itkSingleton.h

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/*=========================================================================
 *
 *  Copyright NumFOCUS
 *
 *  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.txt
 *
 *  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.
 *
 *=========================================================================*/
#ifndef itkSingleton_h
#define itkSingleton_h
 
#include "itkMacro.h"
#include "itkSingletonMacro.h"
#include <map>
#include <functional>
 
#ifndef ITK_FUTURE_LEGACY_REMOVE
template <typename T>
[[deprecated("Preferably use the C++ `[[maybe_unused]]` attribute instead!")]] inline void
Unused(const T &){};
#endif
 
namespace itk
{
class ITKCommon_EXPORT SingletonIndex
{
public:
  using Self = SingletonIndex;
 
#ifndef ITK_LEGACY_REMOVE
  using SingletonData [[deprecated("The internal representation of the singleton data is private, and may not "
                                   "correspond with SingletonData anymore.")]] =
    std::map<std::string, std::tuple<void *, std::function<void(void *)>, std::function<void()>>>;
#endif
 
  // obtain a global registered in the singleton index under the
  // globalName, if unknown then nullptr will be returned.
  template <typename T>
  T *
  GetGlobalInstance(const char * globalName)
  {
    return static_cast<T *>(this->GetGlobalInstancePrivate(globalName));
  }
 
 
  // It is assumed that the global will remain valid until the start
  // of globals being destroyed.
  template <typename T>
  void
  SetGlobalInstance(const char * globalName, T * global, std::function<void()> deleteFunc)
  {
    this->SetGlobalInstancePrivate(globalName, GlobalObject{ global, std::move(deleteFunc) });
  }
 
#ifndef ITK_FUTURE_LEGACY_REMOVE
  template <typename T>
  [[deprecated("Prefer calling the SetGlobalInstance(globalName, global, deleteFunc) overload (without the unused func "
               "parameter)!")]] bool
  SetGlobalInstance(const char *                globalName,
                    T *                         global,
                    std::function<void(void *)> itkNotUsed(func),
                    std::function<void()>       deleteFunc)
  {
    this->SetGlobalInstance(globalName, global, std::move(deleteFunc));
    // Just returns true for backward compatibility (legacy only).
    return true;
  }
#endif
 
  static Self *
  GetInstance();
  static void
  SetInstance(Self * instance);
  ~SingletonIndex();
private:
  // Internal struct to store the instance pointer and the delete function object of a global object.
  struct GlobalObject
  {
    void *                Instance{};
    std::function<void()> DeleteFunc{};
  };
 
  // may return nullptr if string is not registered already
  //
  // access something like a std::map<std::string, void *> or
  // registered globals, it may be possible to restrict the held
  // classes to be derived from itk::LightObject, so dynamic cast can
  // work, and could use some type of Holder<T> class for intrinsic types
  void *
  GetGlobalInstancePrivate(const char * globalName);
 
  // global is added or set to the singleton index under globalName
  void
  SetGlobalInstancePrivate(const char * globalName, GlobalObject globalObject);
 
  std::map<std::string, GlobalObject> m_GlobalObjects;
  static Self *                       m_Instance;
  //  static SingletonIndexPrivate * m_GlobalSingleton;
};
 
 
// A wrapper for a global variable registered in the singleton index.
template <typename T>
T *
Singleton(const char * globalName, std::function<void()> deleteFunc)
{
  [[maybe_unused]] static SingletonIndex * singletonIndex = SingletonIndex::GetInstance();
  T *                                      instance = SingletonIndex::GetInstance()->GetGlobalInstance<T>(globalName);
  if (instance == nullptr)
  {
    instance = new T;
    SingletonIndex::GetInstance()->SetGlobalInstance<T>(globalName, instance, std::move(deleteFunc));
  }
  return instance;
}
 
 
#ifndef ITK_FUTURE_LEGACY_REMOVE
template <typename T>
[[deprecated("Prefer calling the Singleton(globalName, deleteFunc) overload (without the unused func parameter)!")]] T *
Singleton(const char * globalName, std::function<void(void *)> itkNotUsed(func), std::function<void()> deleteFunc)
{
  return Singleton<T>(globalName, std::move(deleteFunc));
}
#endif
 
} // end namespace itk
 
#endif