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.. _classes_section_internals:
CLASSES Section
===============
The CLASSES section contains CLASS definitions which are only important for Autodesk products, some DXF entities require
a class definition or AutoCAD will not open the DXF file.
The CLASSES sections was introduced with DXF AC1015 (AutoCAD Release R13).
.. seealso::
DXF Reference: `About the DXF CLASSES Section`_
Documentation of `ezdxf` :class:`~ezdxf.sections.classes.ClassesSection` class.
The CLASSES section in DXF files holds the information for application-defined classes whose instances appear in
the BLOCKS, ENTITIES, and OBJECTS sections of the database. It is assumed that a class definition is permanently
fixed in the class hierarchy. All fields are required.
**Update 2019-03-03:**
Class names are not unique, Autodesk Architectural Desktop 2007 uses the same name, but with different CPP class names
in the CLASS section, so storing classes in a dictionary by name as key caused loss of class entries in ezdxf, using
a tuple of (name, cpp_class_name) as storage key solved the problem.
CLASS Entities
--------------
.. seealso::
DXF Reference: `Group Codes for the CLASS entity`_
CLASS entities have no handle and therefore ezdxf does not store the CLASS entity in the drawing entities database!
.. code-block:: none
0
SECTION
2 <<< begin CLASSES section
CLASSES
0 <<< first CLASS entity
CLASS
1 <<< class DXF entity name; THIS ENTRY IS MAYBE NOT UNIQUE
ACDBDICTIONARYWDFLT
2 <<< C++ class name; always unique
AcDbDictionaryWithDefault
3 <<< application name
ObjectDBX Classes
90 <<< proxy capabilities flags
0
91 <<< instance counter for custom class, since DXF version AC1018 (R2004)
0 <<< no problem if the counter is wrong, AutoCAD doesn't care about
280 <<< was-a-proxy flag. Set to 1 if class was not loaded when this DXF file was created, and 0 otherwise
0
281 <<< is-an-entity flag. Set to 1 if class reside in the BLOCKS or ENTITIES section. If 0, instances may appear only in the OBJECTS section
0
0 <<< second CLASS entity
CLASS
...
...
0 <<< end of CLASSES section
ENDSEC
.. _About the DXF CLASSES Section: http://help.autodesk.com/view/OARX/2018/ENU/?guid=GUID-6160F1F1-2805-4C69-8077-CA1AEB6B1005
.. _Group Codes for the CLASS entity: http://help.autodesk.com/view/OARX/2018/ENU/?guid=GUID-DBD5351C-E408-4CED-9336-3BD489179EF5
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