8.3.3.4 IfcReference

8.3.3.4.1 Semantic definition

This entity is used to refer to a value of an attribute on an instance. It may refer to the value of a scalar attribute or a value within a collection-based attribute. Referenced attributes may be direct values, object references, collections, inverse object references, and inverse collections. References may be chained to form a path of object-attribute references.

Figure 8.3.3.4.A — An example of chained references to refer to a core layer thickness

8.3.3.4.2 Entity inheritance

- IfcReference

8.3.3.4.3 Attributes

#	Attribute	Type	Description
IfcReference (5)
1	TypeIdentifier	OPTIONAL IfcIdentifier	Optional identifier of the entity or type such as 'IfcMaterialLayerSet'. For entity, type, or select-based references within a collection, this resolves the reference to such type. If omitted, the type is assumed to be the same as the declared referencing attribute. EXAMPLE IfcRelAssociatesMaterial.RelatingMaterial may be resolved to IfcMaterialLayerSet.
2	AttributeIdentifier	OPTIONAL IfcIdentifier	Optionally identifies a direct or inverse attribute within an entity such as 'MaterialLayers'. If TypeIdentifier is specified and refers to an entity, the attribute must exist within the referenced entity. A null value indicates a reference to the type or entity itself, such as for indicating that the type of a value must match a specified constraint.
3	InstanceName	OPTIONAL IfcLabel	Optionally identifies an instance within a collection according to name. If the instance has an attribute called 'Name', such attribute is used for comparison; otherwise the first STRING-based attribute of the entity is used. EXAMPLE IfcRoot-based entities such as IfcPropertySet use the Name attribute; IfcRepresentation entities use the RepresentationIdentifier attribute.
4	ListPositions	OPTIONAL LIST [1:?] OF IfcInteger	Optionally identifies an instance within a collection according to position starting at 1. For referencing single-level collections, this attribute contains a single member; for referencing multi-level collections, then this LIST attribute contains multiple members starting from the outer-most index.
5	InnerReference	OPTIONAL IfcReference	Optional reference to an inner value for ENTITY, SELECT, SET, or LIST attributes. A path may be formed by linking IfcReference instances together.

Table 8.3.3.4.B

8.3.3.4.4 Formal representation

ENTITY IfcReference;
	TypeIdentifier : OPTIONAL IfcIdentifier;
	AttributeIdentifier : OPTIONAL IfcIdentifier;
	InstanceName : OPTIONAL IfcLabel;
	ListPositions : OPTIONAL LIST [1:?] OF IfcInteger;
	InnerReference : OPTIONAL IfcReference;
END_ENTITY;