writestruct() fails to write empty child element to XML file

I need to write a empty, nested element within an XML file. An XML example of what I'd like to achieve is:

<?xml version="1.0" encoding="UTF-8"?>
<Problem>
    <elementA>14</elementA>
    <elementB>
        <child/>
    </elementB>
</Problem>

The nested element in this case is called "child". I expected the following MATLAB code to produce this result:

s.elementA = 14;
s.elementB.child = [];
writestruct(s,'test.xml','StructNodeName','Problem')

However, this generates the following XML, whereupon the "child" element is entirely missing:

<?xml version="1.0" encoding="UTF-8"?>
<Problem>
    <elementA>14</elementA>
    <elementB/>
</Problem>

How can I produce a result with an empty, nested element?

Answer 1

[Voss]

s.elementA = 14;
s.elementB.child = struct();
writestruct(s,'test.xml','StructNodeName','Problem')

type('test.xml')

<?xml version="1.0" encoding="UTF-8"?>
<Problem>
    <elementA>14</elementA>
    <elementB>
        <child/>
    </elementB>
</Problem>

[Matt]

Thank you, this is cleaner than the other solution I posted.

[Voss]

You're welcome!

Answer 2

[Matt]

Well, not even 15 minutes and I found the solution myself. I find it strange that the following must be done to achieve a nested, empty element of an XML, but here is the approach:

s.elementA = 14;
s.elementB.child.dummy = [];
writestruct(s,'test.xml','StructNodeName','Problem')

Including the dummy field informs MATLAB that child is a struct (and not an empty double, as specified in my question). That child is now explictly a struct causes the creation of an empty "child" element in the XML.

<?xml version="1.0" encoding="UTF-8"?>
<Problem>
    <elementA>14</elementA>
    <elementB>
        <child/>
    </elementB>
</Problem>

I hope someone else finds this answer useful.