boost/units/detail/push_front_or_add.hpp

   1 // Boost.Units - A C++ library for zero-overhead dimensional analysis and
   2 // unit/quantity manipulation and conversion
   3 //
   4 // Copyright (C) 2003-2008 Matthias Christian Schabel
   5 // Copyright (C) 2008 Steven Watanabe
   6 //
   7 // Distributed under the Boost Software License, Version 1.0. (See
   8 // accompanying file LICENSE_1_0.txt or copy at
   9 // http://www.boost.org/LICENSE_1_0.txt)
  10
  11 #ifndef BOOST_UNITS_DETAIL_PUSH_FRONT_OR_ADD_HPP
  12 #define BOOST_UNITS_DETAIL_PUSH_FRONT_OR_ADD_HPP
  13
  14 #include <boost/mpl/plus.hpp>
  15 #include <boost/mpl/front.hpp>
  16 #include <boost/mpl/push_front.hpp>
  17 #include <boost/mpl/pop_front.hpp>
  18 #include <boost/type_traits/is_same.hpp>
  19
  20 #include <boost/units/units_fwd.hpp>
  21 #include <boost/units/detail/push_front_if.hpp>
  22
  23 namespace boost {
  24
  25 namespace units {
  26
  27 template<class Item, class Next>
  28 struct list;
  29
  30 namespace detail {
  31
  32 template<class T>
  33 struct is_empty_dim;
  34
  35 /// add an instantiation of dim to Sequence.
  36 template<bool>
  37 struct push_front_or_add_impl;
  38
  39 template<>
  40 struct push_front_or_add_impl<true>
  41 {
  42     template<typename Sequence, typename T>
  43     struct apply
  44     {
  45         typedef typename mpl::plus<T, typename Sequence::item>::type item;
  46         typedef typename push_front_if<!is_empty_dim<item>::value>::template apply<
  47             typename Sequence::next,
  48             item
  49         > type;
  50     };
  51 };
  52
  53 template<>
  54 struct push_front_or_add_impl<false>
  55 {
  56     template<typename Sequence, typename T>
  57     struct apply
  58     {
  59         typedef list<T, Sequence> type;
  60     };
  61 };
  62
  63 template<typename Sequence, typename T>
  64 struct push_front_or_add
  65 {
  66     typedef typename push_front_or_add_impl<boost::is_same<typename T::tag_type, typename Sequence::item::tag_type>::value>::template apply<
  67         Sequence,
  68         T
  69     >::type type;
  70 };
  71
  72 template<typename T>
  73 struct push_front_or_add<dimensionless_type, T>
  74 {
  75     typedef list<T, dimensionless_type> type;
  76 };
  77
  78 } // namespace detail
  79
  80 } // namespace units
  81
  82 } // namespace boost
  83
  84 #endif