gSOAP with OPC XML-DA add attribute

Question

I'm trying to develop a soap client that will access data over the OPC XML-DA specification: Here

I've used the tools provided by gSOAP to generate the gSOAP header file from the OPC Foundations WSDL. (Relevant parts below)

I can't seem to get gSOAP to properly add an attribute to the tag. (See Output section below). Is there a builtin way to do this, or will the WSDL/gSOAP header need to be modified?

WSDL extract:

<s:complexType name="ItemValue">
<s:sequence>
<s:element minOccurs="0" maxOccurs="1" name="DiagnosticInfo" type="s:string" />
<s:element minOccurs="0" maxOccurs="1" name="Value" />
<s:element minOccurs="0" maxOccurs="1" name="Quality" type="s0:OPCQuality" />
</s:sequence>
<s:attribute name="ValueTypeQualifier" type="s:QName" />
<s:attribute name="ItemPath" type="s:string" />
<s:attribute name="ItemName" type="s:string" />
<s:attribute name="ClientItemHandle" type="s:string" />
<s:attribute name="Timestamp" type="s:dateTime" />
<s:attribute name="ResultID" type="s:QName" />
</s:complexType>

Generated gSOAP header

class ns1__ItemValue
{ public:
/// Element DiagnosticInfo of type xs:string.
    char*                                DiagnosticInfo                 0;  ///< Optional element.
/// Element 'Value' has no type or ref: assuming XML content.
    _XML                                 Value                          0;  ///< Optional element.
/// Element Quality of type "http://opcfoundation.org/webservices/XMLDA/1.0/":OPCQuality.
    ns1__OPCQuality*                     Quality                        0;  ///< Optional element.
/// Attribute ValueTypeQualifier of type xs:QName.
   @_QName                               ValueTypeQualifier             0;  ///< Optional attribute.
/// Attribute ItemPath of type xs:string.
   @char*                                ItemPath                       0;  ///< Optional attribute.
/// Attribute ItemName of type xs:string.
   @char*                                ItemName                       0;  ///< Optional attribute.
/// Attribute ClientItemHandle of type xs:string.
   @char*                                ClientItemHandle               0;  ///< Optional attribute.
/// Attribute Timestamp of type xs:dateTime.
   @time_t*                              Timestamp                      0;  ///< Optional attribute.
/// Attribute ResultID of type xs:QName.
   @_QName                               ResultID                       0;  ///< Optional attribute.
/// A handle to the soap struct that manages this instance (automatically set)
    struct soap                         *soap                          ;
};

Generated Code

class SOAP_CMAC ns1__ItemValue
{
public:
    char *DiagnosticInfo;   /* optional element of type xsd:string */
    char *Value;    /* optional element of type xsd:anyType */
    class ns1__OPCQuality *Quality; /* optional element of type ns1:OPCQuality */
    char *ValueTypeQualifier;   /* optional attribute */
    char *ItemPath; /* optional attribute */
    char *ItemName; /* optional attribute */
    char *ClientItemHandle; /* optional attribute */
    time_t *Timestamp;  /* optional attribute */
    char *ResultID; /* optional attribute */
    struct soap *soap;  /* transient */
public:
    virtual int soap_type() const { return 18; } /* = unique id SOAP_TYPE_ns1__ItemValue */
    virtual void soap_default(struct soap*);
    virtual void soap_serialize(struct soap*) const;
    virtual int soap_put(struct soap*, const char*, const char*) const;
    virtual int soap_out(struct soap*, const char*, int, const char*) const;
    virtual void *soap_get(struct soap*, const char*, const char*);
    virtual void *soap_in(struct soap*, const char*, const char*);
             ns1__ItemValue() : DiagnosticInfo(NULL), Value(NULL), Quality(NULL), ValueTypeQualifier(NULL), ItemPath(NULL), ItemName(NULL), ClientItemHandle(NULL), Timestamp(NULL), ResultID(NULL), soap(NULL) { }
    virtual ~ns1__ItemValue() { }
};

Output

<ns1:Items
    ClientItemHandle="Channel1.Device1"
    ItemName="Channel_1.Device_1.Tag_1"
    ValueTypeQualifier="xsd:unsignedInt">
    <Value
        xmlns="http://opcfoundation.org/webservices/XMLDA/1.0/">
        5
    </Value>
</ns1:Items>

Needed Output

<ns1:Items
    ClientItemHandle="Channel1.Device1"
    ItemName="Channel_1.Device_1.Tag_1"
    ValueTypeQualifier="xsd:unsignedInt">
    <Value
        xmlns="http://opcfoundation.org/webservices/XMLDA/1.0/"
        xsi:Type="xsd:unsignedInt">
        5
    </Value>
</ns1:Items>

Output means the XML that is generated and sent over the wire to the remote server.

