整理

include/boost/iterator/min_category.hpp

@@ -0,0 +1,83 @@
+// Copyright Andrey Semashev 2025.
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// https://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_
+#define BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_
+
+#include <type_traits>
+
+namespace boost {
+namespace iterators {
+namespace detail {
+
+template<
+    typename T1,
+    typename T2,
+    bool GreaterEqual = std::is_convertible< T1, T2 >::value,
+    bool LessEqual = std::is_convertible< T2, T1 >::value
+>
+struct min_category_impl
+{
+    static_assert(GreaterEqual || LessEqual, "Iterator category types must be related through convertibility.");
+};
+
+template< typename T1, typename T2 >
+struct min_category_impl< T1, T2, true, false >
+{
+    using type = T2;
+};
+
+template< typename T1, typename T2 >
+struct min_category_impl< T1, T2, false, true >
+{
+    using type = T1;
+};
+
+template< typename T1, typename T2 >
+struct min_category_impl< T1, T2, true, true >
+{
+    static_assert(std::is_same< T1, T2 >::value, "Iterator category types must be the same when they are equivalent.");
+    using type = T1;
+};
+
+} // namespace detail
+
+//
+// Returns the minimum iterator category type in the list
+// or fails to compile if any of the categories are unrelated.
+//
+template< typename... Categories >
+struct min_category;
+
+template< typename T >
+struct min_category< T >
+{
+    using type = T;
+};
+
+template< typename T1, typename T2, typename... Tail >
+struct min_category< T1, T2, Tail... >
+{
+    using type = typename min_category<
+        typename iterators::detail::min_category_impl< T1, T2 >::type,
+        Tail...
+    >::type;
+};
+
+// Shortcut to slightly optimize compilation speed
+template< typename T1, typename T2 >
+struct min_category< T1, T2 >
+{
+    using type = typename iterators::detail::min_category_impl< T1, T2 >::type;
+};
+
+template< typename... Categories >
+using min_category_t = typename min_category< Categories... >::type;
+
+} // namespace iterators
+} // namespace boost
+
+#endif // BOOST_ITERATOR_MIN_CATEGORY_HPP_INCLUDED_