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Question regarding the * symbol in an ifc-step file

1. M by Martin156131 on 9 Sep 2022, edited 10 Sep 2022 #

   So for example here there is * as the first attribute of IfcSIUnit:

   

   The first attribute as in the documentation should be a IfcDimensionalExponents

   

   However instead of #28= IFCSIUNIT(#30,.LENGTHUNIT.,$,.METRE.); it is #28= IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);

   So in the wiki about the step file it's reads:

   Explicit attributes which got re-declared as derived in a subtype are encoded as "*" in the position of the supertype attribute.

   However i don't unterstand the use of * at all.

   The ifc file is from here https://github.com/buildingSMART/Sample-Test-Files/blob/master/IFC%204.0/BuildingSMARTSpec/air-terminal-element.ifc

2. T by theoryshaw on 9 Sep 2022 # +1 votes

   This was discussed a little here:

   https://community.osarch.org/discussion/comment/12231/#Comment_12231

   Generically, what would this be called in computer science?

3. M by Moult on 10 Sep 2022 #

   I've called them "derived attributes", because their value is automatically derived by the rules in the schema.

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