tornavis/source/blender/blenlib/BLI_implicit_sharing_ptr.hh

/* SPDX-FileCopyrightText: 2023 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */

#pragma once

/** \file
 * \ingroup bli
 */

#include <memory>
#include <utility>

#include "BLI_implicit_sharing.hh"
#include "BLI_struct_equality_utils.hh"

namespace blender {

/**
 * #ImplicitSharingPtr is a smart pointer that manages implicit sharing. It's designed to work with
 * types that derive from #ImplicitSharingMixin. It is fairly similar to #std::shared_ptr but
 * requires the reference count to be embedded in the data.
 */
template<typename T> class ImplicitSharingPtr {
 private:
  const T *data_ = nullptr;

 public:
  ImplicitSharingPtr() = default;

  explicit ImplicitSharingPtr(const T *data) : data_(data) {}

  /* Implicit conversion from nullptr. */
  ImplicitSharingPtr(std::nullptr_t) : data_(nullptr) {}

  ImplicitSharingPtr(const ImplicitSharingPtr &other) : data_(other.data_)
  {
    this->add_user(data_);
  }

  ImplicitSharingPtr(ImplicitSharingPtr &&other) : data_(other.data_)
  {
    other.data_ = nullptr;
  }

  ~ImplicitSharingPtr()
  {
    this->remove_user_and_delete_if_last(data_);
  }

  ImplicitSharingPtr &operator=(const ImplicitSharingPtr &other)
  {
    if (this == &other) {
      return *this;
    }

    this->remove_user_and_delete_if_last(data_);
    data_ = other.data_;
    this->add_user(data_);
    return *this;
  }

  ImplicitSharingPtr &operator=(ImplicitSharingPtr &&other)
  {
    if (this == &other) {
      return *this;
    }

    this->remove_user_and_delete_if_last(data_);
    data_ = other.data_;
    other.data_ = nullptr;
    return *this;
  }

  const T *operator->() const
  {
    BLI_assert(data_ != nullptr);
    return data_;
  }

  const T &operator*() const
  {
    BLI_assert(data_ != nullptr);
    return *data_;
  }

  operator bool() const
  {
    return data_ != nullptr;
  }

  const T *get() const
  {
    return data_;
  }

  const T *release()
  {
    const T *data = data_;
    data_ = nullptr;
    return data;
  }

  void reset()
  {
    this->remove_user_and_delete_if_last(data_);
    data_ = nullptr;
  }

  bool has_value() const
  {
    return data_ != nullptr;
  }

  uint64_t hash() const
  {
    return get_default_hash(data_);
  }

  BLI_STRUCT_EQUALITY_OPERATORS_1(ImplicitSharingPtr, data_)

 private:
  static void add_user(const T *data)
  {
    if (data != nullptr) {
      data->add_user();
    }
  }

  static void remove_user_and_delete_if_last(const T *data)
  {
    if (data != nullptr) {
      data->remove_user_and_delete_if_last();
    }
  }
};

/**
 * Utility struct to allow used #ImplicitSharingPtr when it's necessary to type-erase the backing
 * storage for user-exposed data. For example, #blender::Vector, or #std::vector might be used to
 * store an implicitly shared array that is only accessed with #Span or #MutableSpan.
 *
 * This class handles RAII for the sharing info and the exposed data pointer.
 * Retrieving the data with write access and type safety must be handled elsewhere.
 */
class ImplicitSharingPtrAndData {
 public:
  ImplicitSharingPtr<ImplicitSharingInfo> sharing_info;
  const void *data = nullptr;

  ImplicitSharingPtrAndData() = default;
  ImplicitSharingPtrAndData(ImplicitSharingPtr<ImplicitSharingInfo> sharing_info, const void *data)
      : sharing_info(std::move(sharing_info)), data(data)
  {
  }

  ImplicitSharingPtrAndData(const ImplicitSharingPtrAndData &other)
      : sharing_info(other.sharing_info), data(other.data)
  {
  }

  ImplicitSharingPtrAndData(ImplicitSharingPtrAndData &&other)
      : sharing_info(std::move(other.sharing_info)), data(std::exchange(other.data, nullptr))
  {
  }

  ImplicitSharingPtrAndData &operator=(const ImplicitSharingPtrAndData &other)
  {
    if (this == &other) {
      return *this;
    }
    std::destroy_at(this);
    new (this) ImplicitSharingPtrAndData(other);
    return *this;
  }

  ImplicitSharingPtrAndData &operator=(ImplicitSharingPtrAndData &&other)
  {
    if (this == &other) {
      return *this;
    }
    std::destroy_at(this);
    new (this) ImplicitSharingPtrAndData(std::move(other));
    return *this;
  }

  ~ImplicitSharingPtrAndData()
  {
    this->data = nullptr;
  }

  bool has_value() const
  {
    return this->sharing_info.has_value();
  }
};

}  // namespace blender