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Extracted min_category as a variadic metafunction.

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The new min_category is similar to minimum_category but accepts variable
number of iterator categories on input instead of just two, and also does
not depend on Boost.MPL for lambda placeholders. The existing minimum_category
trait has been reimplemented in terms of min_category and deprecated.

We don't yet emit deprecation warnings as there is still code that uses
the old trait which we first need to update. Eventually, minimum_category
will emit warnings and will be removed.
This commit is contained in:
Andrey Semashev
2025-01-27 00:04:13 +03:00
parent b30cbf634c
commit dec7d0f24c
8 changed files with 165 additions and 45 deletions
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83
include/boost/iterator/min_category.hpp Normal file
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@@ -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_

49
include/boost/iterator/minimum_category.hpp
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@@ -4,61 +4,26 @@
#ifndef BOOST_ITERATOR_MINIMUM_CATEGORY_HPP_INCLUDED_
#define BOOST_ITERATOR_MINIMUM_CATEGORY_HPP_INCLUDED_
#include <type_traits>
#include <boost/mpl/arg_fwd.hpp>
#include <boost/iterator/min_category.hpp>
namespace boost {
namespace iterators {
namespace detail {
template <class T1, class T2, bool GreaterEqual, bool LessEqual>
struct minimum_category_impl;
template <class T1, class T2>
struct minimum_category_impl<T1, T2, true, false>
{
using type = T2;
};
template <class T1, class T2>
struct minimum_category_impl<T1, T2, false, true>
{
using type = T1;
};
template <class T1, class T2>
struct minimum_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 category type or fails to compile
// if T1 and T2 are unrelated.
//
// Deprecated metafunction for selecting minimum iterator category,
// use min_category instead.
template< class T1 = mpl::arg<1>, class T2 = mpl::arg<2> >
struct minimum_category
struct minimum_category :
public min_category<T1, T2>
{
static_assert(
std::is_convertible<T1, T2>::value || std::is_convertible<T2, T1>::value,
"Iterator category types must be related through convertibility.");
using type = typename boost::iterators::detail::minimum_category_impl<
T1,
T2,
std::is_convertible<T1, T2>::value,
std::is_convertible<T2, T1>::value
>::type;
};
template <>
struct minimum_category< mpl::arg<1>, mpl::arg<2> >
{
template <class T1, class T2>
struct apply : minimum_category<T1, T2>
struct apply :
public min_category<T1, T2>
{};
};