T3, typename U1
    , typename U2, typename U3, typename U4, typename U5
    >
struct resolve_bind_arg<
      bind3< F,T1,T2,T3 >, U1, U2, U3, U4, U5
    >
{
    typedef bind3< F,T1,T2,T3 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(4, bind3)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(4, bind3)

template<
      typename F, typename T1, typename T2, typename T3
    >
struct bind< F,T1,T2,T3,na,na >
    : bind3< F,T1,T2,T3 >
{
};

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    >
struct bind4
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
        typedef aux::replace_unnamed_arg< T3,n3 > r3;
        typedef typename r3::type a3;
        typedef typename r3::next n4;
        typedef aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > t3;
        ///
        typedef aux::replace_unnamed_arg< T4,n4 > r4;
        typedef typename r4::type a4;
        typedef typename r4::next n5;
        typedef aux::resolve_bind_arg< a4,U1,U2,U3,U4,U5 > t4;
        ///
     public:
        typedef typename apply_wrap4<
              f_
            , typename t1::type, typename t2::type, typename t3::type
            , typename t4::type
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    , typename U1, typename U2, typename U3, typename U4, typename U5
    >
struct resolve_bind_arg<
      bind4< F,T1,T2,T3,T4 >, U1, U2, U3, U4, U5
    >
{
    typedef bind4< F,T1,T2,T3,T4 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(5, bind4)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(5, bind4)

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    >
struct bind< F,T1,T2,T3,T4,na >
    : bind4< F,T1,T2,T3,T4 >
{
};

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    , typename T5
    >
struct bind5
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef aux::replace_unnamed_arg< F, mpl::arg<1> > r0;
        typedef typename r0::type a0;
        typedef typename r0::next n1;
        typedef typename aux::resolve_bind_arg< a0,U1,U2,U3,U4,U5 >::type f_;
        ///
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
        typedef aux::replace_unnamed_arg< T3,n3 > r3;
        typedef typename r3::type a3;
        typedef typename r3::next n4;
        typedef aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > t3;
        ///
        typedef aux::replace_unnamed_arg< T4,n4 > r4;
        typedef typename r4::type a4;
        typedef typename r4::next n5;
        typedef aux::resolve_bind_arg< a4,U1,U2,U3,U4,U5 > t4;
        ///
        typedef aux::replace_unnamed_arg< T5,n5 > r5;
        typedef typename r5::type a5;
        typedef typename r5::next n6;
        typedef aux::resolve_bind_arg< a5,U1,U2,U3,U4,U5 > t5;
        ///
     public:
        typedef typename apply_wrap5<
              f_
            , typename t1::type, typename t2::type, typename t3::type
            , typename t4::type, typename t5::type
            >::type type;

    };
};

namespace aux {

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    , typename T5, typename U1, typename U2, typename U3, typename U4
    , typename U5
    >
struct resolve_bind_arg<
      bind5< F,T1,T2,T3,T4,T5 >, U1, U2, U3, U4, U5
    >
{
    typedef bind5< F,T1,T2,T3,T4,T5 > f_;
    typedef typename apply_wrap5< f_,U1,U2,U3,U4,U5 >::type type;
};

} // namespace aux

BOOST_MPL_AUX_ARITY_SPEC(6, bind5)
BOOST_MPL_AUX_TEMPLATE_ARITY_SPEC(6, bind5)

/// primary template (not a specialization!)

template<
      typename F, typename T1, typename T2, typename T3, typename T4
    , typename T5
    >
struct bind
    : bind5< F,T1,T2,T3,T4,T5 >
{
};

