object_storage.h

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/*
 * Copyright 2021 MusicScience37 (Kenta Kabashima)
 *
 * 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 <algorithm>
#include <array>
#include <cstddef>
#include <new>
#include <utility>
 
#include "num_collect/util/cache_line.h"
 
#ifndef NDEBUG
#include <cassert>
#endif
 
namespace num_collect::util {
 
template <typename T, std::size_t MinimumAlignment = cache_line>
class alignas(std::max(MinimumAlignment, alignof(T))) object_storage {
public:
    object_storage() noexcept = default;
 
    object_storage(const object_storage&) = delete;
    object_storage(object_storage&&) = delete;
    auto operator=(const object_storage&) = delete;
    auto operator=(object_storage&&) = delete;
 
    ~object_storage() noexcept
#ifndef NDEBUG
    {
        assert(!has_object_);
    }
#else
        = default;
#endif
 
    template <typename... Args>
    void emplace(Args&&... args) {
#ifndef NDEBUG
        assert(!has_object_);
#endif
        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay,hicpp-no-array-decay)
        ::new (storage_.data()) T(std::forward<Args>(args)...);
#ifndef NDEBUG
        has_object_ = true;
#endif
    }
 
    void reset() noexcept {
        get_pointer()->~T();
#ifndef NDEBUG
        has_object_ = false;
#endif
    }
 
    [[nodiscard]] auto get_ref() noexcept -> T& { return *get_pointer(); }
 
    [[nodiscard]] auto get_ref() const noexcept -> const T& {
        return *get_pointer();
    }
 
    [[nodiscard]] auto get_pointer() noexcept -> T* {
#ifndef NDEBUG
        assert(has_object_);
#endif
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-reinterpret-cast)
        return std::launder(reinterpret_cast<T*>(storage_.data()));
    }
 
    [[nodiscard]] auto get_pointer() const noexcept -> const T* {
#ifndef NDEBUG
        assert(has_object_);
#endif
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-reinterpret-cast)
        return std::launder(reinterpret_cast<const T*>(storage_.data()));
    }
 
private:
    std::array<char, sizeof(T)> storage_{};
 
#ifndef NDEBUG
    bool has_object_{false};
#endif
};
 
}  // namespace num_collect::util