blender-archive/source/blender/blenlib/BLI_resource_scope.hh

/*
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

#pragma once

/** \file
 * \ingroup bli
 *
 * A `ResourceScope` takes ownership of arbitrary data/resources. Those resources will be
 * destructed and/or freed when the `ResourceScope` is destructed. Destruction happens in reverse
 * order. That allows resources do depend on other resources that have been added before.
 *
 * A `ResourceScope` can also be thought of as a dynamic/runtime version of normal scopes in C++
 * that are surrounded by braces.
 *
 * The main purpose of a `ResourceScope` is to allow functions to inject data into the scope of the
 * caller. Traditionally, that can only be done by returning a value that owns everything it needs.
 * This is fine until one has to deal with optional ownership. There are many ways to have a type
 * optionally own something else, all of which are fairly annoying. A `ResourceScope` can be used
 * to avoid having to deal with optional ownership. If some value would be owned, it can just be
 * added to the resource scope, otherwise not.
 *
 * When a function takes a `ResourceScope` as parameter, it usually means that its return value
 * will live at least as long as the passed in resources scope. However, it might also live longer.
 * That can happen when the function returns a reference to statically allocated data or
 * dynamically allocated data depending on some condition.
 */

#include "BLI_linear_allocator.hh"
#include "BLI_utility_mixins.hh"
#include "BLI_vector.hh"

namespace blender {

class ResourceScope : NonCopyable, NonMovable {
 private:
  struct ResourceData {
    void *data;
    void (*free)(void *data);
    const char *debug_name;
  };

  LinearAllocator<> m_allocator;
  Vector<ResourceData> m_resources;

 public:
  ResourceScope() = default;

  ~ResourceScope()
  {
    /* Free in reversed order. */
    for (int64_t i = m_resources.size(); i--;) {
      ResourceData &data = m_resources[i];
      data.free(data.data);
    }
  }

  /**
   * Pass ownership of the resource to the ResourceScope. It will be destructed and freed when
   * the collector is destructed.
   */
  template<typename T> T *add(std::unique_ptr<T> resource, const char *name)
  {
    BLI_assert(resource.get() != nullptr);
    T *ptr = resource.release();
    if (ptr == nullptr) {
      return nullptr;
    }
    this->add(
        ptr,
        [](void *data) {
          T *typed_data = reinterpret_cast<T *>(data);
          delete typed_data;
        },
        name);
    return ptr;
  }

  /**
   * Pass ownership of the resource to the ResourceScope. It will be destructed when the
   * collector is destructed.
   */
  template<typename T> T *add(destruct_ptr<T> resource, const char *name)
  {
    T *ptr = resource.release();
    if (ptr == nullptr) {
      return nullptr;
    }
    /* There is no need to keep track of such types. */
    if (std::is_trivially_destructible_v<T>) {
      return ptr;
    }

    this->add(
        ptr,
        [](void *data) {
          T *typed_data = reinterpret_cast<T *>(data);
          typed_data->~T();
        },
        name);
    return ptr;
  }

  /**
   * Pass ownership of some resource to the ResourceScope. The given free function will be
   * called when the collector is destructed.
   */
  void add(void *userdata, void (*free)(void *), const char *name)
  {
    ResourceData data;
    data.debug_name = name;
    data.data = userdata;
    data.free = free;
    m_resources.append(data);
  }

  /**
   * Construct an object with the same value in the ResourceScope and return a reference to the
   * new value.
   */
  template<typename T> T &add_value(T &&value, const char *name)
  {
    return this->construct<T>(name, std::forward<T>(value));
  }

  /**
   * Returns a reference to a linear allocator that is owned by the ResourcesCollector. Memory
   * allocated through this allocator will be freed when the collector is destructed.
   */
  LinearAllocator<> &linear_allocator()
  {
    return m_allocator;
  }

  /**
   * Utility method to construct an instance of type T that will be owned by the ResourceScope.
   */
  template<typename T, typename... Args> T &construct(const char *name, Args &&...args)
  {
    destruct_ptr<T> value_ptr = m_allocator.construct<T>(std::forward<Args>(args)...);
    T &value_ref = *value_ptr;
    this->add(std::move(value_ptr), name);
    return value_ref;
  }

  /**
   * Print the names of all the resources that are owned by this ResourceScope. This can be
   * useful for debugging.
   */
  void print(StringRef name) const
  {
    if (m_resources.size() == 0) {
      std::cout << "\"" << name << "\" has no resources.\n";
      return;
    }
    else {
      std::cout << "Resources for \"" << name << "\":\n";
      for (const ResourceData &data : m_resources) {
        std::cout << "  " << data.data << ": " << data.debug_name << '\n';
      }
    }
  }
};

}  // namespace blender