IfcVoxelData explicit attribute definition seems incorrect

[iegorychev]

ENTITY IfcIntegerVoxelData
 SUBTYPE OF (IfcVoxelData);
	Values : ARRAY [1:GridSize] OF IfcInteger;
	Unit : OPTIONAL IfcUnit;
 DERIVE
	GridSize : IfcInteger :=  SIZEOF(SELF\IfcProduct.Representation.Representations[1].Items[1]\IfcVoxelGrid.Voxels);
END_ENTITY;

Usage of DERIVED attribute GridSize seems incorrect here as in the moment of IfcIntegerVoxelData creation GridSize is UNSET and ARRAY can not be instantiated in terms of EXPRESS, ISO 10303 p.11 says:
8.2.1 Array data type
NOTE 1 The bounds may be positive, negative or zero, but may not be indeterminate (?) (see 14.2).

That's why probably all previous IFC schemas use approach with kind of attribute "duplication", as

ENTITY IfcRationalBSplineCurveWithKnots
 SUBTYPE OF (IfcBSplineCurveWithKnots);
	WeightsData : LIST [2:?] OF IfcReal;
 DERIVE
	Weights : ARRAY [0:UpperIndexOnControlPoints] OF IfcReal := IfcListToArray(WeightsData,0,SELF\IfcBSplineCurve.UpperIndexOnControlPoints);

which looks more correct.

[SergejMuhic]

Yes, that is correct. I can confirm that is not only an issue in EXPRESS but also in other languages (obviously 😄 ).

Same problem popped up I tried to compile the toolkit.

As proposed in a separate conversation, let's look at the updated proposal which I would post here once we aligned. After that I will create a PR with the amended code.

[SergejMuhic]

My proposal would be the following:

• adding a data attribute (ValueData) from which the array is derived
• adding a function (IfcListToArrayOfSize) that fills the array

ENTITY IfcIntegerVoxelData
 SUBTYPE OF (IfcVoxelData);
	ValueData : LIST [1:?] OF IfcInteger;
	Unit : OPTIONAL IfcUnit;
 DERIVE
	GridSize : INTEGER := SIZEOF(SELF\IfcProduct.Representation.Representations[1].Items[1]\IfcVoxelGrid.Voxels);
	Values : ARRAY [1:GridSize] OF IfcInteger := IfcListToArrayOfSize(ValueData,1,SELF.GridSize,SELF\IfcProduct.Representation.Representations[1].Items[1]\IfcVoxelGrid.Voxels);
END_ENTITY;

FUNCTION IfcListToArrayOfSize
(Lis : LIST [0:?] OF GENERIC : T;
       Low, U : INTEGER;
	   Condition : LIST[Low:U] OF BOOLEAN) : ARRAY OF GENERIC : T;
   LOCAL
     LisIndex : INTEGER := 1;
     Res : ARRAY [Low:U] OF GENERIC : T;
   END_LOCAL;
      
   IF (SIZEOF(QUERY(item <* Condition | item)) <> SIZEOF(Lis)) THEN
     RETURN(?);
   END_IF;
	  
   Res := [Low : U];
   REPEAT i := 1 TO SIZEOF(Condition);
     IF (Condition[i]) THEN
       Res[i] := Lis[LisIndex];
	   LisIndex := LisIndex + 1;
	 END_IF;
   END_REPEAT;
   RETURN(Res);
END_FUNCTION;

[SergejMuhic]

@iegorychev this was completely reworked. If you want to have a look, there was a bunch of changes.