Class TIfcDerivedUnitElement

Unit

Declaration

type TIfcDerivedUnitElement = class(TIfcPersistent)

Description

A derived unit element is one of the unit quantities which makes up a derived unit.

Spec: https://standards.buildingsmart.org/IFC/RELEASE/IFC4_3/HTML/lexical/IfcDerivedUnitElement.htm

Source: scene/load/ifc/castleifc_ifc_standard_types.inc (line 4468).

Hierarchy

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Overview

Methods

Protected function WantsGlobalId: Boolean; virtual;
Protected class function IfcClassName: String; virtual;

Properties

Published property GlobalId: String read FGlobalId write FGlobalId;
Published property Unit: TIfcNamedUnit read FUnit write FUnit;
Published property Exponent: Integer read FExponent write FExponent;

Description

Methods

Protected function WantsGlobalId: Boolean; virtual;

This item is declared in ancestor TIfcPersistent.

If True, instances of this class will have GlobalId auto-assigned at saving, and they will be saved as part of the "data" list in IFC JSON files.

Strictly following the spec, this should be only defined for TIfcRoot, because only IfcRoot has GlobalId. In reality, we need GlobalId at TIfcPersistent to load some files, and we sometimes need WantsGlobalId = True to save some files.

Source: scene/load/ifc/castleifc_ifc_types.inc (line 52).

Protected class function IfcClassName: String; virtual;

This item is declared in ancestor TIfcPersistent.

Name for the IFC standard, used in IFC files. By default, this is just ClassName with prefix 'T' removed.

Source: scene/load/ifc/castleifc_ifc_types.inc (line 56).

Properties

Published property GlobalId: String read FGlobalId write FGlobalId;

This item is declared in ancestor TIfcPersistent.

Globally unique identifier for the object instance.

Note: Not declared as TGUID, because TGUID cannot be published, but also we don't really need it: for our purposes, this identifier is just an arbitrary string.

Note: IFC 4.3 specification puts this only at IfcRoot, not at every IFC class. But testcase https://github.com/buildingsmart-community/Community-Sample-Test-Files/blob/main/IFC%204.0.2.1%20(IFC%204)/ISO%20Spec%20archive/wall-with-opening-and-window.json (from IFC 4.0) shows this can be present at any IFC class, in particular TIfcGeometricRepresentationContext.

In fact, TIfcGeometricRepresentationContext needs to have GlobalId to be successfully saved (avoid recursively trying to serialize it) in some cases. We indicate such classes with WantsGlobalId.

IFC JSON documentation says this, which seems to (unfortunately it is not unambiguous enough...) allow this at any IFC class: "Whereas the globalId attribute is only available to those elements that are descendent of the IfcRoot entity in the EXPRESS schema, we encourage a broader use of this globalId, to enable referencing between objects in a JSON file when useful.".

Source: scene/load/ifc/castleifc_ifc_types.inc (line 79).

Published property Unit: TIfcNamedUnit read FUnit write FUnit;

The fixed quantity which is used as the mathematical factor.

Source: scene/load/ifc/castleifc_ifc_standard_types.inc (line 4474).

Published property Exponent: Integer read FExponent write FExponent;

The power that is applied to the unit attribute.

Source: scene/load/ifc/castleifc_ifc_standard_types.inc (line 4477).


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