I've been building a parser for STEP-formatted data (specifically the ISO 10303-21 standard), but I've run into a roadblock revolving around a single character - '$'.
A quick Google search reveals that in STEP, this character denotes an 'unset' value, I interpreted this as an uninitialized value, however I don't know exactly what I should do with it.
For example, take the following definitions:
#111=AXIS2_PLACEMENT_3D('Circle Axis2P3D',#109,#110,$) ;
#109=CARTESIAN_POINT('Axis2P3D Location',(104.14,0.,0.)) ;
#110=DIRECTION('Axis2P3D Direction',(1.,-0.,0.)) ;
To me I cannot see how this would even work, as the reference direction is uninitialized, and therefore an x-axis cannot be derived, meaning that anything using this Axis2Placement would also have undefined data.
Normally when I reach this point, I would just define some sort of default data for the given data-type (Vertices (0,0,0), Directions(1,0,0), etc.), however this doesn't seem to work, because there's the chance that my default direction would cause conflicts with the supplied data.
I have Googled what to do in this scenario, only to come up with nothing.
I also have a PDF for a very similar STEP format (ISO-10303-42), but it too doesn't mention any sort of default data, or provide any more insight in to how this works.
So, to explicitly state my question: what do I do with uninitialized data in STEP (ISO 10303-21) data?
You need to be able to represent 'unset' as a distinct value. It doesn't mean the same thing as an uninitialized value or a default value. For example you might represent AXIS2_PLACEMENT_3D as an object with data members that are pointers to point to CARTESIAN_POINT and DIRECTION, and the $ means that that pointer will be null in your representation.
Dealing with null values will depend on the context. It may be some kind of error if the data is really necessary. Or it may be that the data isn't necessary, such as if you don't need the axis to be oriented, and a point and direction are sufficient to represent the data.