/// if_/eval_if specializations
template< template< typename T1, typename T2, typename T3 > class F, typename Tag >
struct quote3;

template< typename T1, typename T2, typename T3 > struct if_;

template<
      typename Tag, typename T1, typename T2, typename T3
    >
struct bind3<
      quote3< if_,Tag >
    , T1, T2, T3
    >
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef mpl::arg<1> n1;
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
        typedef aux::replace_unnamed_arg< T3,n3 > r3;
        typedef typename r3::type a3;
        typedef typename r3::next n4;
        typedef aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > t3;
        ///
        typedef typename if_<
              typename t1::type
            , t2, t3
            >::type f_;

     public:
        typedef typename f_::type type;
    };
};

template<
      template< typename T1, typename T2, typename T3 > class F, typename Tag
    >
struct quote3;

template< typename T1, typename T2, typename T3 > struct eval_if;

template<
      typename Tag, typename T1, typename T2, typename T3
    >
struct bind3<
      quote3< eval_if,Tag >
    , T1, T2, T3
    >
{
    template<
          typename U1 = na, typename U2 = na, typename U3 = na
        , typename U4 = na, typename U5 = na
        >
    struct apply
    {
     private:
        typedef mpl::arg<1> n1;
        typedef aux::replace_unnamed_arg< T1,n1 > r1;
        typedef typename r1::type a1;
        typedef typename r1::next n2;
        typedef aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 > t1;
        ///
        typedef aux::replace_unnamed_arg< T2,n2 > r2;
        typedef typename r2::type a2;
        typedef typename r2::next n3;
        typedef aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > t2;
        ///
        typedef aux::replace_unnamed_arg< T3,n3 > r3;
        typedef typename r3::type a3;
        typedef typename r3::next n4;
        typedef aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > t3;
        ///
        typedef typename eval_if<
              typename t1::type
            , t2, t3
            >::type f_;

     public:
        typedef typename f_::type type;
    };
};

}}

(´・_・`)(´・_・`)(´・_・`)

このファイルのほかに、もう一つ mpl::apply_wrap の実装ファイルにも一部実装があります。このクソ長いコードを、C++11で書くとこうなります。

namespace boost { namespace mpl {

namespace aux {
  template <typename B, typename U, typename R = std::tuple<>>
  struct replace_bind_args;

  template <typename T0, typename ...Tn, typename U, typename ...R>
  struct replace_bind_args<
    std::tuple<T0, Tn...>,
    U,
    std::tuple<R...>
  >: replace_bind_args<
       std::tuple<Tn...>,
       U,
       std::tuple<R..., T0>
     > { };

  template <int N, typename ...Tn, typename U, typename ...R>
  struct replace_bind_args<
    std::tuple<boost::mpl::arg<N>, Tn...>,
    U,
    std::tuple<R...>
  >: replace_bind_args<
       std::tuple<Tn...>,
       U,
       std::tuple<R..., typename std::tuple_element<N-1, U>::type>
     > { };

  template <typename U, typename ...R>
  struct replace_bind_args<std::tuple<>, U, std::tuple<R...>> {
    typedef std::tuple<R...> type;
  };

  template <typename T>
  struct has_apply {
    template <typename U> static boost::mpl::true_ check(const typename U::apply *);
    static boost::mpl::false_ check(...);
    typedef decltype(check((const void *)0)) type;
  };

  template <typename T, bool = has_apply<typename std::tuple_element<0, T>::type>::type::value> struct apply_wrap_tuple;
  template <typename F, typename ...T, bool B>
  struct apply_wrap_tuple<std::tuple<F, T...>, B>: F::template apply<T...> { };
  template <typename F>
  struct apply_wrap_tuple<std::tuple<F>, true>: F::apply { };
}

template <typename F, typename ...T>
struct bind {
  template <typename ...U>
  struct apply {
    typedef typename aux::apply_wrap_tuple<
      typename aux::replace_bind_args<std::tuple<F, T...>, std::tuple<U...>>::type
    >::type type;
  };
};

}}

魔クロフリーの美しい実装ですね。これがC++11の実力です。

まとめ

単に実装並べただけでまとめもクソも無いですが、Variadic Templates の威力は解っていただけたと思います。今から新しくテンプレートでメタメタするコードを書く羽目になった諸兄は、是非 -std=c++11 で Variadic Templates を使いましょう。

明日の担当はεπιστημη先生です。