PyOwnershipManagement.h

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// SPDX-FileCopyrightText: Deutsches Elektronen-Synchrotron DESY, MSK, ChimeraTK Project <chimeratk-support@desy.de>
// SPDX-License-Identifier: LGPL-3.0-or-later
#pragma once
 
#include <list>
#include <memory>
 
namespace ChimeraTK {
 
  /********************************************************************************************************************/
 
  class PyOwnedObject {
   public:
    virtual ~PyOwnedObject() = default;
  };
 
  /********************************************************************************************************************/
 
  class PyOwningObject : public PyOwnedObject {
   public:
    template<class Child, typename... Args>
    Child* make_child(Args... args) {
      auto ptr = std::make_unique<Child>(args...);
      Child* rv = ptr.get();
 
      _children.emplace_back(std::move(ptr));
      return rv;
    }
 
   private:
    // Note about ownership and deinit problem.
    // When naively mapping VariableGroup to Python, we run into a problem at the deinitialization phase:
    // The accessors held by the VariableGroup reference back to their owner (the VariableGroup) and
    // in their C++ destructor, they actually call functions of the owner.
    // On the other hand, if a container in Python is destroyed, Python first releases the container
    // and then the elements. This is different from a C++ class holding the elements as class members.
    // To solve the issue, we decided to take away ownership handling from Python and explicitly take
    // care on the C++ side. This requires some ownership lists (below) and handing out accessors
    // in pybind11 with return_value_policy::reference.
    // Note also, when mapping VariableGroup as PyVariableGroup, and user code subclasses PyVariableGroup,
    // attributes of PyVariableGroup automatically get destroyed before ~PyVariableGroup executes.
    // But this is not enough to cover case of non-subclassed PyVariabledGroup with dynamic attributes!
    // So we still need the general solution described above.
    std::list<std::unique_ptr<PyOwnedObject>> _children;
  };
 
  /********************************************************************************************************************/
} // namespace ChimeraTK