Re: [xml] Making sequence entries option in .xsd files.

[Pete Cordell <petexmldev codalogic com>]

On 08/06/2016 09:40, Steve Underwood wrote:
> On 06/06/2016 07:52 PM, Steve Underwood wrote:
> > Hi,
> >
> > In the .xsd file for the DLMS Smart Grid protocol I find the following
> > section:
> >
> > <xsd:complexTypename="InitiateRequest">
> > <xsd:sequence>
> > <xsd:elementname="dedicated-key"minOccurs="0"type="xsd:hexBinary"/>
> > <xsd:elementname="response-allowed"default="true"type="xsd:boolean"/>
> > <xsd:elementname="proposed-quality-of-service"minOccurs="0"type="Integer8"/>
> >
> > <xsd:elementname="proposed-dlms-version-number"type="Unsigned8"/>
> > <xsd:elementname="proposed-conformance"type="Conformance"/>
> > <xsd:elementname="client-max-receive-pdu-size"type="Unsigned16"/>
> > </xsd:sequence>
> > </xsd:complexType>
> >
> > By the protocol definition, the first 3 items in that sequence should
> > be optional. The minOccurs="0" elements cause items 1 and 3 to be
> > correctly treated as optional. However, the parser in libxml2 2.9.3
> > treats the second item as a required element. Shouldn't the default
> > setting act in a similar way to a minOccurs="0" element, in making the
> > item optional? Is it always necessary to have minOccurs="0" present
> > when default="something" is present, in order to actually implement
> > the default?
> >
> > Regards,
> > Steve
> I have been reading the XSD spec again, and it seems to me the line
> <xsd:elementname="response-allowed"default="true"type="xsd:boolean"/>
> in the above XSD definition needs minOccurs="0" adding to it to make the
> element optional. I can't find anything in the spec that says
> default="xx" has any effect other than to set a default value.
>
> Can anyone confirm or refute this?

You're probably better off asking general XSD questions on the 
xmlschema-dev w3 org mailing list.

But since I'm here, yes, I believe you need minOccurs="0".  If I 
remember correctly, for reasons best known to itself, the XSD spec says 
if you have default="true" on an element (attributes are treated 
differently) then it means if your XML instance includes an empty 
element (i.e. <response-allowed/> or 
<response-allowed></response-allowed>), then the value returned by your 
processor should be "true", not the empty string.

Attributes behave more as you'd expect, but probably still need to be 
marked 'optional'.

Hope that helps,

Pete Cordell
Codalogic Ltd
Read & write XML in C++, http://www.xml2cpp.com