1 // Copyright David Abrahams 2003. Use, modification and distribution is
2 // subject to the Boost Software License, Version 1.0. (See accompanying
3 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
4 #ifndef BOOST_ITERATOR_MINIMUM_CATEGORY_HPP_INCLUDED_
5 # define BOOST_ITERATOR_MINIMUM_CATEGORY_HPP_INCLUDED_
6 
7 # include <boost/static_assert.hpp>
8 # include <boost/type_traits/is_convertible.hpp>
9 # include <boost/type_traits/is_same.hpp>
10 
11 # include <boost/mpl/placeholders.hpp>
12 # include <boost/mpl/aux_/lambda_support.hpp>
13 
14 namespace boost {
15 namespace iterators {
16 namespace detail {
17 
18 template <bool GreaterEqual, bool LessEqual>
19 struct minimum_category_impl;
20 
21 template <class T1, class T2>
22 struct error_not_related_by_convertibility;
23 
24 template <>
25 struct minimum_category_impl<true,false>
26 {
27     template <class T1, class T2> struct apply
28     {
29         typedef T2 type;
30     };
31 };
32 
33 template <>
34 struct minimum_category_impl<false,true>
35 {
36     template <class T1, class T2> struct apply
37     {
38         typedef T1 type;
39     };
40 };
41 
42 template <>
43 struct minimum_category_impl<true,true>
44 {
45     template <class T1, class T2> struct apply
46     {
47         BOOST_STATIC_ASSERT((is_same<T1,T2>::value));
48         typedef T1 type;
49     };
50 };
51 
52 template <>
53 struct minimum_category_impl<false,false>
54 {
55     template <class T1, class T2> struct apply
56       : error_not_related_by_convertibility<T1,T2>
57     {
58     };
59 };
60 
61 } // namespace detail
62 
63 //
64 // Returns the minimum category type or fails to compile
65 // if T1 and T2 are unrelated.
66 //
67 template <class T1 = mpl::_1, class T2 = mpl::_2>
68 struct minimum_category
69 {
70     typedef boost::iterators::detail::minimum_category_impl<
71         ::boost::is_convertible<T1,T2>::value
72       , ::boost::is_convertible<T2,T1>::value
73     > outer;
74 
75     typedef typename outer::template apply<T1,T2> inner;
76     typedef typename inner::type type;
77 
78     BOOST_MPL_AUX_LAMBDA_SUPPORT(2,minimum_category,(T1,T2))
79 };
80 
81 template <>
82 struct minimum_category<mpl::_1,mpl::_2>
83 {
84     template <class T1, class T2>
85     struct apply : minimum_category<T1,T2>
86     {};
87 
88     BOOST_MPL_AUX_LAMBDA_SUPPORT_SPEC(2,minimum_category,(mpl::_1,mpl::_2))
89 };
90 
91 } // namespace iterators
92 
93 } // namespace boost
94 
95 #endif // BOOST_ITERATOR_MINIMUM_CATEGORY_HPP_INCLUDED_
96