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) 2007-2008 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/// \file
12/// \brief base unit (meter, kg, sec...).
13/// \details base unit definition registration.
14
15#ifndef BOOST_UNITS_BASE_UNIT_HPP
16#define BOOST_UNITS_BASE_UNIT_HPP
17
18#include <boost/units/config.hpp>
19#include <boost/units/heterogeneous_system.hpp>
20#include <boost/units/static_rational.hpp>
21#include <boost/units/units_fwd.hpp>
22#include <boost/units/unit.hpp>
23#include <boost/units/detail/dimension_list.hpp>
24#include <boost/units/detail/ordinal.hpp>
25#include <boost/units/detail/prevent_redefinition.hpp>
26
27namespace boost {
28
29namespace units {
30
31/// This must be in namespace boost::units so that ADL
32/// will work with friend functions defined inline.
33/// Base dimensions and base units are independent.
34/// INTERNAL ONLY
35template<long N> struct base_unit_ordinal { };
36
37/// INTERNAL ONLY
38template<class T, long N> struct base_unit_pair { };
39
40/// INTERNAL ONLY
41template<class T, long N>
42struct check_base_unit {
43 enum {
44 value =
45 sizeof(boost_units_unit_is_registered(units::base_unit_ordinal<N>())) == sizeof(detail::yes) &&
46 sizeof(boost_units_unit_is_registered(units::base_unit_pair<T, N>())) != sizeof(detail::yes)
47 };
48};
49
50/// Defines a base unit. To define a unit you need to provide
51/// the derived class (CRTP), a dimension list and a unique integer.
52/// @code
53/// struct my_unit : boost::units::base_unit<my_unit, length_dimension, 1> {};
54/// @endcode
55/// It is designed so that you will get an error message if you try
56/// to use the same value in multiple definitions.
57template<class Derived,
58 class Dim,
59 long N
60#if !defined(BOOST_UNITS_DOXYGEN) && !defined(BOOST_BORLANDC)
61 ,
62 class = typename detail::ordinal_has_already_been_defined<
63 check_base_unit<Derived, N>::value
64 >::type
65#endif
66>
67class base_unit :
68 public ordinal<N>
69{
70 public:
71 /// INTERNAL ONLY
72 typedef void boost_units_is_base_unit_type;
73 /// INTERNAL ONLY
74 typedef base_unit this_type;
75 /// The dimensions of this base unit.
76 typedef Dim dimension_type;
77
78 /// Provided for mpl compatability.
79 typedef Derived type;
80
81 /// The unit corresponding to this base unit.
82#ifndef BOOST_UNITS_DOXYGEN
83 typedef unit<
84 Dim,
85 heterogeneous_system<
86 heterogeneous_system_impl<
87 list<
88 heterogeneous_system_dim<Derived,static_rational<1> >,
89 dimensionless_type
90 >,
91 Dim,
92 no_scale
93 >
94 >
95 > unit_type;
96#else
97 typedef detail::unspecified unit_type;
98#endif
99
100 private:
101 /// Check for C++0x. In C++0x, we have to have identical
102 /// arguments but a different return type to trigger an
103 /// error. Note that this is only needed for clang as
104 /// check_base_unit will trigger an error earlier
105 /// for compilers with less strict name lookup.
106 /// INTERNAL ONLY
107 friend BOOST_CONSTEXPR Derived*
108 check_double_register(const units::base_unit_ordinal<N>&)
109 { return(0); }
110
111 /// Register this ordinal
112 /// INTERNAL ONLY
113 friend BOOST_CONSTEXPR detail::yes
114 boost_units_unit_is_registered(const units::base_unit_ordinal<N>&)
115 { return(detail::yes()); }
116
117 /// But make sure we can identify the current instantiation!
118 /// INTERNAL ONLY
119 friend BOOST_CONSTEXPR detail::yes
120 boost_units_unit_is_registered(const units::base_unit_pair<Derived, N>&)
121 { return(detail::yes()); }
122};
123
124} // namespace units
125
126} // namespace boost
127
128#endif // BOOST_UNITS_BASE_UNIT_HPP
129

source code of boost/libs/units/include/boost/units/base_unit.hpp