IfcPropertySingleValue
实体定义
具有单个值的属性IfcPropertySingleValue定义了一个指定了单个（数字或描述性）值的属性对象。它定义了一个属性-单值组合，其中提供了属性名称、可选描述和可选的NominalValue以及度量值类型。此外，可以通过指定单位来覆盖项目单位上下文中指定的默认单位。
单位由单位属性处理，各种单值属性示例见表：
1、如果未给出Unit属性，则该单位已由IfcMeasureValue或IfcDerivedMeasureValue类型暗示。可以在项目级别全局定义的IfcUnitAssignment（IfcProject.UnitsContext）中找到关联的单元。
2、如果给定了单位属性，则单位属性指定的单位将覆盖全局指定的单位。
IFC1中增加的新实体
IFC2x更改实体已从IfcSimpleProperty重命名。
IFC2x3更改属性NominalValue已被设置为可选，并与基于文件的交换具有向上兼容性。
Attribute inheritance
| # | Attribute | Type | Cardinality | Description | C |
|---|---|---|---|---|---|
| IfcPropertyAbstraction | |||||
| HasExternalReferences | IfcExternalReferenceRelationship @RelatedResourceObjects |
S[0:?] | Reference to an external reference, e.g. library, classification, or document information, that is associated to the property definition. | X | |
| IfcProperty | |||||
| 1 | Name | IfcIdentifier | [1:1] | Name for this property. This label is the significant name string that defines the semantic meaning for the property. | X |
| 2 | Description | IfcText | [0:1] | Informative text to explain the property. | X |
| PartOfPset | IfcPropertySet @HasProperties |
S[0:?] | Reference to the IfcPropertySet by which the IfcProperty is referenced. | X | |
| PropertyForDependance | IfcPropertyDependencyRelationship @DependingProperty |
S[0:?] | The property on whose value that of another property depends. | X | |
| PropertyDependsOn | IfcPropertyDependencyRelationship @DependantProperty |
S[0:?] | The relating property on which the value of the property depends. | X | |
| PartOfComplex | IfcComplexProperty @HasProperties |
S[0:?] | Reference to the IfcComplexProperty in which the IfcProperty is contained. | X | |
| HasConstraints | IfcResourceConstraintRelationship @RelatedResourceObjects |
S[0:?] | User-defined constraints for the property. | X | |
| HasApprovals | IfcResourceApprovalRelationship @RelatedResourceObjects |
S[0:?] | User-defined approvals for the property. | X | |
| IfcSimpleProperty | |||||
| IfcPropertySingleValue | |||||
| 3 | NominalValue | IfcValue | [0:1] | Value and measure type of this property.
NOTE By virtue of the defined data type, that is selected from the SELECT IfcValue, the appropriate unit can be found within the IfcUnitAssignment, defined for the project if no value for the unit attribute is given. |
X |
| 4 | Unit | IfcUnit | [0:1] | Unit for the nominal value, if not given, the default value for the measure type (given by the TYPE of nominal value) is used as defined by the global unit assignment at IfcProject. | X |
EXPRESS Specification
ENTITY IfcPropertySingleValue
SUBTYPE OF (IfcSimpleProperty);
NominalValue : OPTIONAL IfcValue;
Unit : OPTIONAL IfcUnit;
END_ENTITY;
###############################