#### 5.1.3.4 IfcObjectDefinition

| Définition d'objet
| Objektdefinition

An IfcObjectDefinition is the generalization of any semantically treated thing or process, either being a type or an occurrences. Object defintions can be named, using the inherited Name attribute, which should be a user recognizable label for the object occurrance. Further explanations to the object can be given using the inherited Description attribute. A context is a specific kind of object definition as it provides the project or library context in which object types and object occurrences are defined.

Objects are independent pieces of information that might contain or reference other pieces of information. There are four essential kinds of relationships in which object definitons (by their instantiable subtypes) can be involved:

- Assignment of other objects - an assignment relationship (IfcRelAssigns) that refers to other types of objects and creates a bi-directional association. The semantic of the assignment is established at the level of the subtypes of the general IfcRelAssigns relationship. There is no dependency implied a priori by the assignment.
- Association to external resources - an association relationship (IfcRelAssociates) that refers to external sources of information (most notably a classification or document) and creates a uni-directional association. There is no dependency implied by the association.
- Aggregation of other objects - an aggregation relationship (IfcRelAggregates) that establishes an unordered, spatial whole/part relation and creates a bi-directional relation. There is an implied dependency established.
- Nesting of other objects - a nesting relationship (IfcRelNests) that establishes an ordered, non-spatial whole/part relation and creates a bi-directional relation. There is an implied dependency established.
- Declaration within a context - a relationship (IfcRelDeclares) of the uppermost object definition within the object definition tree (e.g. the summary object within an object nesting tree) to the context (a project or project library). It applies the units, representation context and other context information to this object definition and all dependent ones.
  - EXCEPTION The link between the uppermost object in the spatial structure tree, that is IfcSite or ifcBuilding, and the context provided by IfcProject is created using the IfcRelAggregates relationship. See IfcProject for more information.

> HISTORY New abstract entity in IFC2x3.

> IFC4 CHANGE The new subtype IfcContext and the relationship to context HasContext has been added . The decomposition relationship is split into ordered nesting (Nests, IsNestedBy) and un-ordered aggregating (Decomposes, IsDecomposedBy).

XSD Specification:

<xs:element name="IfcObjectDefinition" type="ifc:IfcObjectDefinition" abstract="true" substitutionGroup="ifc:IfcRoot" nillable="true"/>
<xs:complexType name="IfcObjectDefinition" abstract="true">
<xs:complexContent>
<xs:extension base="ifc:IfcRoot"/>
</xs:complexContent>
</xs:complexType>

EXPRESS Specification:

ENTITY IfcObjectDefinition

| ABSTRACT SUPERTYPE OF(ONEOF(IfcContext, IfcObject, IfcTypeObject))

| SUBTYPE OF (IfcRoot);

| INVERSE

HasAssignments | : | SET OF IfcRelAssigns FOR RelatedObjects;
HasContext | : | SET [0:1] OF IfcRelDeclares FOR RelatedDefinitions;
IsDecomposedBy | : | SET OF IfcRelAggregates FOR RelatingObject;
Decomposes | : | SET [0:1] OF IfcRelAggregates FOR RelatedObjects;
HasAssociations | : | SET OF IfcRelAssociates FOR RelatedObjects;

END_ENTITY;

Attribute Definitions:

HasAssignments | : | Reference to the relationship objects, that assign (by an association relationship) other subtypes of IfcObject to this object instance. Examples are the association to products, processes, controls, resources or groups.
HasContext | : | References to the context providing context information such as project unit or representation context. It should only be asserted for the uppermost non-spatial object. IFC4 CHANGE The inverse attribute datatype has been added.
IsDecomposedBy | : | References to the decomposition relationship being an aggregation. It determines that this object definition is whole within an unordered whole/part decomposition relationship. An object definitions can be aggregated by several other objects (occurrences or parts). IFC4 CHANGE The inverse attribute datatype has been changed from the supertype IfcRelDecomposes to subtype IfcRelAggregates.
Decomposes | : | References to the decomposition relationship being an aggregation. It determines that this object definition is a part within an unordered whole/part decomposition relationship. An object definitions can only be part of a single decomposition (to allow hierarchical strutures only). IFC4 CHANGE The inverse attribute datatype has been changed from the supertype IfcRelDecomposes to subtype IfcRelAggregates.
HasAssociations | : | Reference to the relationship objects, that associates external references or other resource definitions to the object.. Examples are the association to library, documentation or classification.

Inheritance Graph:

ENTITY IfcObjectDefinition

| ENTITY IfcRoot

GlobalId | : | IfcGloballyUniqueId;
OwnerHistory | : | OPTIONAL IfcOwnerHistory;
Name | : | OPTIONAL IfcLabel;
Description | : | OPTIONAL IfcText;

| ENTITY IfcObjectDefinition

| INVERSE

HasAssignments | : | SET OF IfcRelAssigns FOR RelatedObjects;
HasContext | : | SET [0:1] OF IfcRelDeclares FOR RelatedDefinitions;
IsDecomposedBy | : | SET OF IfcRelAggregates FOR RelatingObject;
Decomposes | : | SET [0:1] OF IfcRelAggregates FOR RelatedObjects;
HasAssociations | : | SET OF IfcRelAssociates FOR RelatedObjects;

END_ENTITY;

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