folly/folly/container/Enumerate.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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
 *
 *     http://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.
 */

#pragma once

#include <iterator>
#include <memory>

#include <folly/CPortability.h>
#include <folly/portability/SysTypes.h>

/**
 * Similar to Python's enumerate(), folly::enumerate() can be used to
 * iterate a range with a for-range loop, and it also allows to
 * retrieve the count of iterations so far. Can be used in constexpr
 * context.
 *
 * For example:
 *
 * for (auto&& [index, element] : folly::enumerate(vec)) {
 *   // index is a const reference to a size_t containing the iteration count.
 *   // element is a reference to the type contained within vec, mutable
 *   // unless vec is const.
 * }
 *
 * If the binding is const, the element reference is too.
 *
 * for (const auto&& [index, element] : folly::enumerate(vec)) {
 *   // element is always a const reference.
 * }
 *
 * It can also be used as follows:
 *
 * for (auto&& it : folly::enumerate(vec)) {
 *   // *it is a reference to the current element. Mutable unless vec is const.
 *   // it->member can be used as well.
 *   // it.index contains the iteration count.
 * }
 *
 * As before, const auto&& it can also be used.
 */

namespace folly {

namespace detail {

template <class T>
struct MakeConst { … };
MakeConst<T &>;
MakeConst<T *>;

template <class Iterator>
class Enumerator { … };

template <class Range>
class RangeEnumerator { … };

} // namespace detail

template <class Range>
constexpr detail::RangeEnumerator<Range> enumerate(Range&& r) { … }

} // namespace folly