NXDL: Common base class for NXroot and NXobject #1672

Problem

NXroot is the only base class that does not extend NXobject. This is on purpose: NXobject declares items the root must not have:

• NeXus groups: NXcollection, NXdata, NXlog, NXnote, NXparameters, GROUPNAME_log;
• NeXus fields: FIELDNAME_set, FIELDNAME_errors, FIELDNAME_weights, FIELDNAME_mask, identifierNAME.

But NXobject is also where NeXus declares what applies to every NeXus group and field (#1668, #1669). Since NXroot does not extend it, none of that reaches NXroot:

• @NX_class is declared twice: required in NXobject and again in NXroot.
• @default is declared twice: once in NXobject and once in NXroot, each with its own documentation.
• The fieldAttributes of NXobject (@target, @units, @long_name, …) are copied into NXroot; a test keeps both copies the same.
• Anything added to NXobject for every NeXus group and field later on has to be copied to NXroot by hand, or the root silently misses it.

Proposal

Add a new base class (name to be decided, e.g. NXnode) that holds only what applies to every NeXus group and field, the root included:

• NeXus attributes of every NeXus group: @NX_class (required, valueFrom="class"), @default;
• fieldAttributes: @target, @units, @long_name, …

Then:

• NXobject extends it, adds the NeXus groups and fields listed above, and adds @target for groups;
• NXroot extends it and adds the NeXus attributes of the root (file_name, file_time, NeXus_release, …).

Each shared item is then declared once, and NXroot picks up future additions automatically.

@target for groups stays in NXobject: the root cannot be a content link.

Questions for the NIAC

• The name of the new class.
• Should @default move to the common class, keeping the NXroot documentation (which says it points to an NXentry) as a refinement?
• Does any tool assume that NXobject is the top of every inheritance chain, or that NXroot has no extends?

Related

• Express attributes of any field or group in XSD, applied to XML, not HDF5 #1668, Define NeXus attributes of all fields and groups, and make content links independent of storage #1669: attributes of all fields and groups in NXobject
• add target attribute to NXobject using new field super-type #1558: @target in NXobject