[xsd-users] Extracting

[Boris Kolpackov]

Hi,

SHAH, Sima, FM <Sima.SHAH at rbos.com> writes:

> I have an element defined as an xsi:type.  To extract it, I need to do a
> dynamic cast which throws if not the correct type.
>
> Is there a way I can work out what the type is or a visitor pattern to allow
> me to call a function given the correct type.

You can use dynamic_cast on a pointer instead of a reference. In that case
it will return 0 instead of throwing std::bad_cast. This is an effective
way to test if SomeType is-a SomeOtherType.

One extension that we are thinking about implementing is to allow you to
specify native C++ base classes for XML Schema types. For example, suppose
you have the following schema:

<complexType name="person" nativeBase="person_base">
  <sequence>
    <element name="name" type="xsd:string"/>
  </sequence>
</complexType>

<complexType name="superman">
  <complexContent>
    <extension base="person">
      <attribute name="can-fly" type="xsd:boolean" use="required"/>
    </extension>
  </complexContent>
</complexType>


And the person_base C++ class:

struct person_base
{
  ~person_base ();

  virtual
  ~print () = 0;
};

The generated code for the above schema fragment could look like this:

struct person: xml_schema::type, person_base
{
  string
  name () const;
};

struct superman: person
{
  bool
  can_fly () const;
};


Now, since print is pure virtual, you will need to implement those types:

struct person_impl: person
{
  virtual
  ~print ()
  {
    cout << "person " << name ();
  }
};

struct superman_impl: superman
{
  virtual
  ~print ()
  {
    cout << (can_fly () ? "flying superman " : "superman ") << name ();
  }
};

There would also be a factory mechanism that will allow one to instruct
parsing code to use person_impl and superman_impl for person and
superman. Is anybody interested in such a feature?


hth,
-boris
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