absl/base/internal/iterator_traits.h

// Copyright 2025 The Abseil Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: internal/iterator_traits.h
// -----------------------------------------------------------------------------
//
// Helpers for querying traits of iterators, for implementing containers, etc.

#ifndef ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_
#define ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_

#include <iterator>
#include <type_traits>

#include "absl/base/config.h"
#include "absl/meta/type_traits.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace base_internal {

template <typename Iterator, typename = void>
struct IteratorCategory {};

template <typename Iterator>
struct IteratorCategory<
    Iterator,
    std::void_t<typename std::iterator_traits<Iterator>::iterator_category>> {
  using type = typename std::iterator_traits<Iterator>::iterator_category;
};

template <typename Iterator, typename = void>
struct IteratorConceptImpl : IteratorCategory<Iterator> {};

template <typename Iterator>
struct IteratorConceptImpl<
    Iterator,
    std::void_t<typename std::iterator_traits<Iterator>::iterator_concept>> {
  using type = typename std::iterator_traits<Iterator>::iterator_concept;
};

// The newer `std::iterator_traits<Iterator>::iterator_concept` if available,
// else `std::iterator_traits<Iterator>::iterator_category`.
template <typename Iterator>
using IteratorConcept = typename IteratorConceptImpl<Iterator>::type;

template <typename IteratorTag, typename Iterator>
using IsAtLeastIterator =
    std::is_convertible<IteratorConcept<Iterator>, IteratorTag>;

template <typename Iterator>
using IsAtLeastInputIterator =
    IsAtLeastIterator<std::input_iterator_tag, Iterator>;

template <typename Iterator>
using IsAtLeastForwardIterator =
    IsAtLeastIterator<std::forward_iterator_tag, Iterator>;

}  // namespace base_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_ITERATOR_TRAITS_H_