[xsd-users] Re: cxx-tree --variant-type= ?

[Boris Kolpackov]

Hi Mario,

[CC'ed xsd-users].

Mario Lang <mlang at delysid.org> writes:

> For my MusicXML library, I now rewrote my two --ordered-type(s) with
> boost::variant as a --custom-type.  I find this much more convenient to
> use on the client side, especially since container manipulation (insert,
> push_back, push_front, etc.) functions do not have to be manually implemented.  As
> an added benefit, I get a compile-time checked visitor interface.  The
> transformation was quite mechanical though:
> 
> 1. Remove all direct sequence accessors.
> 2. Redefine content_oder_ and its accessors as variant_sequence, a
> std::vector<boost::variant<...>>.
> 3. Write a serialization_visitor to handle serialization from a
> container of variants.  Something like this:
> 
> class serialization_visitor : public boost::static_visitor<void> {
>   ::xercesc::DOMElement &e;
> 
> public:
>   serialization_visitor(::xercesc::DOMElement &e) : e(e) {}
> 
>   void operator()(const ::musicxml::note &x) const {
>     ::xsd::cxx::xml::dom::create_element("note", e) << x;
>   }
>   void operator()(const ::musicxml::backup &x) const {
>     ::xsd::cxx::xml::dom::create_element("backup", e) << x;
>   }
>   void operator()(const ::musicxml::forward &x) const {
>     ::xsd::cxx::xml::dom::create_element("forward", e) << x;
>   }
>   //...
> };
> 
> 4. Rewrite the parsing code to *not* use element_traits::create,
> instead, push_back directly to variant_.
> 
> This works WONDERFUL.  I really love it.  I wonder, have you ever
> considered to implement this as an alternative for --ordered-type?
> Maybe --variant-type= ?  --variant-type=class[/accessor] maybe?
> So the user could override the name of the variant() accessor.

No, I haven't considered it. To do that would require quite a bit
more thinking because while you only needed to handle this particular
case, if we were to implement this in XSD, it would have to work with
all reasonable use-cases (wildcards, mixed content, polymorphic
types are the ones that came to mind; there are most likely others).

At the same time it should be very easy to implement a class
template (using variadic template arguments, if using C++11) that
would allow you to achieve pretty much what you want:

--custom-type=foo=ordered_as_variant<foo_base, bar, baz, fox>/foo_base

The only minor inconvenience is that you have to specify the
possible element types (bar, baz, and fox in this case) explicitly.
But I think this is a small price to pay.

Boris