boost/iterator/min_category.hpp

0001 // Copyright Andrey Semashev 2025.
0002 //
0003 // Distributed under the Boost Software License, Version 1.0.
0004 // (See accompanying file LICENSE_1_0.txt or copy at
0005 // https://www.boost.org/LICENSE_1_0.txt)
0006
0007 #ifndef BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_
0008 #define BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_
0009
0010 #include <type_traits>
0011
0012 namespace boost {
0013 namespace iterators {
0014 namespace detail {
0015
0016 template<
0017     typename T1,
0018     typename T2,
0019     bool GreaterEqual = std::is_convertible< T1, T2 >::value,
0020     bool LessEqual = std::is_convertible< T2, T1 >::value
0021 >
0022 struct min_category_impl
0023 {
0024     static_assert(GreaterEqual || LessEqual, "Iterator category types must be related through convertibility.");
0025 };
0026
0027 template< typename T1, typename T2 >
0028 struct min_category_impl< T1, T2, true, false >
0029 {
0030     using type = T2;
0031 };
0032
0033 template< typename T1, typename T2 >
0034 struct min_category_impl< T1, T2, false, true >
0035 {
0036     using type = T1;
0037 };
0038
0039 template< typename T1, typename T2 >
0040 struct min_category_impl< T1, T2, true, true >
0041 {
0042     static_assert(std::is_same< T1, T2 >::value, "Iterator category types must be the same when they are equivalent.");
0043     using type = T1;
0044 };
0045
0046 } // namespace detail
0047
0048 //
0049 // Returns the minimum iterator category type in the list
0050 // or fails to compile if any of the categories are unrelated.
0051 //
0052 template< typename... Categories >
0053 struct min_category;
0054
0055 template< typename T >
0056 struct min_category< T >
0057 {
0058     using type = T;
0059 };
0060
0061 template< typename T1, typename T2, typename... Tail >
0062 struct min_category< T1, T2, Tail... >
0063 {
0064     using type = typename min_category<
0065         typename iterators::detail::min_category_impl< T1, T2 >::type,
0066         Tail...
0067     >::type;
0068 };
0069
0070 // Shortcut to slightly optimize compilation speed
0071 template< typename T1, typename T2 >
0072 struct min_category< T1, T2 >
0073 {
0074     using type = typename iterators::detail::min_category_impl< T1, T2 >::type;
0075 };
0076
0077 template< typename... Categories >
0078 using min_category_t = typename min_category< Categories... >::type;
0079
0080 } // namespace iterators
0081 } // namespace boost
0082
0083 #endif // BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_