Answer 1

@Tisho, thanks for your input.

It turns out that the WSDL provided by the OPC Foundation lacked to specify a type on the value element. Our solution was to add a type of s:anyType, this allowed us to use polymorphic types such as:
xsd_unsignedInt
xsd_string
xsd__anyType

Since all of the types inherited from xsd__anyType, and a virtual soap_type function was included, we can use any of the types, and store them in the value, then gSOAP magically uses the soap_type to figure out what type the var is.

Relevant portion of the OPC Foundation's WSDL Modified:

<s:complexType name="ItemValue">
  <s:sequence>
    <s:element minOccurs="0" maxOccurs="1" name="DiagnosticInfo" type="s:string"/>
    <s:element minOccurs="0" maxOccurs="1" name="Value" type="s:anyType"/> <!-- Here -->
    <s:element minOccurs="0" maxOccurs="1" name="Quality" type="s0:OPCQuality"/>
  </s:sequence>
  <s:attribute name="ValueTypeQualifier" type="s:QName"/>
  <s:attribute name="ItemPath" type="s:string"/>
  <s:attribute name="ItemName" type="s:string"/>
  <s:attribute name="ClientItemHandle" type="s:string"/>
  <s:attribute name="Timestamp" type="s:dateTime"/>
  <s:attribute name="ResultID" type="s:QName"/>
</s:complexType>

Here's the new gSOAP generated header

class ns1__ItemValue : public xsd__anyType
{ public:
/// Element DiagnosticInfo of type xs:string.
    char*                                DiagnosticInfo                 0;  ///< Optional element.
/// Element Value of type xs:anyType.
    xsd__anyType*                        Value                          0;  ///< Optional element.
/// Element Quality of type "http://opcfoundation.org/webservices/XMLDA/1.0/":OPCQuality.
    ns1__OPCQuality*                     Quality                        0;  ///< Optional element.
/// Attribute ValueTypeQualifier of type xs:QName.
   @_QName                               ValueTypeQualifier             0;  ///< Optional attribute.
/// Attribute ItemPath of type xs:string.
   @char*                                ItemPath                       0;  ///< Optional attribute.
/// Attribute ItemName of type xs:string.
   @char*                                ItemName                       0;  ///< Optional attribute.
/// Attribute ClientItemHandle of type xs:string.
   @char*                                ClientItemHandle               0;  ///< Optional attribute.
/// Attribute Timestamp of type xs:dateTime.
   @time_t*                              Timestamp                      0;  ///< Optional attribute.
/// Attribute ResultID of type xs:QName.
   @_QName                               ResultID                       0;  ///< Optional attribute.
};

Answer 2

Here is something that might be of help. I cannot test it right now, so it is just a suggestion. Look at 'Void pointers': http://www.cs.fsu.edu/~engelen/soapdoc2.html#tth_sEc11.9

struct myStruct 
{ 
   int __type; // the SOAP_TYPE pointed to by p 
   void *p; 
};

As I understand from docs, this is used to serialize anything, with specifying it's type. So it might work to replace the _XML type of the Value element with this struct. Then you just have to set the __type attribute with the corresponding value:

struct ns1_Value {
   int __type; // the SOAP_TYPE pointed to by p 
   void *p; 
}
struct ns1_Value value; 
int n; 
value.p = &n; 
value.__type = SOAP_TYPE_int; 

Another way to solve the problem could be to serialize raw xml instead of the <Value> element... Or to think of some xs:element that extends xs:anyType, but defines a xs:type.. Actually gSoap is a cool product, but to get the best of it you need to learn how to hack it..