.. role:: blue
   :class: blue

MbRefItem Reference Counter
===========================
.. _MbRefItem_Reference_Counter_:

.. kernel version: 118060

The :blue:`MbRefItem` class is declared in the *reference_item.h* file.

The :blue:`MbRefItem` class is characterized by the number of its owners *useCount* and represents a counter of objects that own the given object.

The geometric objects of the C3D kernel are divided into three groups: two-dimensional geometric objects, three-dimensional geometric objects and topological objects. Geometric objects of these three groups are derived from the :blue:`MbRefItem` and :blue:`TapeBase` classes:

.. figure:: /MODELER/GEOMETRICAL_OBJECTS/pictures/O.2.1.1.png
   :width: 12cm
   :align: center

The :blue:`TapeBase` class provides stream writing and reading for its child class.

The following objects are derived from the :blue:`MbRefItem` and :blue:`TapeBase` classes: 

:ref:`MbSpaceItem <MbSpaceItem_Three-Dimensional_Geometrical_Object_>` is the base abstract class of three-dimensional geometric objects;

:ref:`MbTopItem <MbTopItem_Topological_Object_>` is the base abstract class of topological objects;

:ref:`MbPlaneItem <MbPlaneItem_Two-Dimensional_Geometrical_Object_>` is the base abstract class of two-dimensional geometric objects;

:ref:`MbFunction <MbFunction_Function_>` is the base abstract class of scalar functions.

The reference counter ensures correct work of classes and methods that contain pointers to geometric objects. If a class stores a pointer to a geometric object in its data, then in its constructor it must increase the reference counter of the geometric object by one using the :blue:`AddRef`\() method, and in its destructor it must call the :blue:`Release`\() method of the geometric object, which decreases the reference counter of the geometric object by one and, when the reference counter reaches zero, deletes the geometric object.

The :blue:`DecRef`\() method decreases the reference counter of the geometric object by one. It is intended for internal use in the implementation where a decrease of the counter without ownership release is explicitly required.

The :blue:`MbRefItem` class is processed by the duplication registrar :blue:`MbRegDuplicate` and by the transformation registrar :blue:`MbRegTransform`.

The :blue:`RefType`\() method returns the registration type of the object that uses the reference counter.